3.4.4 Daylighting Control

This group of building descriptors is applicable for spaces that have daylighting controls.

Daylight Modeling Method
Applicability All spaces with daylighting controls
Definition

The method used to model daylighting. Daylighting credits must be calculated based on the local climate and daylight models of the space. Building descriptors are provided in this section for an internal daylighting model and two variations of an external daylighting model:

  1. Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 
  2. External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 
    1. Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 
    2. Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.
      1. The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.
      2. The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 
Other methods may be used by software developers as long as they produce consistent results. Regardless of the method used, it is desirable that all methods have the same key assumptions.
Units List (see above)
Input Restrictions As designed
Baseline Rules Not applicable
Lighting Schedules for Daylighting
Applicability Daylighted spaces that use the schedule adjustment method
Definition A schedule that indicates the reduction in electric lighting for the lighting system that is being controlled. This schedule is applied to the lighting schedule (see above) to produce a schedule for lighting with daylighting controls.
Units Data structure: schedule, fractional
Input Restrictions The schedule of adjustments should account for seasonal variations in the time of day. Since the schedule will apply for both sunny days and overcast days, the adjustments should represent the conservative condition, e.g. the smallest savings.
Baseline Rules Baseline does not have daylighting
Daylight Ratios
Applicability Daylighted spaces that use the daylight ratio method
Definition A matrix of daylight factors for the space that represent the ratio on illumination at the daylighting reference point to the exterior illumination. The simulation engine calculates the daylighting illumination at the reference point based on this information and the exterior illumination and uses the daylighting control building descriptors to determine for each hour how the lighting power is reduced.
Units Data structure: matrix
Input Restrictions The special daylighting program used to calculate the daylight factors should use inputs consistent with those described below for the internal daylight model method.
Baseline Rules The baseline building does not have daylighting
Daylighted Area
Applicability All daylighted spaces
Definition The floor area that is daylighted. Two types of daylighted areas are recognized. The primary daylighted area is the portion that is closest to the daylighting source and receives the most illumination. The secondary daylighted area is an area farther from the daylighting source, but still receives useful daylight.
Units Data structure
Input Restrictions

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking includes ensuring that the following is true:

Sidelit depth is less than or equal to ceiling height. 

Total daylit area is no greater than space area.

Baseline Rules The baseline building does not have daylighting
Reference Position for Illuminance Calculations
Applicability Daylighted spaces that use the internal daylight model method
Definition The position of the daylight reference point within the daylighted space. Lighting controls are simulated so that the illuminance at the reference position is always above the illuminance setpoint. Thus for step switching controls, the combined daylight illuminance plus uncontrolled electric light illuminance at the reference position must be greater than the setpoint illuminance before the controlled stage of lighting can be tuned off. Similarly, dimming controls will be dimmed so that the combination of the daylight illuminance plus the controlled lighting illuminance is equal to the setpoint illuminance.
Units Data structure
Input Restrictions The reference location shall be as far away from daylight apertures as possible (but still within the daylighted area) so that all occupants have sufficient amounts of total illuminance (combined daylight and electric light) under all daylighting conditions.
Baseline Rules The baseline building does not have daylighting.
Illuminance Setpoint
Applicability Daylighted spaces that use the internal daylight model method
Definition The design illuminance for the daylighted space. The daylighting control adjusts the controlled lighting to maintain this level of illuminance at the reference point.
Units Footcandles
Input Restrictions As designed, but should be consistent with the visual tasks in the space and the recommendations of the IESNA.
Baseline Rules Baseline does not have a daylighting control.
Fraction of Controlled Lighting
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition The fraction of the lighting power in the daylighted space that is controlled by daylight. This is applicable when some of the luminaires in the space are controlled by daylighting and others are not. This input can be eliminated if multiple lighting systems are modeled for each space and the system that is controlled by daylight is separately specified.
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules Baseline does not have a daylighting control.
Daylighting Control Type
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

  • Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See [bookref id="example-dimming-daylight-control"].
  • Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See [bookref id="example-dimming-daylight-control"].

[figure title="Example Stepped Daylighting Control" id="example-stepped-daylighting-control"]Example Stepped Daylighting Control[/figure]

[figure title="Example Dimming Daylight Control" id="example-dimming-daylight-control"]Example Dimming Daylight Control[/figure]

Units List (see above)
Input Restrictions As designed
Baseline Rules Baseline does not have a daylighting control.
Minimum Dimming Power Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition The minimum power fraction when controlled lighting is fully dimmed. Minimum power fraction = (Minimum power) / (Full rated power). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules Baseline does not have a daylighting control.
Minimum Dimming Light Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition Minimum light output of controlled lighting when fully dimmed. Minimum light fraction = (Minimum light output) / (Rated light output). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules Baseline does not have a daylighting control.
Number of Control Steps
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Numeric: integer
Input Restrictions Integer less than 10.
Baseline Rules Baseline does not have a daylighting control.
Step Dimming Control Points
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped dimming controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Data structure. Matched pairs of data (light output and fraction of rated power) for the defined number of control steps
Input Restrictions Integer less than 10. More than 10 steps approximate with continuous dimming.
Baseline Rules Baseline does not have a daylighting control.
  • 1. F. Winkelmann & S. Selkowitz. “Daylighting Simulation in DOE-2: Theory, Application, Validation and Applications.” LBL-19829. http://simulationresearch.lbl.gov/dirpubs/19829.pdf
  • 2. Tregenza P., Waters I, “ Daylight coefficients”, Lighting Research and Technology, 15(2), pp. 65- 71, 1983
90.1-2010

This group of building descriptors is applicable for spaces that have daylighting controls.

General Daylighting Requirements for Toplighting

ASHRAE Standard 90.1-2010 requires that spaces have skylights and be daylighted when the space is:
1)    Greater than 5,000 ft²,
2)    Located directly under a roof,
3)    With a ceiling height greater than 15 ft, and
4)    One of the following space types: office, lobby, atrium, concourse, corridor, non-refrigerated warehouse or storage, gymnasium/exercise center, convention center, automotive service, manufacturing, retail, distribution/sorting area, transportation, or workshop.
5)    Located in thermal zones 1 through 5,
6)    Has an allowed interior lighting power density of 0.5 W/ft² or greater, and
7)    Not substantially shaded (see the text below on what qualifies).  Substantial shading occurs when 90% of the qualifying space roof area  is shaded at noon on the summer solstice.

COMNET accredited software shall have the capability to identify these conditions  and model daylighting in the baseline building when they all occur. When required, the baseline building daylighting system shall be defined as follows:

1)    The qualifying space in the baseline building shall have the same physical dimensions and configuration as the rated building. The qualifying space is the entire space, half of which is required to be daylighted.
2)    The qualifying space shall be divided into two equal parts with one part daylighted and the other part not daylighted. For rectangular spaces , the longer dimension shall be bisected creating two equally dimensioned spaces.
3)    The daylighted space (half of the qualifying space) shall have skylights with sizes and spacing defined as follows. These rules result in a skylight area that is equal to approximately 3% of the daylighted area.
     a)    Square skylights shall have a dimension on each side of 0.3  times the ceiling height. There shall be no skylight well.
     b)    Skylights shall be spaced in both directions at a distance of 1.7  times the ceiling height thereby creating a continuous daylighted area in both directions.
     c)    The skylight grid defined above shall be centered in the daylighted portion of the qualifying space such that the edge of skylights are no further than 0.7 times the ceiling height from the edges of the daylighted portion.
4)    The skylights shall have a visible transmittance of 40%. The U-factor and SHGC shall be equal to the prescriptive requirements for plastic skylights on a curb for the applicable climate zone and building classification (nonresidential, residential or semi-heated).
5)    The illuminance setpoint shall be:
     a)    40 footcandles for lobby, atrium, concourse, corridor, gymnasium/exercise center, convention center, automotive service, manufacturing, and retail.
     b)    15 footcandles for non-refrigerated warehouse or storage, gymnasium/exercise center, and distribution/sorting area, transportation, or workshop.
6)    The daylighting reference point shall be positioned in the center of the daylighted space.
7)    The electric lighting system shall be automatically controlled by step switching in response to available daylight with the first control step at 67% of design lighting power and the second control step at 33% of design power. 
     a)    Daylighting Control Type = Step Switching
     b)    Number of Control Steps = Three (100%, 67% and 33%)
8)    The electric lighting controls shall capable of being easily and accurately calibrated . In particular:
     a)    The light sensor shall be remote from where calibration adjustments are made, and
     b)    The calibration adjustments shall be readily accessible.

General Daylighting Requirements for Sidelighting

ASHRAE Standard 90.1-2010 requires that primary sidelighted areas have automatic daylighting controls. Primary sidelighted areas that require such controls are defined as follows:
1)    The depth of the primary sidelighted area is equal to the window head height, where the window head height is the distance from the floor to the top of the window.
2)    The width of the primary sidelighted area is the width of the window plus 2 ft on each side. 
3)    The area of the primary sidelighted area in an enclosed space is equal to or greater than 250 ft². The area is the depth of the primary sidelighted area times the combined width.
4)    The windows creating the primary sidelighted area have an effective aperture of 0.1 or greater. The effective aperture is the product of the window wall ratio and the visible transmission of the window divided by the primary sidelighted area.
5)    The windows creating the primary sidelighted area are not significantly shaded. Significant shading is deemed to occur when structures exist that have a profile angle of 45 degrees or greater. The profile angle is the angle between the horizon and a line from the top of the window to the top of adjacent structures.

COMNET accredited software shall have the capability to identify these conditions  and model daylighting controls in the baseline building when they all conditions occur. When required, the baseline building daylighting system shall have the following characteristics:

1)    The physical configuration of the enclosed space and the windows that create the primary sidelighted area shall be the same as the rated building, following the rules of Section 6.5.7.
2)    The windows shall have a U-factor, SHGC and VT  equal to the prescriptive requirements for the applicable climate zone and building classification (nonresidential, residential or semi-heated).
3)    The illuminance setpoint shall be:
     a)    40 footcandles for lobby, atrium, concourse, corridor, gymnasium/exercise center, convention center, automotive service, manufacturing, and retail.
     b)    15 footcandles for non-refrigerated warehouse or storage, gymnasium/exercise center, and distribution/sorting area, transportation, or workshop.
4)    The daylighting reference point shall be positioned mid distance in the width of the space and located two thirds of the way to the back of the daylighted space.
5)    The electric lighting system shall be automatically controlled by step switching in response to available daylight with the first control step at 67% of design lighting power and the second control step at 33% of design power
     a)    Daylighting Control Type = Step Switching
     b)    Number of Control Steps = Three (100%, 67% and 33%)
6)    The electric lighting controls shall capable of being easily and accurately calibrated. In particular:
     a)    The light sensor shall be remote from where calibration adjustments are made, and
     b)    The calibration adjustments shall be readily accessible.

Daylight Modeling Method
Applicability All spaces with daylighting controls
Definition

The method used to model daylighting. Daylighting credits must be calculated based on the local climate and daylight models of the space. Building descriptors are provided in this section for an internal daylighting model and two variations of an external daylighting model:

  1. Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 
  2. External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 
    1. Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 
    2. Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.
      1. The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.
      2. The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 
Other methods may be used by software developers as long as they produce consistent results. Regardless of the method used, it is desirable that all methods have the same key assumptions.
Units List (see above)
Input Restrictions As designed
Baseline Rules When the baseline building is required to have daylight controls, the baseline building shall use the same daylighting method as the rated building. If the rated building does not have daylighting, then an internal daylighting model shall be used for the baseline building.
Lighting Schedules for Daylighting
Applicability Daylighted spaces that use the schedule adjustment method
Definition A schedule that indicates the reduction in electric lighting for the lighting system that is being controlled. This schedule is applied to the lighting schedule (see above) to produce a schedule for lighting with daylighting controls.
Units Data structure: schedule, fractional
Input Restrictions The schedule of adjustments should account for seasonal variations in the time of day. Since the schedule will apply for both sunny days and overcast days, the adjustments should represent the conservative condition, e.g. the smallest savings.
Baseline Rules This is applicable when the baseline building is required to have daylilghting (see general requirements above) and when the rated building also has daylighting and uses the schedule adjustment method. In these cases, a daylight schedule is also developed for the baseline building using procedures identical to those used for the rated building.
Daylight Ratios
Applicability Daylighted spaces that use the daylight ratio method
Definition A matrix of daylight factors for the space that represent the ratio on illumination at the daylighting reference point to the exterior illumination. The simulation engine calculates the daylighting illumination at the reference point based on this information and the exterior illumination and uses the daylighting control building descriptors to determine for each hour how the lighting power is reduced.
Units Data structure: matrix
Input Restrictions The special daylighting program used to calculate the daylight factors should use inputs consistent with those described below for the internal daylight model method.
Baseline Rules This is applicable when the baseline building is required to have daylilghting (see general requirements above) and when the rated building also has daylighting and uses the daylight ratio method. In these cases, a (set of) daylight ratio is also developed for the baseline building using procedures identical to those used for the rated building.
Daylighted Area
Applicability All daylighted spaces
Definition The floor area that is daylighted. Two types of daylighted areas are recognized. The primary daylighted area is the portion that is closest to the daylighting source and receives the most illumination. The secondary daylighted area is an area farther from the daylighting source, but still receives useful daylight.
Units Data structure
Input Restrictions

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking includes ensuring that the following is true:

Sidelit depth is less than or equal to ceiling height. 

Total daylit area is no greater than space area.

Baseline Rules Defaults shall be used to define the daylighted area for the baseline building, both under skylights and primary sidelighted area.
Reference Position for Illuminance Calculations
Applicability Daylighted spaces that use the internal daylight model method
Definition The position of the daylight reference point within the daylighted space. Lighting controls are simulated so that the illuminance at the reference position is always above the illuminance setpoint. Thus for step switching controls, the combined daylight illuminance plus uncontrolled electric light illuminance at the reference position must be greater than the setpoint illuminance before the controlled stage of lighting can be tuned off. Similarly, dimming controls will be dimmed so that the combination of the daylight illuminance plus the controlled lighting illuminance is equal to the setpoint illuminance.
Units Data structure
Input Restrictions The reference location shall be as far away from daylight apertures as possible (but still within the daylighted area) so that all occupants have sufficient amounts of total illuminance (combined daylight and electric light) under all daylighting conditions.
Baseline Rules This is applicable to the baseline building when the baseline building has daylighting and the internal daylight model method is used (see rules above). In these cases, the reference position shall be in the center of toplighted spaces. For primary sidelighted areas, the reference position shall be in the center of the width and located two thirds of the way toward the back of the daylighted space (away from the window).
Illuminance Setpoint
Applicability Daylighted spaces that use the internal daylight model method
Definition The design illuminance for the daylighted space. The daylighting control adjusts the controlled lighting to maintain this level of illuminance at the reference point.
Units Footcandles
Input Restrictions As designed, but should be consistent with the visual tasks in the space and the recommendations of the IESNA.
Baseline Rules The illuminance setpoint shall be 40 footcandles for lobby, atrium, concourse, corridor, gymnasium/exercise center, convention center, automotive service, manufacturing, and retail; and 15 footcandles for non-refrigerated warehouse or storage, gymnasium/exercise center, and distribution/sorting area, transportation, or workshop.
Fraction of Controlled Lighting
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition The fraction of the lighting power in the daylighted space that is controlled by daylight. This is applicable when some of the luminaires in the space are controlled by daylighting and others are not. This input can be eliminated if multiple lighting systems are modeled for each space and the system that is controlled by daylight is separately specified.
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules For the baseline building, the daylighted areas are separated, therefore, all lighting in the daylighted areas of the baseline building shall be controlled.
Daylighting Control Type
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

  • Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See Figure 6.4.4-1.
  • Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See Figure 6.4.4-2.
  • Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See Figure 6.4.4-1.
  • Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See Figure 6.4.4-2.

Example Stepped Daylighting Control

 

Figure 6.4.4-1: Example Stepped Daylighting Control

 

Example Dimming Daylight Control

 

Figure 6.4.4-2: Example Dimming Daylight Control

Units List (see above)
Input Restrictions As designed
Baseline Rules When the baseline building requires daylighting, stepped switching control shall be used.
Minimum Dimming Power Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition The minimum power fraction when controlled lighting is fully dimmed. Minimum power fraction = (Minimum power) / (Full rated power). See Figure 6.4.4-2.
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules Not applicable when the baseline building is required to have daylighting since stepped controls are used.
Minimum Dimming Light Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition Minimum light output of controlled lighting when fully dimmed. Minimum light fraction = (Minimum light output) / (Rated light output). See Figure 6.4.4-2.
Units Numeric: fraction
Input Restrictions As designed
Baseline Rules Not applicable when the baseline building is required to have daylighting since stepped controls are used.
Number of Control Steps
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Numeric: integer
Input Restrictions Integer less than 10.
Baseline Rules When the baseline building is required to have daylighting, three control steps shall be used: 100%, 67% and 33% of maximum lighting power.
Step Dimming Control Points
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped dimming controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Data structure. Matched pairs of data (light output and fraction of rated power) for the defined number of control steps
Input Restrictions Integer less than 10. More than 10 steps approximate with continuous dimming.
Baseline Rules Not applicable when the baseline building is required to have daylighting since stepped controls are used.
  • 1. F. Winkelmann & S. Selkowitz. “Daylighting Simulation in DOE-2: Theory, Application, Validation and Applications.” LBL-19829. http://simulationresearch.lbl.gov/dirpubs/19829.pdf
  • 2. Tregenza P., Waters I, “ Daylight coefficients”, Lighting Research and Technology, 15(2), pp. 65- 71, 1983
90.1-2016 BM

This group of building descriptors is applicable for spaces that have daylighting controls.

Daylight Modeling Method

Applicability

All spaces with daylighting controls

Definition

The method used to model daylighting. Building descriptors are provided in this section for an internal daylighting model, two variations of an external daylighting model, and a simplified approach based on power adjustment factors (PAFs):

1.Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 

2.External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 

A.Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 

B.Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.

a.The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.

b.The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 

3.Power adjustment factors. With this method, the lighting power in qualifying daylighted zones may be reduced using power adjustment factors (PAFs). The procedure is similar to how PAFs are applied for occupant sensors and timeclocks.

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

The baseline building has no daylight controls.

 

Lighting Schedules for Daylighting

Applicability

Daylighted spaces that use the schedule adjustment method to model daylighting

Definition

A schedule that indicates the reduction in electric lighting for the lighting system that is being controlled. This schedule is applied to the lighting schedule (see above) to produce a schedule for lighting with daylighting controls.

Units

Data structure: schedule, fractional

Input Restrictions

The schedule of adjustments should account for seasonal variations in the time of day. Since the schedule will apply for both sunny days and overcast days, the adjustments should represent the conservative condition, e.g. the smallest savings.

Baseline Rules

This is applicable when the baseline building is required to have daylilghting (see general requirements above) and when the rated building also has daylighting and uses the schedule adjustment method. In these cases, a daylight schedule is also developed for the baseline building using procedures identical to those used for the rated building.

 

Daylight Ratios

Applicability

Daylighted spaces that use the daylight ratio method

Definition

A matrix of daylight factors for the space that represent the ratio on illumination at the daylighting reference point to the exterior illumination. The simulation engine calculates the daylighting illumination at the reference point based on this information and the exterior illumination and uses the daylighting control building descriptors to determine for each hour how the lighting power is reduced.

Units

Data structure: matrix

Input Restrictions

The special daylighting program used to calculate the daylight factors should use inputs consistent with those described below for the internal daylight model method.

Baseline Rules

This is applicable when the baseline building is required to have daylilghting (see general requirements above) and when the rated building also has daylighting and uses the daylight ratio method. In these cases, a (set of) daylight ratio is also developed for the baseline building using procedures identical to those used for the rated building.

 

Daylighted Area

Applicability

Daylighted spaces that use the PAF method to calculate daylighting benefit. It is not necessary to enter the daylight area when the internal or external models are used since the daylighted area will be equal to the space area.

Definition

The floor area that is daylighted. With the PAF method, two types of sidelighted areas are recognized: the primary daylighted area is the portion that is closest to the daylighting source and receives the most illumination; and the secondary daylighted area is an area farther from the daylighting source, but still receives useful daylight. A third type is the daylighted area under a skylight.

Units

ft²

Input Restrictions

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking should ensure that the sidelit depth is less than or equal to ceiling height and the total daylighted area under skylights is no greater than space area.

Baseline Rules

The baseline building does not have daylighting.

 

Reference Position for Illuminance Calculations

Applicability

Daylighted spaces that use the internal daylight model method

Definition

The position of the daylight reference point within the daylighted space. Lighting controls are simulated so that the illuminance at the reference position is always above the illuminance setpoint. Thus for step switching controls, the combined daylight illuminance plus uncontrolled electric light illuminance at the reference position must be greater than the setpoint illuminance before the controlled stage of lighting can be tuned off. Similarly, dimming controls will be dimmed so that the combination of the daylight illuminance plus the controlled lighting illuminance is equal to the setpoint illuminance.

Units

Data structure

Input Restrictions

The reference location shall be as far away from daylight apertures as possible (but still within the daylighted area) so that all occupants have sufficient amounts of total illuminance (combined daylight and electric light) under all daylighting conditions.

Baseline Rules

This is applicable to the baseline building when the baseline building has daylighting and the internal daylight model method is used (see rules above). In these cases, the reference position shall be in the center of toplighted spaces. For primary sidelighted areas, the reference position shall be in the center of the width and located two thirds of the way toward the back of the daylighted space (away from the window).

 

Illuminance Setpoint

Applicability

Daylighted spaces that use the internal daylight model method

Definition

The design illuminance for the daylighted space. The daylighting control adjusts the controlled lighting to maintain this level of illuminance at the reference point.

Units

Footcandles

Input Restrictions

As designed, but should be consistent with the visual tasks in the space and the recommendations of the IESNA.

Baseline Rules

The illuminance setpoint shall be 40 footcandles for lobby, atrium, concourse, corridor, gymnasium/exercise center, convention center, automotive service, manufacturing, and retail; and 15 footcandles for non-refrigerated warehouse or storage, gymnasium/exercise center, and distribution/sorting area, transportation, or workshop.

 

Fraction of Controlled Lighting

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method

Definition

The fraction of the lighting power in the daylighted space that is controlled by daylight. This is applicable when some of the luminaires in the space are controlled by daylighting and others are not. This input can be eliminated if multiple lighting systems are modeled for each space and the system that is controlled by daylight is separately specified.

Units

Numeric: fraction

Input Restrictions

As designed

Baseline Rules

For the baseline building, the daylighted areas are separated, therefore, all lighting in the daylighted areas of the baseline building shall be controlled.

 

Daylighting Control Type

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method

Definition

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

  • Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See Figure 3.4.4-1.
  • Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See Figure 3.4.4-2.
  • Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See Figure 3.4.4-1.
  • Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See Figure 3.4.4-2.

daylight_control_stepped.jpg

Figure 3.4.4-1: Example Stepped Daylighting Control

daylight_control_continuous.jpg

Figure 3.4.4-2: Example Dimming Daylight Control

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

The baseline building does not have daylighting.

 

Minimum Dimming Power Fraction

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls

Definition

The minimum power fraction when controlled lighting is fully dimmed. Minimum power fraction = (Minimum power) / (Full rated power). See Figure 3.4.4-2.

Units

Numeric: fraction

Input Restrictions

As designed

Baseline Rules

The baseline building does not have daylighting.

 

Minimum Dimming Light Fraction

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls

Definition

Minimum light output of controlled lighting when fully dimmed. Minimum light fraction = (Minimum light output) / (Rated light output). See Figure 3.4.4-2.

Units

Numeric: fraction

Input Restrictions

As designed

Baseline Rules

The baseline building does not have daylighting.

 

Number of Control Steps

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped controls

Definition

Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.

Units

Numeric: integer

Input Restrictions

Integer less than 10.

Baseline Rules

The baseline building does not have daylighting.

 

Step Dimming Control Points

Applicability

Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped dimming controls

Definition

Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.

Units

Data structure. Matched pairs of data (light output and fraction of rated power) for the defined number of control steps

Input Restrictions

Integer less than 10. More than 10 steps approximate with continuous dimming.

Baseline Rules

The baseline building does not have daylighting.

 

Power Adjustment Factors (PAF)

Applicability

Daylighted spaces that are modeled using the power adjustment factor

Definition

The power reduction associated with the space and the type of control. The PAFs apply to three daylight configurations: primary sidelighted areas, secondary sidelighted areas, and toplighted areas. These daylight configurations are defined in Standard 90.1-2016-BM. To qualify, primary sidelighted areas must have an effective aperture greater than 0.15, secondary sidelighted areas must have an effective aperture greater than 0.30, and toplighted areas must have an effective aperture of greater than 0.01. Effective aperture is defined in Standard 90.1-2016-BM. The PAFs are shown in the table below and depend on the type of daylighting control and the size of the daylighted area.

Table 3.4.4-1: Daylighting Power Adjustment Factors

Daylighting Configuration

Effective Aperture

Daylighted Area

PAF

Control Type

Primary sidelighted areas

> 0.15

≥ 250 ft²

0.10 (note 1)

Automatic bi-level or multi-level switching

< 250 ft²

0.10

< 250 ft²

0.10

Automatic continuous daylight dimming

≥ 250 ft²

0.20

Secondary sidelighted areas

> 0.30

n. a.

0.10

Toplighted areas

> 0.01

< 900 ft²

0.20

≥ 900 ft²

0.10

Notes:                                                             

1.Only applicable to retail display areas

2.Other PAFs are applicable for an office, conference room, meeting room, classroom (lecture/ training), retail sales area, lobby, atrium, dining area, corridors/ stairways, gym/ pool, mall concourse, and parking garage

Notes from Table 9.6.2: Control factors may not be used if controls are used to satisfy exceptions to Section 5.5.4.2.3.

 

Units

Unitless

Input Restrictions

As designed

Baseline Rules

The baseline building does not have daylighting.

 

  • 1. F. Winkelmann & S. Selkowitz. “Daylighting Simulation in DOE-2: Theory, Application, Validation and Applications.” LBL-19829. http://simulationresearch.lbl.gov/dirpubs/19829.pdf
  • 2. Tregenza P., Waters I, “ Daylight coefficients”, Lighting Research and Technology, 15(2), pp. 65- 71, 1983
Building EQ

This group of building descriptors is applicable for spaces that have daylighting controls.

Daylight Modeling Method
Applicability All spaces with daylighting controls
Definition

The method used to model daylighting. Daylighting credits must be calculated based on the local climate and daylight models of the space. Building descriptors are provided in this section for an internal daylighting model and two variations of an external daylighting model:

  1. Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 
  2. External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 
    1. Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 
    2. Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.
      1. The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.
      2. The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 
Other methods may be used by software developers as long as they produce consistent results. Regardless of the method used, it is desirable that all methods have the same key assumptions.
Units List (see above)
Input Restrictions As designed
Lighting Schedules for Daylighting
Applicability Daylighted spaces that use the schedule adjustment method
Definition A schedule that indicates the reduction in electric lighting for the lighting system that is being controlled. This schedule is applied to the lighting schedule (see above) to produce a schedule for lighting with daylighting controls.
Units Data structure: schedule, fractional
Input Restrictions The schedule of adjustments should account for seasonal variations in the time of day. Since the schedule will apply for both sunny days and overcast days, the adjustments should represent the conservative condition, e.g. the smallest savings.
Daylight Ratios
Applicability Daylighted spaces that use the daylight ratio method
Definition A matrix of daylight factors for the space that represent the ratio on illumination at the daylighting reference point to the exterior illumination. The simulation engine calculates the daylighting illumination at the reference point based on this information and the exterior illumination and uses the daylighting control building descriptors to determine for each hour how the lighting power is reduced.
Units Data structure: matrix
Input Restrictions The special daylighting program used to calculate the daylight factors should use inputs consistent with those described below for the internal daylight model method.
Daylighted Area
Applicability All daylighted spaces
Definition The floor area that is daylighted. Two types of daylighted areas are recognized. The primary daylighted area is the portion that is closest to the daylighting source and receives the most illumination. The secondary daylighted area is an area farther from the daylighting source, but still receives useful daylight.
Units Data structure
Input Restrictions

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking includes ensuring that the following is true:

Sidelit depth is less than or equal to ceiling height. 

Total daylit area is no greater than space area.

Reference Position for Illuminance Calculations
Applicability Daylighted spaces that use the internal daylight model method
Definition The position of the daylight reference point within the daylighted space. Lighting controls are simulated so that the illuminance at the reference position is always above the illuminance setpoint. Thus for step switching controls, the combined daylight illuminance plus uncontrolled electric light illuminance at the reference position must be greater than the setpoint illuminance before the controlled stage of lighting can be tuned off. Similarly, dimming controls will be dimmed so that the combination of the daylight illuminance plus the controlled lighting illuminance is equal to the setpoint illuminance.
Units Data structure
Input Restrictions The reference location shall be as far away from daylight apertures as possible (but still within the daylighted area) so that all occupants have sufficient amounts of total illuminance (combined daylight and electric light) under all daylighting conditions.
Illuminance Setpoint
Applicability Daylighted spaces that use the internal daylight model method
Definition The design illuminance for the daylighted space. The daylighting control adjusts the controlled lighting to maintain this level of illuminance at the reference point.
Units Footcandles
Input Restrictions As designed, but should be consistent with the visual tasks in the space and the recommendations of the IESNA.
Fraction of Controlled Lighting
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition The fraction of the lighting power in the daylighted space that is controlled by daylight. This is applicable when some of the luminaires in the space are controlled by daylighting and others are not. This input can be eliminated if multiple lighting systems are modeled for each space and the system that is controlled by daylight is separately specified.
Units Numeric: fraction
Input Restrictions As designed
Daylighting Control Type
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

  • Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See [bookref id="example-dimming-daylight-control"].
  • Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See [bookref id="example-dimming-daylight-control"].

[figure title="Example Stepped Daylighting Control" id="example-stepped-daylighting-control"]Example Stepped Daylighting Control[/figure]

[figure title="Example Dimming Daylight Control" id="example-dimming-daylight-control"]Example Dimming Daylight Control[/figure]

Units List (see above)
Input Restrictions As designed
Minimum Dimming Power Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition The minimum power fraction when controlled lighting is fully dimmed. Minimum power fraction = (Minimum power) / (Full rated power). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Minimum Dimming Light Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition Minimum light output of controlled lighting when fully dimmed. Minimum light fraction = (Minimum light output) / (Rated light output). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Number of Control Steps
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Numeric: integer
Input Restrictions Integer less than 10.
Step Dimming Control Points
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped dimming controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Data structure. Matched pairs of data (light output and fraction of rated power) for the defined number of control steps
Input Restrictions Integer less than 10. More than 10 steps approximate with continuous dimming.
  • 1. F. Winkelmann & S. Selkowitz. “Daylighting Simulation in DOE-2: Theory, Application, Validation and Applications.” LBL-19829. http://simulationresearch.lbl.gov/dirpubs/19829.pdf
  • 2. Tregenza P., Waters I, “ Daylight coefficients”, Lighting Research and Technology, 15(2), pp. 65- 71, 1983
Energy Star

This group of building descriptors is applicable for spaces that have daylighting controls.

Daylight Modeling Method
Applicability All spaces with daylighting controls
Definition

The method used to model daylighting. Daylighting credits must be calculated based on the local climate and daylight models of the space. Building descriptors are provided in this section for an internal daylighting model and two variations of an external daylighting model:

  1. Internal daylighting model. With this method, the simulation model has the capability to model the daylighting contribution for each hour of the simulation and make an adjustment to the lighting power for each hour, taking into account factors such as daylighting availability, geometry of the space, daylighting aperture, control type and the lighting system. The assumption is that the geometry of the space, the reflectance of surfaces, the size and configuration of the daylight apertures, and the light transmission of the glazing are taken from other building descriptors. 
  2. External daylighting model. An external daylighting model may be used in combination with an hourly simulation program to calculate daylighting savings as long as it produces consistent results and makes use of the key assumptions described below for internal daylighting models. Exterior daylight models include, but are not limited to, the following types of methods: 
    1. Schedule adjustments. With this method, a space is modeled in a stand alone daylighting program to determine the amount of interior daylight available different times of the year and for different times of the day. In addition this program has an electric lighting model that calculates the electricity savings by hour based on interior illuminance and the daylighting control type (switching, dimming etc.). These savings values are converted into a schedule of electric lighting power reduction multipliers. This lighting power reduction schedule is applied to the proposed design energy simulation model and results in reduced electric lighting energy consumption and reduced internal heat gain, both of which are reflected in the proposed design energy consumption. 
    2. Daylight ratio. With this method, an outside program pre-calculates a relationship between outdoor daylight conditions (illuminances or luminances) and interior illuminance. Within the rating software, interior illuminance is calculated from the daylighting ratios and the daylight conditions derived from data on the local weather file. The remainder of the calculations are the same as for an internally calculated daylight model where the interior illuminances are compared to an illuminance setpoint and electric lighting power is calculated based on control type. The two most widely used methods of pre-calculating daylighting ratios are the modified daylight factor method and the daylight coefficients method.
      1. The modified daylight factor method uses pre-calculated diffuse and direct illuminance daylight factors and multiplies these by diffuse and direct beam outdoor illuminance from the weather file to calculate interior illuminance.1  Daylight factors are calculated from a simulation of the space that relies on user entered information about the space modeled such as orientation, geometry, material properties (transmittances and reflectances) etc. For any given hour, the interior illuminance at the reference point is calculated by the direct beam angle specific daylight factor multiplied by the outdoor direct beam and clear sky illuminance and this is added to the overcast daylight factor multiplied by the overcast sky illuminance. Outdoor direct beam, clear sky and overcast sky illuminances are calculated from the weather data used in the proposed building energy simulation.
      2. The daylight coefficients method is essentially a similar but more accurate method that relates internal illuminance to the luminance of patches of the sky.2  The sky is divided up into patches as defined by altitude and azimuth. The daylight coefficients are ratios of interior illuminance to luminance for patches or areas on the sky dome. An outside daylight simulation program uses information about the space modeled: its orientation, geometry, material properties (transmittances and reflectances) etc, and calculates daylight coefficients for each sky patch. The precalculated daylight coefficients are then used to calculate interior illuminances for each hour. The illuminance for a location with the space at any point in time is the product of the luminance for each sky patch multiplied by the specific daylight coefficient for each sky patch integrated over the entire sky dome. The luminance for each sky patch is calculated from the weather data used in the proposed building energy simulation. 
Other methods may be used by software developers as long as they produce consistent results. Regardless of the method used, it is desirable that all methods have the same key assumptions.
Units List (see above)
Input Restrictions As designed
Lighting Schedules for Daylighting
Applicability Daylighted spaces that use the schedule adjustment method
Definition A schedule that indicates the reduction in electric lighting for the lighting system that is being controlled. This schedule is applied to the lighting schedule (see above) to produce a schedule for lighting with daylighting controls.
Units Data structure: schedule, fractional
Input Restrictions The schedule of adjustments should account for seasonal variations in the time of day. Since the schedule will apply for both sunny days and overcast days, the adjustments should represent the conservative condition, e.g. the smallest savings.
Daylight Ratios
Applicability Daylighted spaces that use the daylight ratio method
Definition A matrix of daylight factors for the space that represent the ratio on illumination at the daylighting reference point to the exterior illumination. The simulation engine calculates the daylighting illumination at the reference point based on this information and the exterior illumination and uses the daylighting control building descriptors to determine for each hour how the lighting power is reduced.
Units Data structure: matrix
Input Restrictions The special daylighting program used to calculate the daylight factors should use inputs consistent with those described below for the internal daylight model method.
Daylighted Area
Applicability All daylighted spaces
Definition The floor area that is daylighted. Two types of daylighted areas are recognized. The primary daylighted area is the portion that is closest to the daylighting source and receives the most illumination. The secondary daylighted area is an area farther from the daylighting source, but still receives useful daylight.
Units Data structure
Input Restrictions

The default primary daylight area for sidelighting is a band near the window with a depth equal to the distance from the floor to the top of the window. The default secondary daylight area for sidelighting is a band beyond the primary daylighted area that extends a distance double the distance from the floor to the top of the window. Other daylight areas may be defined with appropriate documentation.

The default primary daylight area for toplighting is a band around the skylight well that has a depth equal to the 70% of the ceiling height. The default secondary daylight area for toplighting is a band beyond the primary daylighted area that extends 140% of the ceiling height.

Daylighted areas may not overlap or extend beyond partitions higher than 5 ft.

Error checking includes ensuring that the following is true:

Sidelit depth is less than or equal to ceiling height. 

Total daylit area is no greater than space area.

Reference Position for Illuminance Calculations
Applicability Daylighted spaces that use the internal daylight model method
Definition The position of the daylight reference point within the daylighted space. Lighting controls are simulated so that the illuminance at the reference position is always above the illuminance setpoint. Thus for step switching controls, the combined daylight illuminance plus uncontrolled electric light illuminance at the reference position must be greater than the setpoint illuminance before the controlled stage of lighting can be tuned off. Similarly, dimming controls will be dimmed so that the combination of the daylight illuminance plus the controlled lighting illuminance is equal to the setpoint illuminance.
Units Data structure
Input Restrictions The reference location shall be as far away from daylight apertures as possible (but still within the daylighted area) so that all occupants have sufficient amounts of total illuminance (combined daylight and electric light) under all daylighting conditions.
Illuminance Setpoint
Applicability Daylighted spaces that use the internal daylight model method
Definition The design illuminance for the daylighted space. The daylighting control adjusts the controlled lighting to maintain this level of illuminance at the reference point.
Units Footcandles
Input Restrictions As designed, but should be consistent with the visual tasks in the space and the recommendations of the IESNA.
Fraction of Controlled Lighting
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition The fraction of the lighting power in the daylighted space that is controlled by daylight. This is applicable when some of the luminaires in the space are controlled by daylighting and others are not. This input can be eliminated if multiple lighting systems are modeled for each space and the system that is controlled by daylight is separately specified.
Units Numeric: fraction
Input Restrictions As designed
Daylighting Control Type
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method
Definition

The type of control that is used to control the electric lighting in response to daylight available at the reference point. The options are:

  • Step Switching controls have discrete steps of light output, where the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Step Dimming controls also have discrete steps of light output but typically the intermediate steps of light output are associated with higher levels of fraction of rated power. When the lights are fully off or fully on, the fraction of rated power matches the fraction of light output. See [bookref id="example-stepped-daylighting-control"].
  • Continuous Dimming controls have a fraction to rated power to fraction of rated output that is a linear interpolation of the minimum power fraction at the minimum diming light fraction to rated power (power fraction = 1.0) at full light output. See [bookref id="example-dimming-daylight-control"].
  • Continuous Dimming + Off controls are the same as continuous dimming controls except that these controls can turn all the way off when none of the controlled light output is needed. See [bookref id="example-dimming-daylight-control"].

[figure title="Example Stepped Daylighting Control" id="example-stepped-daylighting-control"]Example Stepped Daylighting Control[/figure]

[figure title="Example Dimming Daylight Control" id="example-dimming-daylight-control"]Example Dimming Daylight Control[/figure]

Units List (see above)
Input Restrictions As designed
Minimum Dimming Power Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition The minimum power fraction when controlled lighting is fully dimmed. Minimum power fraction = (Minimum power) / (Full rated power). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Minimum Dimming Light Fraction
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and dimming controls
Definition Minimum light output of controlled lighting when fully dimmed. Minimum light fraction = (Minimum light output) / (Rated light output). See [bookref id="example-dimming-daylight-control"].
Units Numeric: fraction
Input Restrictions As designed
Number of Control Steps
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Numeric: integer
Input Restrictions Integer less than 10.
Step Dimming Control Points
Applicability Daylighted spaces that use the internal daylight model method or the daylight ratio method and stepped dimming controls
Definition Number of control steps. For step switching, this term defines even steps of light output and even steps of rated power fractions. For step dimming, identifies number of steps that require fraction of rated light output and rated power fraction.
Units Data structure. Matched pairs of data (light output and fraction of rated power) for the defined number of control steps
Input Restrictions Integer less than 10. More than 10 steps approximate with continuous dimming.
  • 1. F. Winkelmann & S. Selkowitz. “Daylighting Simulation in DOE-2: Theory, Application, Validation and Applications.” LBL-19829. http://simulationresearch.lbl.gov/dirpubs/19829.pdf
  • 2. Tregenza P., Waters I, “ Daylight coefficients”, Lighting Research and Technology, 15(2), pp. 65- 71, 1983