Daylighting
Daylighting is the direct or indirect use of light from the sun to illuminate the interior of buildings and offset the use and power consumption of artificial lights. Providing sunlight through windows and skylights satisfies a deep need in humans to connect to both the outdoors and the diurnal rhythm, which also seems to increase human performance and health. The use of artificial lighting in buildings can be reduced when daylight is available, which can be due to manual or automatic controls of the artificial lighting. Most non-residential building energy codes already require some automatic daylighting control. Still, many opportunities exist to increase daylight utilization in many buildings, and the modeler can help the design team identify and analyze these options. Building energy modeling software has a variety of algorithms to account for the daylight provided to the interior of the building. It can simulate energy savings due to automatic controls. While well-designed automatic daylighting systems almost always save energy overall, end-use energy consumption changes. When modeling daylighting, the annual cooling energy decreases, and the annual heating energy increases due to reduced heat from using less artificial lighting.
How It Works
Daylight Sources
Beams of direct light from the sun can enter a building through fenestration, such as windows and skylights. Additionally, light enters buildings after bouncing and scattering through other indirect routes, including:
- Sky (whether clear, cloudy, or overcast)
- Ground reflection
- Building reflection
Building reflection includes the reflection of sunlight from other buildings as well as portions of the modeled building, such as wings, overhangs, and fins.
The lighting from these indirect paths, since they are more diffuse, is often very desirable for illuminating the inside of buildings. Direct sunlight, if directly properly, can illuminate further into the space, but efforts must be made to combat the glare and provide more diffuse illumination.
Daylight Illuminance
Some terms that are important to understand related to illuminance, which is usually measured in footcandles (lumens/ft2) or lux (lumens/m2) from the sun and sky, are:
- Direct normal illuminance
- Direct horizontal illuminance
- Diffuse horizontal illuminance
- Global horizontal illuminance (direct and diffuse)
These are illustrated in the following diagram:
The total illuminance, which includes direct and diffuse components, is highly variable. The exact position of the sun and, thus, the angle of direct sunlight entering through windows and skylights is exactly known based on location and hour of the year, and building energy models can accurately estimate this, presuming no clouds are blocking the path. The diffuse components, since they are impacted by the cloudiness of the sky, can vary significantly, and typical hourly weather files have been selected to be similar to the most typical conditions, including temperature, humidity, and cloud cover. Building energy models can also accurately estimate the diffuse illumination in the space, but they are based on typical weather conditions. Many weather files include measures of atmospheric moisture and turbidity called aerosol optical depth, which impacts the amount of beam and diffuse light. Some BEM software has a way to add or modify these values for local conditions.
Sun Path Charts
The following chart shows the global horizontal illuminance from a typical weather file on a spherical sun path chart, which shows the angle of the sun at different times of day throughout the year. The following diagram is from the Center for the Built Environment's (CBE) Clima Tool, but other software can produce similar diagrams.
More information on how to read the chart can be found here
Global Horizontal Illuminance
Another view of the annual global horizontal illuminance and the impact of both latitude and local climate is shown below.
Illuminance on Vertical Surfaces
The amount of illuminance on vertical surfaces is even more highly variable and depends on the direction that the surface is facing. The following diagram shows this for San Francisco on a clear day in both June and December.
Building energy modeling software will calculate these values for the exact orientation of surfaces containing windows and skylights.
Daylighting Design Metrics
Sunlight consists of a wide spectrum outside the visible wavelengths, including ultraviolet and infrared. The amount of heat that daylight adds to the space can be expressed similarly to the heat that artificial lighting adds to the space. Fluorescent light provides approximately 100 lumens/watt, and LED systems can provide up to 140 lumens/watt with expectations that much higher efficacy will be available in the future. All of this heat from artificial lights ends up in the space. Daylight is comparable to this, with effectively 108 lumens/watt. Specularly selective high-efficiency windows can filter out certain wavelengths from entering the building, which can effectively result in 210 lumens/wall.
The upper limit for human comfort for illuminance is subjective but is typically in the range of 300 to 500 footcandles. Example design criteria for target illuminance for different activities and people between the ages of 25 and 65 years are:
| Category | Activity | Illuminance Target (footcandles) |
| H | Circulation orientation | 2 |
| K | Public areas | 5 |
| M | Simple tasks | 10 |
| P | Large tasks good contrast | 30 |
| Q | Medium tasks | 40 |
| R | Small tasks good contrast | 50 |
| T | Small tasks poor contrast | 100 |
Some common daylighting design metrics that are often discussed include
- Spatial Daylight Autonomy (sDA) is the fraction of the area with sufficient daylight based on the threshold described in the subscripts.
- Annual Sunlight Exposure (ASE) is the fraction of the area with too much daylight based on the threshold described in the subscripts.
These are expressed using a threshold based on the illumination in lux and the hours they apply. They are calculated based on annual estimates, usually based on simulation. The values for each indicate the amount of the area of the room that meets the threshold described in the subscripted values. The first subscripted value is the amount of illumination expressed in lux, and the second is the fraction of hours that meet or exceed the amount of illumination. For Spatial Daylight Autonomy (sDA), the larger the value, the better the space is illuminated for people to take advantage of the daylight. A typical threshold for sDA is 300 lux or more for 50% or more of the hours shown as sDA300,50%. For LEED, the target values for sDA300,50% are 40%, 55%, or 75% of the floorspace meeting this requirement. Annual Sunlight Exposure (ASE) indicates too much light exposure; the lower the number, the better, and indicates what portion of the space has too much glare or uncomfortably bright conditions. A typical threshold for ASE is 1000 lux for 250 hours, shown as ASE1000,250, and for LEED, values greater than 10% usually the designer needs to identify how glare will be addressed.
Another metric used in USGBC’s LEED program is based on the hour’s fraction of the floor area with illuminance between 300 and 3000 lux is based on computer simulation and described as:
“Perform computer simulations for illuminance at 9 a.m. and 3 p.m. on a clear-sky day at the equinox for each regularly occupied space. Healthcare projects should use the regularly occupied spaces located in the perimeter area determined under EQ Credit Quality Views.
Demonstrate illuminance levels are between 300 lux and 3,000 lux at both 9 a.m. and 3 p.m. Spaces with view-preserving automatic (with manual override) glare-control devices may demonstrate compliance for only the minimum 300 lux illuminance level.
Calculate illuminance intensity for sun (direct component) and sky (diffuse component) for clear-sky conditions as follows:
● Use typical meteorological year data, or an equivalent, for the nearest available weather station.
● Select one day within 15 days of September 21 and one day within 15 days of March 21 that represent the clearest sky condition.
● Use the average of the hourly value for the two selected days.
Exclude blinds or shades from the model. Include any permanent interior obstructions. Moveable furniture and partitions may be excluded.”
For most buildings, LEED points can be earned when 55%, 75%, or 90% of the regularly occupied floor areas meet these criteria. The following diagram illustrates this procedure:
Daylighting Strategies
Building Energy Modeling can help evaluate many different design options that impact daylighting, including:
- Building form and orientation
- Space dimensions - Ceiling height, space depth
- Space planning - Consider whether the direct sun is ok or not
- Size and location of openings
- Shading devices - Fixed or variable
- Light redirecting devices - e.g., light shelves
- Glazing selection - Visible light transmittance (VLT), Diffusion
- Surface reflectances - Bouncing, diffusing surfaces, indoors and outdoors
The two major categories of fenestration openings to get light into the space are called top lighting and side lighting. Top lighting comprises skylights, clerestories, sawtooth openings, and roof monitors. Top lighting tries to prevent direct sunlight from entering the space by either using diffusing glass or by bouncing the sunlight off walls and other surfaces. Side lighting consists of windows, which may be augmented by overhangs, other shading devices, and light shelves to push daylight further into the space. Since the angle of the sun varies throughout the year, overhangs can help mitigate excessive sunlight during the summer months and allow sunlight deeper into the space during the winter months. The goal with any side lighting is to try to illuminate deep into the space while providing good views but without introducing excess heat or glare. To achieve this, high windows are often used with overhangs or light shelves on the southern exposure with operable shades on the east and west exposures.
In general, daylighting design principles that can be evaluated with building energy modeling include:
- Avoid direct sun
- Except in circulation/transitory spaces
- Introduce daylight:
- As high in the space as possible
- On two sides, if possible
- Use light-colored surfaces to reflect & distribute light
- Keep sunlit surfaces out of the line of sight
- Use architectural shielding, overhangs, light shelves, fins, shades, and/or blinds
- Provide adjustable blinds or louvers where there is potential for glare
- Reflected sunlight from cars and buildings can be as annoying as direct sunlight
Many resources that provide more detailed information about daylighting are available. See the resources section below.
Information Needed for the Model
There are three main algorithms used in building energy modeling to estimate the distribution of daylight in a building:
- Split-flux method
- Radiosity
- Ray tracing
Split-Flux Method
The split-flux method is the simplest of the three algorithms, requires the least amount of input, and is the fastest to execute but is also the least accurate. The “procedure separates the light reaching the point being considered into three components, these being (a) light directly from the sky, (b) light after reflected by external, and (c) internal surfaces.” Where fs is from the sky, fg is from the ground, Rfw is reflected on the floor and walls below mid-height, and Rcw is reflected on the ceiling and walls above mid-height. The method assumed the interior of a room was similar to a sphere with no internal partitions and could be applied to rooms.
(Bryan, H. J. and Clear, R. D. “A Procedure for Calculating Interior Daylight Illumination with a Programmable Calculator". LBNL. October 1980. LBL-11186. )
The required inputs for the split-flux method include
- reflectivity of the surfaces
- size of the window or skylight
- location and size of overhangs or fins
- optical characteristics of the window or skylights like absorptance and transmittance
- sky clearness
- ground reflectance
- location of the daylight sensor
- view factors to the ground
- lighting control approach
- window shading details and daylighting control
Certain implementations of the split-flux method may have lower accuracy when modeling
- light shelves
- light scoops or pipes
- skylights with deep wells
- roof monitors
- rooms with internal obstructions
- deep sensor locations
- light through internal windows such as from atriums
- complicated blinds and shades that are highly directional
Radiosity Method
The radiosity method uses an energy balance similar to heat transfer but instead is applied to light. A lumped parameter network is solved similarly to an electrical network. The inputs it requires are similar to the split-flux method. The computational effort is also significantly greater than split-flux but not as much as ray-tracing. Although not typically done with building energy modeling software, rendering a scene using a radiosity algorithm results in a nearly realistic portrayal of the lighting in the space.
Ray Tracing Method
Ray tracing, as its name implies, actually tracks the rays of light that are coming into a space and how they are reflected and distributed inside and finally back to an eye’s point of view. Originally developed to provide a realistic rendering of a scene, ray tracing is much more computationally expensive but results in the most accurate approach of the three. Two approaches to ray tracing have been used: tracing from the eye through the space to the light sources and tracing from the light source through the space to the eye. The input for a ray tracing simulation is generally similar to the radiosity and split-flux methods, but the surface optical properties are generally more detailed. Any internal partitions or furniture or any other expected content of the room should be included so that the rendered perspective is as accurate as possible. It is very common to use ray tracing on just a few example zones and presume that the daylighting characteristics can be extrapolated to the entire building.
Bidirectional Scattering Distribution Functions
Typically, daylighting algorithms are tightly coupled with fenestration algorithms. One implication is that even though BEM software may have multiple daylighting and multiple fenestration algorithm options, the combinations may be more limited, so care should be taken by the modeler before investing significant time to ensure that the fenestration being evaluated is supported with a specific daylighting algorithm.
One particular fenestration modeling challenge is for shades and blinds that have more complicated light transmissions and reflections that have multiple directional components. These are often described as BSDF (bidirectional scattering distribution functions) and are made up of the transmitted and reflected portions and are needed for Venetian blinds and other window covering that may scatter light in multiple directions. The data to represent the complexity of these window elements are sometimes captured in an XML file referenced as BSDF files or else has to be provided to the BEM software using matrices of numbers representing the optical properties at different angles.
While building energy modeling software could use any of the three algorithms, split-flux and radiosity are the most common. Software that uses ray tracing is often separate and may require duplicate descriptions of the surfaces and their optical properties or else extra steps to integrate data between the ray tracing and BEM programs.
Multipliers for walls and windows may not be compatible with any daylighting algorithm since the specific path and accounting for light entering and being distributed in the room must be performed realistically.
Common Control Options
Ultimately, it is not simply how sunlight can illuminate a room but how much reduction in artificial lighting can be achieved. To do this, the control of the room’s lighting system must be simulated. This starts with the lighting sensor, sometimes called a control point or reference point, which is the “eye” in the room to determine how much artificial light is needed. These sensors are one end of the path being traced for ray tracing. For actual buildings, multiple sensors are often deployed in a room, each controlling different banks of lights. This is most commonly arranged in a strip of lights and a corresponding sensor close to the window and another strip of lights and sensor further from the window. The sensors are often located at desk height but may also be on the ceiling. The same system that controls the artificial lights for a room may also control any automatic shading devices to cover windows fully or partially when too much glare is detected.
To control interior lights, the sensors can typically control the lights using stepped dimming, continuous dimming, or continuous dimming with a cutoff.
(EnergyPlus Input Output Reference Figure 1.74 and Figure 1.73)
With the increase in the number of LED luminaires, continuous dimming has become more common.
Fixture level controls for daylighting are also available so that a room may have slightly different lighting output for each fixture. To simulate this, a large number of sensor points need to be able to be simulated.
Common Measures
Many different design options may be considered to increase the effectiveness of the daylighting system:
- Continuous instead of stepped dimming controls
- Luminaire level controls
- Light shelves
- Automatic shading
- Overhangs and fins
- Top lighting such as skylights, clerestories, sawtooth openings, and roof monitors
- Light baffles and other diffusers
Depending on how complicated the design options being considered, it might make sense to utilize ray-tracing software in conjunction with the building energy modeling software.
Model Output Checks
The variation of lighting power in zones using daylighting should be reviewed to ensure daylighting operates as expected. To do this, look at detailed results on a timestep basis, including the lighting power in the zone with daylighting controls, the global horizontal illuminance from the sun on the outside, and the amount of light transmitted through the windows. While it is not worth trying to recreate the daylighting calculations, it is important that the amplitude and variation of daylight make sense. For two zones facing the same orientation but with different depths and similar-sized windows, confirm that the fraction of lighting power for the same hour is greater for the shallower zone. Compare zones facing different orientations and if they match your intuition on when sunlight should have the greatest and smallest impact on the zone. Look at both cloudy and sunny days. On shorter and longer days of the year, near the solstices, confirm the daily lighting power in zones with daylighting controls and, if possible, confirm the lighting power during the winter solstice but during normal operating hours. If automatic shading controls are included, confirm their operation during these times.
Related Energy Code Requirements
Today, most new buildings are covered by building energy codes that include requirements for providing daylighting controls for specific space types with the presence of windows, skylights, or other fenestration. There may even be requirements for a minimum area for skylights for some types of spaces. In ASHRAE Standard 90.1-2022, the term “daylight” is mentioned over 150 times, including in:
- Section 3 Definitions
- Section 5 Building Envelope
- Section 9 Lighting
- Section 11 Additional Efficiency Requirements
- Section 12 Energy Cost Budget Method
- Appendix G Performance Rating Method
For the ones most related to building energy modeling, Section 12 states in Table 12.5.1 for the proposed design:
“Automatic daylighting controls included in the proposed design may be modeled directly in the building simulation or be modeled in the building simulation through schedule adjustments determined by a separate analysis approved by the authority having jurisdiction. Modeling and schedule adjustments shall separately account for primary sidelighted areas, secondary sidelighted areas, and toplighted areas.”
As well as requiring the budget building:
“Additional interior lighting power for nonmandatory controls allowed under Table 9.5.2.3 shall not be included in the budget building design.”
Appendix G in Table G3.1 for the proposed states:
“g. For lighting controls, at a minimum, the proposed design shall contain the mandatory automatic lighting controls specified in Section 9.4.1 (e.g., automatic daylight responsive controls, occupancy sensors, programmable controls, etc.). These controls shall be modeled in accordance with (h) and (i).
h. Automatic daylighting responsive controls shall be modeled directly in the proposed design or through schedule adjustments determined by a separate daylighting analysis approved by the rating authority. Modeling and schedule adjustments shall separately account for primary sidelighted areas, secondary sidelighted areas, and toplighted areas.”
And for the baseline building model states:
“No additional automatic lighting controls, e.g., automatic controls for daylight utilization and occupancy sensors in space types not listed above, shall be modeled in the baseline building design.”
Additional Resources
2021 ASHRAE Handbook – Fundamentals – Chapter 15 Fenestration - Section 8.1Daylight Prediction
An J, Mason S. Integrating Advanced Daylight Analysis Into Building Energy Analysis. SimBuild 2010. IBPSA-USA. August 2010.
Boubekri M. Daylighting Design: Planning Strategies and Best Practice Solutions. Basel/Berlin/Boston: Birkhäuser; 2014. doi:10.1515/9783038214786
Daylight Harvesting - Wikipedia
Daylight in Buildings - a source book on daylighting systems and components A report of IEA SHC Task 21/ ECBCS Annex 29, July 2000.
Daylighting - Whole Building Design Guide
Daylighting - Wikipedia
Gherri B. Assessment of Daylight Performance in Buildings: Methods and Design Strategies. Southampton: WIT Press; 2015. Accessed December 26 2023.
Glazer J. ASHRAE 1651-RP Development of Maximum Technically Achievable Energy Targets for Commercial Buildings Ultra-Low Energy Use Building Set. December 31, 2015.
Grondzik WT, Kwok AG. Mechanical and Electrical Equipment for Buildings. Thirteenth ed. Hoboken, New Jersey: John Wiley & Sons; 2019.
Heschong L. Visual Delight in Architecture Daylight Vision and View. Milton: Taylor & Francis Group; 2021. . Accessed December 26 2023.
IEA SHC
- https://task21.iea-shc.org/publications
- https://task31.iea-shc.org/publications
- https://task50.iea-shc.org/publications
- https://task61.iea-shc.org/publications
Illuminating Engineering Society of North America Daylighting Committee. Lighting Practice: Designing and Specifying Daylighting for Buildings : An American National Standard. New York, New York: Illuminating Engineering Society; 2020.
Integrated daylighting and lighting in practice. IEA SHC Task 61. June 2021
Johnsen K, Watkins R. Daylight in Buildings. ECBCE Annex 29 / SHC Task 21. 2010.
Konis K, Selkowitz S, Effective Daylighting with High-Performance Facades : Emerging Design Practices. Cham Switzerland: Springer; 2017. doi:10.1007/978-3-319-39463-3
LEED v4.1 Building Design and Construction. Getting started guide for beta participants July 2023.
Luis Filipe Batista Silveira Dos Santos. Efficient Modeling Strategies for Performance-based Building Design Supported by Daylight and Building Energy Simulation. PhD Dissertation. University of California, Berkeley. 2020
Luminaire Level Lighting Control. Zero net Energy Technology Application Guide.
Meek, Christopher. Introduction to Daylighting. Lighting Design Lab. May 19, 2020.
O'Conner J, Lee ES, Rubinstein FM, Selkowitz SE. Tips for Daylighting with Windows: The Integrated Approach. 1997 LBNL-39945.
Papamichael K, Hathaway NG. Daylighting Harvesting for Commercial Buildings. University of California. 2018.
Phillips D Gardner C. Daylighting: Natural Light in Architecture. Amsterdam: Elsevier Architectural Press; 2004.
Reinhart CF, Stein R. Daylighting Handbook. Volume I Fundamentals Designing with the Sun. United States: Christoph Reinhart; 2014.
Reinhart CF, Stein R. Daylighting Handbook Volume II Daylight Simulations Dynamic Façades. United States: Christoph Reinhart; 2018.
Ticleanu C., Howlett G, Villa L, Le Gall G. Daylight calculation methods in BS EN 17037. 2023. CIBSE RI07.
Waskett RK Society of Light and Lighting. Lighting Guide 10 : Daylighting : A Guide for Designers. New ed. London: Chartered Inst. of Building Services Engineers; 2014.
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