The basics of daylighting

A design strategy needs to be considered in order to provide optimal daylighting in a space.

By Alexander Sassoon, EIT, LEED AP BD+C, WELL AP, P2S, Long Beach, Calif. June 20, 2017

Learning Objectives

  • How to identify successful daylighting design.
  • Explore the elements of daylighting.
  • Learn how to integrate daylighting and lighting control systems.

Effective daylight design in buildings creates a comfortable and productive environment for occupants. Effective daylighting can reduce energy use, but also must limit HVAC loads. The U.S Green Building Council’s LEED rating system and the International WELL Building Institute’s WELL Building Standard are two well-known rating systems that reward outstanding daylighting design.

Daylighting design

It’s important to first identify the elements of successful daylighting design. Buildings that are daylit provide adequate lighting to perform most visual tasks and limit glare. Luminance is the measurement of task lighting, and Illuminance is a measurement of direct glare. The unit of measurement for luminance is a footcandle (fc), or lux, and the unit for illuminance is candelas per square meter (cd/m2). Uniformity glare is also important, which is the ratio of luminances in adjacent spaces.

People need between 5 and 10 fc for circulation and orientation in a space, between 30 and 50 fc to perform most visual tasks, and between 50 and 100 fc to perform difficult tasks involving low contrast or high accuracy. People also need uniform light. The difference between lighting in task areas and adjacent areas (uniformity glare ratio) should be close to 3:1, and the uniformity glare ratio between task areas and remote areas should be close to 5:1. At no point should the brightest spot be more than 10 times brighter than the darkest spot, or it will become a source of indirect glare.

There are two ways to bring daylight into a building: from the walls or from the ceiling. Once daylight is brought into a building, it’s important to make sure that glare wasn’t introduced.

Sidelighting refers to daylighting from windows. Windows can bring twice as much daylight into a space. For example, if the top of the window is 10 ft above the floor, daylight will penetrate up to 20 ft into the room. Window daylighting has several advantages and disadvantages. Windows allow occupants to look out into the world, contributing to visual comfort. The LEED V4 Indoor Environmental Quality credit calls for view glazing for 75% of the regularly occupied floor area. But, windows reduce insulation efficiency and allow solar heat gain, resulting in increased HVAC energy. Windows on all but the north side of a building present glare conditions unless controlled.

Toplighting is the application of daylighting from the ceiling. This includes skylights, solar collectors, clerestories, or sawtooth structures. Toplighting can typically supply light to an area equal to the mounting height. A skylight aperture mounted 20 ft above the floor can provide daylight to a 20×20-ft area below the skylight. Skylights get more sun exposure than windows, leading to increased heat gain, but solar tubes can reduce this. Toplighting can penetrate deep into buildings, but it favors only 1- or 2-story buildings and does not provide views to the outdoors.

After bringing daylight into the space, reducing glare is just as important. There are three types of glare: direct glare, indirect glare, and uniformity-related glare. Unshielded light fixtures and low-incident sunlight (such as at sunrise and sunset) cause direct glare. Reflections off of interior surfaces cause indirect glare. Uniformity glare occurs when illuminance varies greatly between adjacent areas of a room.

The only way to limit direct glare is to reduce incident light coming through windows. Static exterior shading reduces heat load and glare throughout the year, but it is not adjustable. Interior shading, such as adjustable blinds or shades, also reduce direct glare and heat gain. Electrochromic glass also is a popular and affordable option that alters the visible transmittance of glaring by up to 90%. The Illuminating Engineering Society of North America (IESNA) and others recommend automatic shading, since occupants often leave shades closed.

Reducing indirect glare hinges on reducing the reflectivity of interior surfaces. Selecting satin instead of glossy finishes, for instance, goes a long way toward reducing glare. Materials with low light-reflectance values (LRV) reduce glare but also reduce daylight penetration. Rotating highly reflective surfaces is also important to reduce undesirable indirect glare. The WELL Building Standard recommends orienting computer screens so they are perpendicular to windows that are within 15 ft of them and allowing a minimum LRV of 50% for furniture.

Daylighting design strategies

There are several design factors that can reduce uniformity-related glare. Providing multiple daylight sources can balance daylight penetration and uniformity. Examples of this are installing windows on different walls or installing a window and a skylight. Reducing the "lease depth"—the distance from the window to the nearest interior partition—allows light to enter the space. Feature 61 in the WELL Building Standard requires that 75% of regularly occupied spaces be within 25 ft of view windows. Balancing surface reflectance and minimizing opaque interior partitions can also improve light uniformity.

There are other tradeoffs to balance besides glare. Along with visible light, infrared light also enters buildings, contributing to solar heat gain. This can be an advantage in hot climates, where it contributes to passive design strategies. But the improper application of daylighting in cooler climates can waste energy. Selecting glazing with the right balance between visual transmittance and solar heat-gain coefficient is difficult and often requires coordination between daylight and energy models.

The location and orientation of glazing and the facade design can also significantly impact heat gain. North-facing glazing experiences lower heat gain than south-, east-, or west-facing glazing. Since sidelighting penetrates the space based on the top-of-sill height, the bottom-of-sill height can be raised to reduce solar heat gain. Floor-to-ceiling windows significantly contribute to heat gain while providing limited daylighting. Clerestories and skylights bring daylight deep into spaces, but do not offer outside views. Another way to reduce solar heat gain is to provide an exterior shade or awning above the glazing.

To analyze daylighting beyond rules of thumb, specialized software is necessary. LEED and WELL Building Standard guidelines both prescribe software modeling to receive credits for a daylighting strategy. There are two types of analysis: discrete and continuous. Discrete analysis simulates lighting at a particular hour of the year. Continuous analysis simulates lighting throughout the year using 10 hours of each day. The analysis then generates statistics on annual performance based on defined criteria.

Discrete analysis provides direct luminance and illuminance values on a surface. These values help to analyze glare and daylighting potential. The summer and winter solstices (June 21 and Dec. 21) and spring and fall equinoxes (March 21 and Sept. 21) are common dates to use. These are days where the sun is highest, lowest, and at the midpoint in the sky, making them ideal analysis days. Typical times are 8 a.m., noon, and 4 p.m., representing both east, south, and west exposures. These simulations give the designer information about extremes, but provide little about annual performance. Their upside is they can be performed much faster than continuous-analysis simulations.

Continuous analysis provides statistics to determine if there is glare or adequate daylight. To do this, continuous analysis performs many discrete analyses. Simulations analyze conditions during every hour between 8 a.m. and 5 p.m. on a daily basis. In this case, the continuous analysis is performing 3,650 discrete analyses, which takes some time.

There are three common types of continuous analysis, with each serving a different purpose:

  • Daylight factor (DF) is the ratio of indoor to outdoor illuminance. Because it measures relative brightness, it is useful for analyzing light uniformity. It is also easy to use DF to estimate illuminance levels and validate using physical models or existing buildings.
  • Spatial daylight autonomy (sDA) is the percentage of a space that can be daylit most of the time. For instance, a value of sDA300/50 at 55% means that at least 55% of the area is lit to 300 Lux (28 fc) at least 50% of the time.
  • Annual sunlight exposure (ASE) is the percentage of a space that experiences too much light, resulting in glare conditions. A value of ASE1000/250 at 10% means that no more than 10% of the space experiences more than 1000 Lux (93 fc) for more than 250 hours/year.

Other methods include useful daylight illuminance (UDI) and averages of discrete time analyses. UDI is the percentage of time when useful daylight is available. Averages of discrete time analyses include annual footcandle levels or luminance levels. Both give more granular information than the sDA and ASE values, making them useful for accent daylighting. But they are more difficult to interpret and have limited direct value to daylighting design.

Integrating lighting control systems

After developing a successful daylighting plan for the project, integrating the lighting control system is the next step. Without this, there will be very little energy savings and general occupant confusion. The two primary lighting control components for daylighting coordination are photosensors and zoning.

Photosensors measure light and control light fixtures in response to available daylight. There are two types of photosensors—closed-loop and open-loop. This refers to the type of feedback control and impacts the installation and zoning capabilities of the system. Closed-loop photosensors measure the amount of light in a space, from both artificial and natural lighting. This makes it easy to set a specific minimum light level, but the photosensor only controls one zone of lighting. Open-loop photosensors only measure daylight and can control multiple zones. This limits their ability to set a specific light level and relies on the space not changing after commissioning.

Advanced lighting controls are necessary to set up control zones that can respond to both daylight and occupant needs. For instance, take a room that is daylit from two directions and requires a "projector screen" zone. A typical room controller or panel-based lighting control system requires at least three zones for that room, and they may overlap. But if each fixture is its own zone, they can maintain programmatic features and allow fine-grain control of daylit spaces. Individual fixture controls use software to create digital zones. These zones can then overlap with each other and respond to sensor and user interactions. Individual fixture controls also often include their own daylight and occupancy sensors. This allows them to be independent but still controllable as a group, giving users the best of each method.

Successful daylighting requires close coordination between architecture, structural, mechanical, and lighting design. Without coordination, daylighting is likely to upset both occupants and owners as well as waste energy. Successful daylighting also complements the lighting design and improves occupant comfort. Reducing lighting power and heat gain with daylighting also reduces energy use in the building.

Artificial lighting uses around 25% of the building’s energy, and daylighting can reduce that use by up to 80%. This limits solar gains and lowers HVAC loads that offset the internal lighting load. Apart from energy savings, daylighting contributes to the health and well-being of occupants. It helps increase productivity and reduce absenteeism while providing high-quality light for task and ambient uses. Daylighting is valuable and necessary for efficient building design.


Alexander Sassoon is an electrical design engineer and project manager at P2S Engineering Inc.