November 24, 2025
Designing With Daylight: A Control-Based Approach
by Craig DiLouie, LC, CLCP and C. Webster Marsh, CLCP of the Lighting Controls Association
Intelligent daylighting strategies reduce energy use and enhance comfort
Daylighting, or the use of daylight as a primary source of general lighting in interior spaces, has grown in importance in mainstream construction due to the sustainable design movement, energy code requirements, and evidence-based human-centric design for health and wellbeing.
Daylight-responsive lighting controls (aka, daylighting or daylight harvesting) are interior automatic controls that reduce electric lighting energy consumption in response to daylight contribution levels. This control strategy has been a staple in commercial buildings due to energy codes. According to a U.S. General Services Administration case study Integrated Daylighting Systems published in March 2013, “Energy savings potential for daylighting varies and has been measured within a range of 40%-80% daily savings, based on the time of year and orientation with annual averages as high as 40%-50%. Daylight savings are typically estimated to be between 20%-60% of lighting energy and normally assume that daylight dimming is only a portion of an integrated lighting control system.”
Based on an updated version of EE201: Daylight-responsive Lighting Control, a course in NEMA Academy, this article describes a process for designing and applying lighting control solutions that utilize daylight-responsive lighting controls.
DAYLIGHT
Exterior daylight has three components: the sun, the sky, and light reflected from exterior surfaces. Interior daylight has two additional components: fenestration and light reflected from interior surfaces. Daylight often enters a building via fenestration, which distributes light in a space as toplighting (e.g., a skylight) or sidelighting (e.g., a window).
Good daylighting design provides diffuse illumination to serve as a primary source of general illumination while mitigating glare and unwanted heat gain. Additionally, it focuses on providing balanced illumination from multiple sources and relatively uniform brightness, while preserving views of the outside world.
DAYLIGHT VERSUS DIRECT SUNLIGHT
While daylight is highly desirable in a building as an illumination source, direct sunlight, that is, an unobstructed view of the sun, generally should be avoided, since direct sunlight is a major source of glare. This means the daylight itself needs to be controlled via a daylight management strategy.
Glazing, shading, and various daylight management devices can be used to diffuse the light as broadly and uniformly as possible throughout the space. These devices may be manually controlled, such as window blind cords, or automatically controlled, such as daylight sensors and motorized window shades. Devices for automatic control offer the added potential for integrated control of electric lighting, which is what daylight-responsive lighting controls are designed for.
DAYLIGHT-RESPONSIVE CONTROLS
Daylight-responsive control is a strategy designed to take advantage of available daylight to reduce electric lighting energy consumption. Daylight-response occurs when a light sensor (typically referred to as a photosensor or daylight sensor) measures incoming daylight or reflected light from a surface and provides this information to a controller to adjust the electric light source (e.g., shades or luminaires). Control methods range from simply turning the luminaire Off to dimming the luminaire to maintain a desired light level on the workplane.
Note that electrical lighting changes should be inconspicuous to users with minimal effect on visual comfort or task performance. Avoiding noticeable and objectionable changes to the overall illumination in the space, may lead to occupant dissatisfaction and attempts to manually override the daylight-responsive controls, thereby impacting the system’s performance.
DAYLIGHT-RESPONSIVE CONTROL BENEFITS
The primary benefit of daylight-responsive lighting controls is the generation of ongoing lighting energy savings by reducing waste. Actual energy savings in a given building will depend on factors such as daylight availability via fenestration and the actual daylight contribution, in addition to the characteristics of the daylight-responsive lighting controls.
Other benefits include occupant comfort, as controls can adjust luminaire output to maintain design light levels with uniformity, and the potential positive impact on luminaire service life, as reducing output helps maintain the lumen output of LED sources.
ENERGY CODES
Prevailing commercial building energy codes require daylight responsive controls, notably model codes such as the International Energy Conservation Code (IECC) and ANSI/ASHRAE/IES 90.1 (ASHRAE 90.1) and progressive state-based codes such as California’s Title 24, Part 6 code. Although not addressed in this learning module, Leadership in Energy and Environmental Design (LEED) and the WELL Building Standard (WELL) also recognize daylight, exterior views, and glare control as necessary aspects of a quality lighting design.
These codes typically define daylight availability in specified primary and sometimes secondary zones around fenestrations and require separate control of general lighting in these daylight zones. The controls must be automatic and may use continuous dimming, step dimming, or multilevel switching as the light reduction method, depending on the applicable code. Continuous dimming is generally recommended for regularly occupied spaces.
In addition to defining the dimensions of daylight zones and the lighting that must be controlled, codes may include additional requirements for how control zones—defined by total luminaire wattage controlled simultaneously via a control output—within the daylight zones are configured. For example, the floor area of the primary and secondary daylight zones may be designated as control zones.
DESIGN AND APPLICATION
Proper design and application of daylight-responsive lighting controls are similar to other control systems, with key decision points specifically related to daylight availability. While the applicable energy code is a good starting point for requirements and restrictions, the best designs must also respond to the specific project’s needs.
Daylight-responsive lighting controls can be effective in virtually any area where ample daylight is present and electrical lighting is operational while the space is occupied.
If the entire space is uniformly skylighted with properly spaced skylights covering about 3-5 percent of the floor area (as in the skylighted space shown below), light reduction can be uniformly applied to all luminaires. In the majority of buildings, however, daylight-response will primarily apply to the perimeter area (typical windows in a commercial building), forming primary and secondary daylight zones.
TYPES OF OUTPUT CONTROL
A key design decision in daylight-responsive lighting controls is control of the luminaire output. Choices include switching, continuous dimming, step dimming, and multilevel switching. The latest energy codes require continuous dimming, and the capability to turn the lighting Off when there is adequate daylight contribution in a daylight zone. Older codes may also allow the step and multilevel switching.
Many LED luminaires, especially more recent products, are dimmable. Continuous dimming, which enables a smooth, continuous change in light level across the given dimming range, is generally recommended for ongoing daylight-response in regularly occupied areas. Energy codes typically have a minimum lighting power level that the system must achieve and the ability to turn the lighting off.
Multilevel Switching requires separate circuiting and control of groups of luminaires or luminaire zones, enabling one or more steps between Off and full light output. For example, switching every other luminaire On and leaving the other half Off provides 50% of the total light for the space.
Step Dimming is similar to multilevel switching, but the change in output applies across all luminaires, which provides a uniform visual appearance.
CLOSED-LOOP VS. OPEN-LOOP
Another key decision-point in designing daylight-responsive lighting controls is whether the daylight-responsive lighting controls use closed-loop or open-loop implementations.
In closed-loop sensing,the daylight sensor is installed where it is exposed to both daylight entering the space and the output of the controlled luminaires. The sensor measures the total light level in the space and provides this information to a controller which then adjusts the electrical lighting to maintain a desired light level.
There is no single ideal choice when it comes to selecting an implementation as long as each daylight sensor is installed in an appropriate location. To improve responsiveness, a designer should consider adding additional sensors and control zones. The most responsive system uses luminaire-level lighting controls (LLLC) in which each luminaire includes both a light sensor and controller.
OPEN-LOOP SENSING
As mentioned above, open-loop sensing is not affected by changes in the luminaire output of the space. The daylight sensor is typically placed at or near fenestration and oriented either toward incoming daylight or the closest reflected surface not exposed to electrical lighting. Ideal applications for open-loop sensing include unshaded apertures, spaces with relatively constant daylighting conditions, or where task surfaces are inconsistent or hard to measure.
Open-loop sensing is more forgiving in terms of sensor placement than closed-loop sensing, since closed-loop sensors are also measuring electric lighting, but it is less preferable if close control of light levels on the task surface (such as the top of a desk or the floor of a lobby) is needed. Open-loop sensing does not measure actual light levels in the space. For example, if manual or automatic shading blocks the fenestration at the location of the senor, luminaire outputs may be adjusted to undesirable levels; for this reason, it is important to provide local overrides (e.g., a keypad).
CLOSED-LOOP SENSING
Closed-loop sensing offers the advantage of being able to measure light levels at the task surface to ensure that adequate light levels are provided, and as such it is well-suited to applications that include windows with shading or spaces with frequently changing lighting conditions.
Good sensor placement and orientation are critical for effective daylight-responsive lighting controls using closed-loop sensing. The sensor should not be aimed in a way that it receives direct light from the sun or luminaires, including light sources from tops of indirect luminaires, and is exposed to light from its own daylight zone only. The sensor should be aimed at and have an unobstructed view of the task surface, at a recommended height above the task surface.
Closed-loop sensors should be placed in locations with consistent task surface conditions. Task surfaces undergoing changing conditions, such as different occupants, or surfaces with significantly different reflectance values, such as white paper set on a dark desktop are not good candidates for a closed-loop sensing implementation. If suitable locations cannot be identified, consideration should be given to an open-loop implementation instead.
DETERMNING DAYLIGHT ZONES
An additional step critical to daylight-responsive lighting controls is determination of the interior daylight zones.
A daylight zone is an area of consistent daylight availability around one or more fenestrations.
In projects required to comply with commercial building energy codes, daylight zones are defined by various factors including fenestration height, location, and transmittance and objects that may obstruct available daylight.
There are two types of daylight zones, based on their fenestration location: toplit (e.g., skylight) and sidelit (e.g., window installed in a wall).
The 2024 IECC, for example, defines the primary daylight zone of vertical wall-mounted fenestration (sidelit) as the adjacent floor area extending laterally into the space one times the fenestration’s height (measurement from the floor to the top of the fenestration) or to the nearest wall. This is provided the area of the fenestration is greater than 24 square feet, and has a visible transmittance of at least 0.20, and where outside obstructions that would block daylight from entering the building are no closer than required dimensions.
The width dimension of sidelit daylight zone is comprised of three dimensions.
1. The width of the windows or vertical fenestration
2. The width of any opaque break between windows is not greater than fenestration height (like a building column or wall element between windows).
3. The dimension of the fenestration height at each end of the last window on each side.
A secondary sidelit daylight zone has the same dimension as a primary sidelight daylight zone and is directly adjacent to the primary sidelit daylight zone further into the space.
DESIGNING CONTROL ZONES
A control zone is a group of one or more luminaires controlled simultaneously and uniformly.
Because a daylight zone often covers a large area where daylight is available, the control zone may match the daylight zone, or multiple control zones may be deployed based on energy code requirements. Additional control zones going beyond minimum code requirements may also be deployed for more accurate control responses and potentially greater user satisfaction and energy savings.
The diagram below shows both a primary daylight zone (Daylight Zone 1) closest to the windows of the space, and a secondary daylight zone (Daylight Zone 2) further from the windows.
It is best practice to define the both primary and secondary (if required) daylight zones before defining the control zones.
As stated earlier, control zones are often nested within daylight zones for improved lighting system responsiveness. For example, in the graphic above, in the left-hand primary daylight zone, each luminaire might be equipped with a light sensor and controller providing independent daylight-response. However, while control zones can be nested, they should not extend past their daylight zone, nor overlap with other daylight control zones.
An additional design consideration involves the type of dimming used by the luminaires. If luminaires use an analog dimming protocol, the lighting circuit wiring will need to align with the control zones, since analog dimming requires one circuit per zone. If luminaires use a digital dimming protocol, circuit wiring can be provided without considering the control zones, as zoning is commonly defined during system programming.
LIGHT SENSORS
With the daylight control zones identified and control zones designed, it is time to select equipment. A primary component of daylight-responsive lighting controls is the light sensor, which measures light levels and provides this information to the lighting controller, which in turn maintains a target level based on its programming. Light sensors may be mounted on the ceiling, wall, within the fenestration, or as an integrated component of luminaires. The visible size of these sensors ranges from a quarter to a standard light switch.
LIGHT SENSORS: RANGE OF RESPONSE
A key characteristic of light sensors is range of response, which describes the range of light levels to which the sensor can actively and accurately respond. A given light sensor may be limited in the range of light levels it is able to detect, so the range of response needs to be matched to the application. For example, an open loop light sensor may need to have a range of 500 – 6,000 foot-candles (about 5,400 to 65,000 lux) while a closed loop sensor may only need a 10 – 100 foot-candle (about 107 to 1100 lux) range. The images below provide typical foot-candle ranges for various environments.
LIGHT SENSORS: DIRECTIONAL SENSITIVITY
Another light sensor characteristic is directional sensitivity (also called spatial response), which describes the sensor’s sensitivity to light from different directions. This is determined by sensor housing and lens designs that directs light to the light-sensing element. The diagram below shows the directional sensitivity of the light sensor from three different directions with respect to the light source, 0°, 45°, and 90°. As shown below, the directional sensitivity can vary greatly and should be taken into account in selecting both a sensor and mounting orientation.
While there is no recognized optimal sensitivity distribution. Wider distributions are less susceptible to changes in surface and environmental reflectances, since they can detect light over a wider range, but they are more likely to detect light from undesirable sources such as bright windows. Sensors with narrower distributions, on the other hand, are more susceptible to localized changes in reflectances, but less likely to detect unwanted light sources.
LIGHT SENSORS: SPECTRAL RESPONSE
Light not visible to the human eye, such as infrared or ultraviolet light, should not be part of determining required interior light levels, so daylight sensors are typically designed to only detect the wavelengths of light visible to the human eye.
Some light sensors can be used to detect which spectral wavelengths are strongest so that tunable-white lighting can be adjusted to match the current color temperature of the incoming daylight which varies over the course of the day.
SETPOINTS AND DEADBAND
Other important parameters of daylight-responsive lighting controls are setpoints and deadband.
A setpoint is a designated light level threshold indicating a lighting level change is needed. For example, a lighting control system may be programmed such that when the light sensor provides a measurement of 40 foot-candles to the system controller, the controller turns office lighting On. Once light levels rise above the 40 foot-candle set point, the controller turns the lighting Off.
To avoid frequent light switching or dimming due to temporary changes in daylight conditions (e.g., a cloud passing in front of the sun) a deadband is programmed. A deadband is a sensor foot-candle range between which the lighting controls will leave lighting levels as they are. As shown in the diagram below, the deadband high setpoint (green line) is the point at which the luminaires will be turned Off or dimmed and the bottom setpoint (red line) is the point at which the luminaires will be turned On or the output increased. In this example, the light remains Off until the daylight level has decreased through the deadband to the low setpoint.
RAMP RATES AND TIME DELAYS
Two more important characteristics of daylight-responsive lighting controls are ramp rates and time delays, both of which ensure that lighting adjustments are smooth and non-intrusive to occupants.
The ramp rate is the speed at which the lighting system changes the light output in response to changing daylight levels. A slower ramp rate creates a gradual transition that is less noticeable to occupants, while faster ramp rates provide a quicker transition for suddenly changing conditions, like someone shutting a set of window shades.
The time delay (or response delay) is the period the system waits for after detecting a change in daylight levels before initiating a lighting adjustment. It prevents the system from reacting to temporary or minor fluctuations in daylight, such as someone walking in front of a window. Common time delays for an office daylight zone can range from 30 seconds to 15 minutes, depending on the needs of the occupants in the space. Longer delays are better for spaces that have slow changing and consistent daylight, while shorter delays offer quicker responsive for spaces that have frequently changing daylight conditions.
A good combination of ramp rate and time delay provides a smooth transition from one lighting level to another, helping to ensure occupant comfort.
CONCLUSION
Daylight-responsive lighting control enables commercial buildings to enjoy the benefits of daylighting while saving energy. Good daylight-responsive lighting control solutions, however, require careful design application.
The Lighting Controls Association is a council of the National Electrical Manufacturers Association that provides education about lighting control technology and application, including articles, videos, design awards, news, resources, and Education Express, a free, 24/7 series of online courses covering everything from technology to design to commissioning.
