August 2007

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Designing a Daylight Harvesting System
Daylighting is the use of daylight as a primary source of general illumination in a space.
(Part 1 of 2)
(Part 2 of 2)

  Craig DiLouie

Craig DiLouie, Principal
ZING Communications, Inc.
Communications Director
Lighting Controls Association

Daylighting is the use of daylight as a primary source of general illumination in a space. Daylighting has become a more important feature of mainstream construction due to the sustainable design movement.

Numerous studies over the last 50 years attest to the importance of daylight in design. Research indicates that daylight can improve user satisfaction/performance and retail sales. These characteristics can make daylighted buildings more valuable and marketable. Daylighting also enables daylight harvesting, an innovative control strategy that can generate 35-60+% energy savings. A daylight harvesting system decreases electric light contribution as the daylight contribution increases.

Figure 1aFigure 1b









Figure 2

Figure 1a, Figure 1b, & Figure 2  A daylight harvesting system decreases electric light contribution as the daylight contribution increases. Courtesy of Leviton.

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Daylight harvesting, also called daylighting control or automatic daylight dimming or switching, uses a ceiling-, wall- or fixture-mounted light sensor to measure the amount of illumination at the task surface in the space or at the daylight aperture, then signals a switch or dimming ballast to adjust light output from the electric lighting system to maintain the desired level of illumination. An effective daylight harvesting control system saves energy while being virtually unnoticed by occupants.

With a daylight harvesting control system, electric lighting is increased or decreased in direct or approximate proportion to the amount of natural light present, resulting in a minimum maintained illumination level in the controlled space. Daylight harvesting controls can be effective in virtually any type of facility where the lights operate much of the time and where a significant quantity of daylight is provided with windows and/or skylights.

Spaces with skylights, and corridors, private offices and open cubicles near windows—particularly those with task lighting—are good candidates for daylight harvesting. If the entire space is uniformly skylighted, energy savings can accrue on the entire lighting load. More commonly, they apply only to the perimeter zone of a vertically glazed installation.

Figure 3. The result of daylight harvesting is energy savings. While the level of savings depends on the application characteristics, savings of 35-60+% have been demonstrated. Courtesy of Lighting Design Lab.

Figure 3. The result of daylight harvesting is energy savings. While the level of savings depends on the application characteristics, savings of 35-60+% have been demonstrated. Courtesy of Lighting Design Lab.


Automatic daylight harvesting control systems are comprised of:

Note that control components may be mounted in the application as separate units or can be consolidate into packages; some dimmable ballasts, for example, can be matched to photosensors that directly control the ballast without the need for additional controls.

Figure 4
Figure 4. Typical daylight harvesting control system. Courtesy of Lawrence Berkeley National Laboratory.

Designing a System

Dimming Versus Switching

The first step in designing a daylight harvesting system is to select the control method. Two control methods are available, dimming and switching.

Dimming: Dimming is continuous over the dimmable ballast’s range, allowing a wide range of light output. Although the cost of dimmable ballasts is falling, dimming can cost about twice as much as switching; however, dimming is preferable for many applications because it can be more acceptable to occupants.

Switching: Switching may be bi-level, with selection of three conditions—ON, 50% light output and OFF—based on separately circuiting ballasts in each fixture or separately circuiting select light fixtures, or multi-level (also called stepped dimming), with selection of four conditions—ON, 66%, 33% and OFF—based on separately circuiting ballasts operating the lamps in three-lamp fixtures. In occupied spaces, multi-level switching may be preferable because it offers smaller changes in light output. According to the New Buildings Institute, in high-ceiling applications, users generally do not notice changes in light level that are less than one-third of the current light level.

Control Method: Open Versus Closed Loop

distech Daylight harvesting controls may be “closed loop” or “open loop” systems. They measure the daylight contribution on the task surface differently.

Closed Loop: Closed-loop systems measure the combined contribution to light level from both daylight and the electric lighting system, then adjust light output to maintain the desired level of illumination. Because the photosensor measures the electric lighting system’s light output, it “sees” the results of its adjustment and may make further adjustments based on this feedback—creating a closed loop.

Open Loop: Open-loop systems measure only the incoming daylight, not the contribution from the electric lighting. The photosensor should not see any electric light and therefore it is mounted outside the building or inside near a daylight aperture. Because there is no feedback, it is an open loop. In the case of a switching system, the photosensor signals the lights to shut off when daylight reaches a predetermined level. In the case of a dimming system, the photosensor measures incoming daylight and signals a controller to proportionately dim the lights based on the estimated daylight contribution.

Comparison: The primary advantage of open-loop systems is that they are able to control multiple zones from a single photosensor, as opposed to closed-loop systems, which require that each zone be controlled by a dedicated photosensor. (In review, a zone is a fixture or group of fixtures that are controlled simultaneously.) Because a single photosensor can be used for control of multiple control zones, open-loop systems are generally economical for control of larger areas with multiple adjacent control zones in dimming applications (e.g., an open office). Open-loop systems are also recommended for high-bay applications with skylights, as the photosensor can be mounted in the lightwell of the skylight, while with a closed-loop system, it may be difficult to find a good photosensor viewing location. Open-loop systems tend to be easier to adjust, requiring setup with a light level reading only during the daytime. In addition, open-loop systems provide greater calibration flexibility than most closed-loop systems, and are more “forgiving” to errors in placement of the sensor or its field of view.

The primary disadvantage of open-loop systems is that they respond only to exterior daylight availability and not actual daylight contribution in a space; if an occupant closes the blinds, the system will not recognize that and dim the lights anyway. For this reason, local overrides are useful. In addition, in applications with a single zone, open-loop systems generally pose a higher initial cost than closed-loop systems.

The primary advantage of closed-loop systems is that they pose a lower initial cost when only a single zone must be controlled, and, unlike open-loop systems, they measure actual light level by sampling the task surface, so they will respond to users opening and closing blinds and other changing conditions. As a result, closed-loop systems are generally economical for control of smaller spaces, or larger spaces with all the lights controlled in a single zone (e.g., a private windowed office). However, there is more setup required; closed-loop systems must be set up with light level readings under both daytime and nighttime (or approximating nighttime—i.e., with blinds closed) conditions.

Figure 5

Figure 5. Open loop control method. Courtesy of Watt Stopper/Legrand.



Figure 6



Figure 6. Closed loop control method. Courtesy of Watt Stopper/Legrand.


Selecting the Photosensor

Automatic daylight harvesting control systems use a photosensor to measure light level on the task surface or entering the space—measuring reflected light but not direct sunlight. The photosensor is a small device that can include a light-sensitive photocell, input optics and an electronic circuit used to convert the photocell signal into an output control signal, all within a housing and with mounting hardware. The visible size of a photosensor on the ceiling ranges from a golf ball to a standard wall switch.

While many photosensors are ceiling- or wall-mounted, photosensors are now available that integrate directly into open and louvered light fixtures by attaching to a lamp via a clip and to the fixture’s dimmable ballast via low-voltage wires. This allows individual fixture control; each fixture requires its own photosensor. Another form of integration with light fixtures is shown in the second photo, where a photosensor is integrated with a pendant direct/indirect light fixture.

Figure 7


Figure 7. Photosensor. Courtesy of Watt Stopper/Legrand.

Figure 8



Figure 8. Fixture-integrated photosensor. One manufacturer, Ledalite, offers indirect fixtures with photosensors and all other hardware, such as dimmable ballast, mounted within the fixture, at no additional cost (subject to change). Courtesy of Lutron Electronics.



Designing a Daylight Harvesting System: Part Two - September Issue


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