Daylight Harvesting is increasingly being incorporated into the design of new buildings to reduce the amount of electric light required during daylight hours. In order to take advantage of this, controls are required to step down the amount of artificial light and lower energy consumption.

Step Controls or Dimming?
Photo sensors are used to sense the amount of daylight entering a space and light fixtures can then be switched or dimmed to the appropriate levels.

Step dimming (or switching) is usually done in 2 or 3 levels (50%, 100% or 1/3, 2/3, 3/3). Sometimes light fixtures can be circuited so that alternate fixtures are on different circuits or relays. This can save significant costs over having to supply 2 ballasts per fixture. The photo sensor then switches the levels according to the amount of daylight.

The other way to reduce artificial light levels is to use dimming ballasts. Using 0-10VDC dimming ballasts instead of the line voltage type avoids expensive dimming panels, although there is some extra wiring involved (the purple and gray control wire have to be connected to each ballast). The big advantage of dimming is that if the system is well designed, the occupant doesn't notice the changes in light levels as the daylight changes and is less likely to override the daylight harvesting control system. As the cost of dimming ballasts come down, this method will become the obvious choice.

Open Loop or Closed Loop?
An open loop daylight control system measures the incoming daylight to a space and ignores the electric lighting contribution within the space. A closed loop system measures both the available daylight and the electric light, then monitors the result of its own adjustments.

Open loop systems are more suitable for switched or step dimming because there are significant light level changes with each step. It is also easier to get the deadbands set correctly (i.e., the light level gained or lost with each step). If step dimming is used in open loop applications, one has to be very careful with sensor placement to avoid cycling between light levels.

Other instances when open loop is preferred are when multiple spaces are controlled from one sensor or when it is not possible to locate a sensor within a space so that it will not see direct light from a fixture.

Closed loop systems are preferred in single spaces because they have the advantage of always maintaining the commissioned light level as lamp output depreciates, furnishings change and blinds are adjusted.

Network Sensors vs. Individual Sensors.
Network sensors have several advantages. Many zones can be controlled by a single sensor thereby reducing the quantity required while still being able to select different light level parameters for each zone (e.g., many different floors of a building on the north facing exposure could be monitored by a single sensor). Also, the light levels the sensors are reading can be monitored on the front end computer and switching or dimming adjustment levels can be made without ladders.

Individual sensors have the advantage of being simpler to commission. All adjustments are done on the sensor behind the faceplate.

CONCLUSION
The most critical factor in designing a successful daylight harvesting control system is the placement of the photo sensors. All Douglas sensors have the ability to adjust the angle of the lens to read the most functional spot in the space. This allows the ideal combination of daylight and electric light in closed loop applications, or the optimum utilization of the incoming daylight in open loop applications. For assistance in sensor placement, please contact your Douglas Representative.