In part 1 of this two-part series on photosensors, I described the major characteristics of photosensors and ended with a problem: suppose we have a classroom in which we want to begin dimming the row of fluorescent lighting fixtures adjacent to a series of windows when daylight levels reach 150 percent of design light levels. What kind of photosensor will we need?

Since students perform stationary, critical tasks, the lighting system will need to maintain a target light level; a closed-loop system may be the appropriate one here. As a closed-loop system, the sensor should have a 60- to 100-degree field of view. Light levels are typically controlled at less than 100 foot-candles (fc), so a sensor with a 1–500 light level response would be appropriate. The sensor should provide automatic photopic correction.

Now that we have selected our photosensor, we encounter another challenge: where are we going to put it?

Photosensor placement is one of the most important aspects of daylight harvesting control and, to a significant extent, what makes it as much an art as a science. Manufacturers provide detailed guidance on proper placement of their products, which may be supported by the general rules of thumb, as follows.

Photosensors should be placed where they can view a good representation of available daylight. The daylight input to the sensor should be stronger than the electric lighting so that variance in daylight levels can be detected easily. The sensor should be placed so that, if it does not receive sufficient light, it can be recalibrated at its location.

Whether the sensor is open- or closed-loop will largely affect its optimal placement. Of the two, closed-loop is more challenging because there is less margin for error. First, the closed-loop photosensor will need to be installed in the zone it controls. The sensor must be aimed at a task surface or an interior wall so that it can measure combined electric light and daylight that is reflected from the surface. The field of view should be unobstructed and large enough to measure light levels on a good representation of the daylight available in the space, but it should not see direct sunlight or have a direct view of light sources in indirect or direct/indirect lighting fixtures. In the latter case, one solution might be to mount the sensor in the lighting fixture itself. If lighting shelves are used to increase daylight penetration from windows, consider putting the sensor above the shelf pointing down so that it detects daylight reflected from the top of the shelf.

If the photosensor is closed-loop and ceiling-mounted, one rule of thumb is to place the photosensor at a distance from the window equal to about two-thirds the depth of the daylight-control zone. If there is one major task area, the sensor could be mounted over the task. Beware of task areas that receive direct sunlight, have sunlight reflected from highly reflective surfaces, or have dark, shaded areas. Similarly, beware of task areas where surfaces are constantly changing—such as high-reflectance papers being placed on a low-reflectance desktop—as this will affect photosensor readings.

If there are several task areas with some distance between them, there are three schools of thought. If we can easily estimate which task area receives the most typical amount of daylight, we could place the sensor over that area. If we want to be conservative, we could instead place the sensor over the task area that gets the least daylight. And if the owner is willing to invest in an optimal solution, multiple sensors could be used that provide multiple inputs to a controller that makes decisions accordingly, or the sensors could each control its own zone in the space.

Open-loop sensors are easier to place, because they are located on top of the building or inside a daylight aperture looking out. The sensor should always face away from the lights being controlled. If the sensor is intended to work with a skylight, the photosensor should be installed under the skylight, not above it. This protects the sensor from the elements and ensures that it sees daylight through the glazing, including any collected dirt. If the sensor is mounted in a skylight well, make sure it is mounted far enough away from the nearest face of the well to prevent being affected by shadows and looking north to prevent the photosensor being overwhelmed from direct daylight. If the sensor is mounted outdoors, beware of obstructions that can cast shadows or otherwise affect performance.

For help, consult the manufacturer. The Sensor Placement and Orientation Tool (SPOT), a photosensor placement software package, provides independent assistance and is available for free at www.archenergy.com/SPOT.

Thanks to Solayappan Alagappan of Philips, Norm Dittmann of PLC-Multipoint, Bob Freshman of Leviton and Daniel Trevino of WattStopper for their assistance in developing this article.


DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.