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System Components

The diagram below schematically shows the basic, simplified situation for photosensor control in an office with a window. The basic components of the system are daylight, luminaires, and a photosensor.

Daylight distribution and the ratio of illuminances between the task and ceiling director impact photosensor performance. An understanding of their relationship is critical for determining proper photosensor location.

Daylight Distribution

When daylight enters a room through vertical windows, vertical surface illuminance is greater than horizontal surface illuminance. Useful daylight entering a room comes from diffuse skylight and from light reflected off the ground, nearby buildings, and landscape features. Direct sunlight is usually too intense for comfort and must be blocked or redirected by window treatments. Because of this, even for a south-facing room under clear sky conditions, daylight enters the room in a horizontal, or even an upward direction due to the window treatments diffusing and/or redirecting the direct sunlight. Consequently, in a daylighted space, the workplane, ceiling, and walls tend to be nearly equally illuminated.


Luminaires are typically mounted in or on the ceiling with the electric light being directed down onto the workplane. Except for the case of pendant-mounted direct/indirect or indirect luminaires, the electric light illuminating the ceiling is that which is reflected from the walls, floors, and other surfaces in the room. Consequently, the ceiling, and to a lesser extent the walls, are illuminated much less than the workplane. Also, the ceiling illuminance varies considerably for different room geometries and surface treatments. Rooms with high ceilings and dark walls and floors will have a much lower ceiling illuminance than rooms with low ceilings and light colored walls and floors even though the workplane illuminance is the same in both cases. In typical offices, the workplane illuminance from electric lighting is five to 10 times higher than the ceiling illuminance. In offices with high ceilings and/or dark walls and floors it is not uncommon to find workplane:ceiling illuminance ratios as high as 20:1.

Photosensor Location

The photosensor is typically mounted on the ceiling and receives light from the workplane below, as well as other room surfaces. For precise control of workplane illuminance, the ideal location for the photosensor is on the workplane, where it would respond directly to workplane illuminance. However, the workplane is not a practical location because the photosensor would likely be disturbed, or be covered up or shaded by activities in the room. So, for practical reasons, photosensors are located on the ceiling to minimize interference from activities in the room.

Controlling workplane illuminance with a sensor located on the ceiling complicates photosensor control. This is because of the variation in light distribution throughout the room, caused by changes in the amount of daylight entering the room, and, to a lesser extent, the activities that take place in the room change. The next section on Feedback Control explains why.

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