Daylight harvesting’s value proposition is fairly simple: as daylight levels increase in a space, electric lighting levels can be automatically reduced to maintain a target task lighting level and save energy. Because this system is automated, a device is needed to tell a controller that there is a high enough light level to warrant reduction of electric lighting. This device is called a photosensor.
Photosensors typically include a light-sensitive photocell, input optics and an electronic circuit used to convert the photocell signal into an output control signal. The visible size of a photosensor in the space ranges from a golf ball to a standard wall switch. It may be connected to the controller using low-voltage wiring or wireless contact, which sends a voltage signal distances up to 500 feet and current signal thousands of feet. It may be mounted on the ceiling, integral to a light fixture, or outside the building. It may be manually commissioned or self-configuring.
Important characteristics include the following elements:
Control method: Most photosensors are open-loop or closed-loop. Closed-loop photosensors are aimed at primary task areas, avoiding direct view of electric lighting sources; these “see” a combination of daylight and electric light. Open-loop photosensors measure only incoming daylight and are, therefore, mounted near a daylight aperture or outside the building.
With closed loop, the photosensor measures overall light levels (but only where located, typically at the ceiling), so it is considered preferable by some when a specific target light level must be closely maintained, such as in small offices. Control is limited to a single zone, however, and the system must be properly set so that transient light level changes (e.g., white sheets of paper being shuffled on and off a dark desktop) do not cause overly frequent dimming or switching.
With open loop, the sensor is not affected by transient light level changes; it measures only incoming daylight. This means a sensor placed outside a window would not know the blinds were closed and would dim the lights inside. As a result, open loop is sometimes preferred for applications where accuracy is less important, such as in an atrium, warehouses with skylights, and spaces where there is no window-shade control.
Dual-loop photosensors, a potentially significant emerging technology, are an interesting new technology that combine open-loop and closed-loop photodiodes looking in different directions. The result is greater accuracy than using open loop alone and greater resistance to transient light level changes than using closed loop alone.
Spatial response: The photosensor’s spatial response, also called its angular sensitivity, describes its sensitivity to light from different directions and defines its field of view—what it “sees,” in effect.
If the field of view is too broad, the photosensor may detect light where it should not, such as from direct sunlight near or outside a window. If the field of view is too narrow, the photosensor may become too sensitive to changes in brightness within a localized area and would raise or lower the lights incorrectly. A sensor placed deep in its housing, for example, will have a restricted field of view. According to the New Buildings Institute, a 60-degree cone of vision is common. One manufacturer suggests a 100-degree field of view for closed-loop photosensors and a 45-degree field of view for open-loop. Some sensors provide an adjustable feature to shield direct sunlight from the field of view.
Light level response: The photosensor may be limited in the range of light levels it is able to detect. Dusk and dawn lighting control is performed at less than 10 foot-candles (fc), daylighted offices are controlled at less than 100 fc, atrium spaces are controlled at less than 1,000 fc, and skylight sensors see up to 10,000 fc of sunlight. In each case, the relationship between the photosensor input and output signal should be linear.
Photopic correction: The photosensor’s spectral response describes its sensitivity to optical radiation of different wavelengths. If the sensor is able to respond to ultraviolet and infrared radiation, it might control the lights unnecessarily. As a result, filters are used that attempt to sift out these wavelengths so that the sensor focuses on the visible light spectrum the same way the human eye does. These sensors are fairly effective when mixing daylight and an electric lighting source.
In the second and final part of this series on photosensors, we will discuss photosensor placement. Until then, think about this: we have a classroom in which we want to specify daylight harvesting. What kind of photosensor will we need?