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.

1. Photosensors | isweek - Industry sourcing
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.

2. 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?
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