Photosensors: Technology and Major Trends

Daylighting is going mainstream and daylight harvesting, the energy-saving lighting control strategy that actually makes daylighting “sustainable,” is beginning to catch up. Of particular interest is the fact that daylight harvesting is transitioning from being something encouraged by energy programs to something required by energy codes and standards—not just California’s Title 24, but IECC 2009 and, likely, ASHRAE 90-1.2010 as well. It is likely, in fact, that in the future, most commercial buildings that have windows and skylights will be required to have some type of daylight harvesting control in the adjacent area.

Daylight harvesting’s value proposition is fairly simple: As daylight levels increase in a space, electric light levels can be automatically reduced to maintain a target task light level and save energy. All automatic daylight harvesting control systems need a device that can measure light levels and signal a controller to dim or switch the lights in response to daylight contribution. This device is called a photosensor.

The photosensor is a small device that can include a light-sensitive photocell, input optics and an electronic circuit used to convert the photocell signal into an output control signal, all within a housing and with mounting hardware.

Photosensors may be mounted on walls, ceilings and even as a part of light fixtures. Fixture-integrated sensors may be installed as part of the original fixture or installed later in the field by attaching to a lamp via a clip and being wired directly to the ballast. The visible size of a photosensor ranges from a golf ball to a standard wall switch.

As daylight harvesting grows in importance, photosensors are becoming more sophisticated. But buyer beware: There is no standard defining how photosensors should operate. When selecting a photosensor, important questions to ask about a given product include: Is it compatible with the given controller? What control method does the system use? What is the sensor’s spatial response? What range of light levels can it “see”? How accurate is its signal? Is it photopic-corrected? How far is it to be installed from its controller? How is it commissioned? How many zones can it support? What are the configuration options? Is it capable of operating reliably within the given environment—heat, cold, moisture? Are there listing or compliance requirements such as UL or ROHS?

“Studies have shown the importance of using daylight harvesting strategies in commercial spaces, particularly with the growing emphasis on architectural daylighting design, but have also illustrated the importance of choosing the right product for the application,” says Daniel Trevino, LEED-AP, Daylighting Product Manager for WattStopper. “This helps maximize energy savings while avoiding user complaints.”

“Using the correct photosensor for a particular application is one of the most critical design aspects,” says Bob Freshman, Marketing Manager for Leviton Lighting Management Systems. “The sensor that is used should be appropriate for the size of the space and the environment in which the sensor is located.”

Control algorithm
Daylight harvesting controls may be “open loop” or “closed loop” systems. Each measures the daylight contribution on the task surface differently. Dual loop is now emerging as a potentially significant technology.

Closed-loop systems measure the combined contribution to light level from both daylight and the electric lighting system. Because the photosensor measures the electric lighting system’s light output, it “sees” the results of the controller’s adjustment and may make signal further adjustments based on this feedback—creating a closed loop.

With closed loop, the photosensor measures actual light levels, so it is sometimes considered more accurate than open loop, Closed loop is considered preferable by some for applications where a specific target light level must be maintained, such as small offices. But control is limited to a single zone and the system must be properly set so that transient light level changes (e.g., white sheets of paper shuffled on and off a dark desk). do not cause overly frequent dimming or switching.

Open-loop systems measure only the incoming daylight, not the contribution from the electric lighting. The photosensor should not see any electric light and therefore it is mounted outside the building or inside near a daylight aperture facing away from the controlled lighting. Because there is no feedback, it is an open loop.

With open loop, the sensor is not affected by transient light level changes but it does not measure actual light levels. This means that a sensor placed outside a window would not know that the blinds were closed, and dim the lights inside anyway. As a result, open loop is often preferable for applications where accuracy is less important, such as hallways and atria.

iSweek（http://www.isweek.com/）- Industry sourcing & Wholesale industrial products