Daylight responsive control
 
  
  

The photocell may either be mounted externally or facing out of the window to sense the daylight only, or mounted inside the space to measure the total daylight and electric light (constant lux). In this latter case, the photoelectric sensor can either control several luminaires, or be mounted on each luminaire for individual control.
 

Example of
luminaire-mounted
photocell 

Most common daylight responsive control systems fall into three categories:

  • On/Off systems: a photoelectric switching causes sudden and noticeable changes in lighting level and can lead to occupant complaints. This type of system is best used for areas which are well daylit and where switching frequency will be low, for example close to windows. In addition, it is important to incorporate time delays into the control system to avoid repeated rapid switching caused, for example, by fast moving clouds.
  • Stepping systems: same systems as above but with one or more intermediate lighting positions between the On and Off positions.
  • Dimming systems: They ensure that the sum of daylight and electric lighting always reaches the design level by sensing the total light in the controlled area and adjusting the output of the electric lighting to top-up the daylight as necessary. If daylight alone reaches the design level the electric lighting is dimmed to extinction. Unlike photoelectric switching, photoelectric dimming control is relatively unobtrusive. The energy saving potential of dimming control is greater than for simple photoelectric switching and the mode of control is more likely to be acceptable to the occupants. The developments in electronic ballasts have facilitated the use of photoelectric dimming systems for fluorescent lighting, particularly in commercial interiors.

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Generally speaking, if sufficient daylight is available to meet lighting requirements for a significant part of the day, energy savings can be considerable. Research has shown that the probability of switching on electric lighting on first entering a space correlates closely with the daylight availability, but switching off rarely occurs until the last occupant has left.

Whether or not a daylight responsive lighting control system is suitable depends on the daylight availability within the room. The most appropriate applications are of course in spaces with much daylight.

Want to know more about photosensor ? Visit the Photosensor Tutorial, an interactive tutorial to answer questions about how photosensors work and how they are used.

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