How Can I Determine Photocell Settings With A Foot Candle Meter?

If a customer has a foot candle meter and wishes to use it to determine their daylighting settings they can do so by following these steps:

  • Measure the foot candle reading on the work surface.
  • Measure the foot candle reading at the sensor with the foot candle meter pointing down if the sensor has a downward facing photocell.
  • Calculate the ratio between these two numbers.
  • Choose the desired foot candle reading at the work surface.
  • Multiply the desired foot candle reading by the ratio to get the effective set point.
  • Choose a Sunlight Discount Factor and Set Point that when multiplied together will result in a number close to the effective set point.  If the sensor contains dimming the Sunlight Discount Factor should be chosen to reflect the characteristics of the room.

Notes:  Depending on the time of day the measurements are taken you may get different results for the ratio.  Ideally the ratio should be taken when there is sufficient sunlight such that the lights could be turned off and still satisfy the desired foot candle number at the work surface.  

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