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In Part One of this blog, I described a common explanation for photosensor  failure – being mounted too high in a space.  When I ask why the sensor won’t work properly because it’s too high, the inevitable answer is “the inverse square law, of course!”  A look of smug satisfaction usually accompanies the recall of this tidbit from high school physics.  Case closed!  Everybody knows there is no defense against the infallible inverse square law of photosensor failure.  It’s as good as the Latvian Gambit in chess.  Or for you Trekkies, the Corbomite Maneuver.

Having taught Daylighting at the University of Colorado for many years, I’ve had the opportunity to ponder this control challenge.  And I don’t buy the ISL explanation for a nanosecond (as we’re talking about photons, a minute would be entirely inappropriate).  Here’s why.

It’s true that our most common light sources, whether the sun, an incandescent bulb, or a fluorescent lamp, emit light that weakens as the inverse square of the distance from the light source.  Twice as far from the light source, one fourth the light.  Three times as far, one ninth the light.  Pretty basic stuff.

Oops, that’s all for Part Two.  Continue on to Part Three for the conclusion.


We have recently experienced some problems with daylight photosensors inside buildings not properly controlling the electric lights within their zones.  Most often, the sensors fail to detect daylight accurately and they therefore leave lights on or fail to dim them adequately.  This is a significant problem because electric lights account for a large share of energy use in a school building.  When we have both daylight and electric light, we are wasting resources.

As we began investigating the issue, we heard a variety of explanations.  Some made sense, others less so.  One very common excuse was that “the sensors are only rated for a certain distance above the floor.  Higher than that, they don’t work.”

This response drives me nuts!!!  I can just imagine the picture inside the mind of the sales rep, electrician, or even electrical engineer.  Itty bitty photons leave the surface of a desk, heading upward.  They valiantly attempt to reach the photosensor 16 feet above the floor, like Thomas the Tank Engine climbing the hill on the island of Sodor.  But the higher they get, the more they fight gravity, saying to themselves “I think I can, I think I can.”  Somehow knowing the photosensor is only rated for 12 feet, they fall back just short of the sensor.  And the lights stay on.

If you want to know what’s wrong with this scenario, look for Part Two early next week.