In Parts One and Two of this blog, I described the basic situation of a photosensor mounted high in a space not working and the Inverse Square Law being invoked as the obvious reason why.  Now let’s follow this to a logical conclusion.

The oft cited explanation is that the photosensor is seeing a light source such as a desk in its field of view.  That desk is a light source by virtue of its reflecting daylight from its top surface.  The desk isn’t truly a point source, but we can overlook that for now.  If we are twice as far from that desk, our photosensor will receive approximately one-fourth the light from it.  And the less light the photosensor receives, the harder it is to calibrate accurately.  That makes sense, doesn’t it?

Not really.  Something that does not change no matter how far the photosensor is above the floor is the sensor’s field of view.  Imagine I have a photosensor mounted high on a ceiling pointed downward.  If it has a field of view of 30 degrees and is 16 feet high, it “sees” a floor area of 57.74 square feet.   If I were to move that photosensor down to 8 feet, it would “see” only 14.43 s.f of floor.  That photosensor at 8 feet above the floor would probably only see one student desk.   But when the same photosensor moves to 16 feet above the floor there would be four desks in its field of view.  The math is pretty obvious.  When the photosensor is twice as far away, each desk contributes one-fourth the reflected light.  But the four desks in the field of view perfectly balance that loss, and the photosensor receives the same amount of light!  It should work just as well at 16 ft. as it does at 8 ft.!

If you want to learn more about what is really going wrong with photosensor performance, check out Part Four.

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