Re: Lighting myths revisited

> > Thus far, I have seen no mention of the Law of Inverse Squares as it
> > lighting.  Very briefly, the intensity of light (any light) shining on
> > surface varies inversely as the square of the distance from the light
to the
> > surface. This is true, regardless of the efficiency of the reflector,
> > (Unless, of course, you have a coherent source of light, such as a
laser, but
> > they are really lousy lights for growing plants.  :-D  )  

Light is electromagnetic radiation. Once you leave the 'near field'
propagation of EMR follows the inverse square law loss in free space.
Propagation through any material increases this loss.

Items such as reflectors concentrate EMR and provide 'gain'. A reflector
providing 10 db gain, for instance, will produce a magnitude of radiation
at a given point equal to a transmission source of 10 times the power with
a spherical radiation pattern. 

With proper reflectors the effective gain can be very high - on the order
of 1000:1 or 10,000:1. An example of this would be - a twenty watt lamp
with a 30 db reflector can produce a light field intensity equal to a
20,000 watt lamp of the same efficiency with no reflector. To produce 30 db
of gain one would likely require a parabolic reflector with a point source
light. I have much experience with lower frequency EMR so am not aware of
actual gains produced by practical reflectors.

Being from RF, where we are normally dealing with large distances, I have
forgotten many of the details of the 'near field', such as how far the near
field exists. This is the part of the argument dealing with a 'point'
source and a 'linear' source. At a given distance from a linear source it
will behave the same as a point source. I suppose that aquariums are in an
area where we are not far enough away from the light source to always
experience inverse square propagation loss.

Patrick L. Martin        pmartin at netcom_com