From: Richard M. Koolish (koolish@bbn.com)
Date: 12/11/01-05:53:42 PM Z
Daren writes:
> Sandy,
>
> If what you have just stated is true then when shooting with my softbox
> the inverse square law shouldn't apply?? That's absurd!! And what about
> flourecent banks used for lighting?? Are these not effected by inverse
> square law??? Something is more than a bit fishy here.
>
> Light falls off at a regular rate regardless of relative distance and
> size of light source to subject (point source vs. soft source). The
> idea that a soft or broad source will create light in any way different
> than a point source is wrong. Light is light, we obviously have
> different wave legnths in the elctromagnetic spectrum, but a photon is
> just a photon. The photon doesn't know the difference between "soft",
> "hard", "bright" or "dim", it is simply a photon. Besides, the ONLY
> difference between a point source and a soft source is distance from
> source to subject and size relative to one another. Unless acted upon
> by a great force (large amounts of gravity) or another substance (glass,
> etc.) light has no "choice" but to obey the laws of physics, that
> includes the inverse square law. Light is not magic, it can and must be
> described in a rational way.
The inverse square law applies to point sources because light spreads
out in a sphere, and the surface area of spheres increases as the
square of the radius.
Now consider a light source that is a line, like a fluorescent tube.
Close to the source, the light spreads out in a cylinder, not a sphere.
The surface area of a cylinder of constant height increases linearly
with the radius, so the intensity falls of as 1/R. This is strictly
true for an infinite line source, and is basically true close to a finite
line source. As you move farther away, the line starts to look like a
point.
This archive was generated by hypermail 2b30 : 01/02/02-04:47:33 PM Z CST