Leo Lichtman wrote:

wrote: A lens can't collect and focus more energy
than is intercepted by it's total area. (clip)
^^^^^^^^^^^^^^^^^^
That is true. However, a given light source at a greater distance produces
a proportionately smaller image. so the brightness of the image remains the
same. I'm not making this up. Lay out a diagram of a lens and do the ray
tracing, and you'll see.

Or, as experimental evidence, think about what i said about taking a
photograph. Let's say you take a picture of two billboards. One is 100
feet from you, and the other is 1000 feet. They are in the same picture,
and they are both correctly exposed. The more distant billboard has the
same illumination falling on it, but, being 10x farther away, the lens will
receive 1/100 as much light. Inside the camera, the image of the more
distant billboard will be 1/10 as large, so it will have 1/100 as much area.
So the brightness of the image will come out the SAME.



Brightness is a slightly misleading term here, what we need to worry
about is energy absorbed by tissue.

Consider a match and a nice big wood bonfire, both will burn at the same
temperature and be emitting the same infra red wavelengths, if you look
at them they will have the same brightness, but one will be much bigger
than the other.

Now hold your hand an inch from the burning match, then try the same
thing with the bonfire. Obviously the match won't be a problem and the
bonfire will burn you - to put it another way, the energy absorbed from
the match isn't enough to damage the skin, the energy absorbed from the
bonfire is enough to blister it, keep it there long enough and the
tissue will be destroyed.

It all comes down to how much energy, how long ?.



--
scruttocks
k12rs r80g/s