Bill Funk wrote:
On Mon, 07 Aug 2006 16:12:21 -0700, "Roger N. Clark (change username
to rnclark)" wrote:


Bill Funk wrote:

On Mon, 07 Aug 2006 12:27:11 -0700, "Roger N. Clark (change username
to rnclark)" wrote:



Volker Hetzer wrote:


Roger N. Clark (change username to rnclark) wrote:



A good way to make a light trap is a cone painted with
glossy black paint. The reflections from the painted surface
go deeper into the cone.

So the camera looks into the pointy end?
Otherwise it seems to me that the light gets reflected back.

Yes. Think of an ice-cream cone. Look at the inside,
and paint it glossy black. A machined metal (like aluminum)
works very well.

Roger


Wouldn't machined (or even moreso, polished) metal reflect much more
light?
Why did you pick glossy black?

You need something to absorb the light. Each reflection
absorbs ~95% of the light, and the remaining gets reflected
further into the cone. If you use flat black, each reflection
scatters some light out of the cone. A polished aluminum
surface would reflect most light so most light would never be absorbed,
and eventually, after reaching the bottom, would reflect back out.
Flat black gives you both controlled absorption and controlled
reflection.

Roger


If you're really after a light trap, wouldn't a baffle, where any
reflections send the light to something that absorbs light be better
than any reflective surfaces?

Yes, that is what you want, but there is no surface that
absorbs all light. So you must make a trap that any
light that does get reflected does so in a direction
away from your view. The is what the cone accomplishes.
The angle of the cone must be steep enough for any
reflected light to get reflected further into the cone.
If, for example, light was reflected 5 times, and the
reflectance was 0.02, then the light reaching the bottom
would be 0.02*0.02*0.02*0.02*0.02 times the original
light level. The more specular the reflection of the
glossy paint, the less light will be scattered back
to you. In practice, it works very well.

Roger