"Roger N. Clark (change username to rnclark)" writes:

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.

If the sides are steep enough and clean enough that the reflection
always goes further towards the bottom of the trap, it doesn't matter
how reflective the walls are. No black paint is needed.

For a demonstration, buy a pack of double-edged razor blades. Look at
the cutting edges, and note that they have shiny ground and polished
metal just behind the cutting edge. Now get the blades very clean,
wiping off any excess oil that they were packaged in, and stack them in
a pile. Carefully line up all the blades in the stack and clamp them
in that position with a C clamp, a nut and bolt, or whatever you want.

Now look at one of the two edges of the stack of blades that is made up
of cutting edges. It will be very, very black. The cutting edges
stacked beside each other form very deep narrow "V" shapes, and
virtually all light that strikes these edges of the stack falls into one
of these V slots (since the cutting edges themselves are so thin that
they have near-zero area). And even though the reflectivity of the
metal is high, probably 80 or 90%, each time the light reflects some of
the energy is absorbed and the rest is sent deeper into the V, never to
return back into the world.

One of my professors had assembled a stack like this, and liked to show
it to people. Apparently a V-groove absorber like this is good for
absorbing laser beams that have substantial power, since the device is
all metal and the dumped energy just heats the metal. You don't want to
get it hot enough to warp the metal, but it can handle more power
without damage than black paint would.

Dave