[ATM] Measuring scattered light off optical surfaces

[Dominic-Luc Webb]

On Wed, 10 Jan 2007, Don Clement wrote:

> It appears that scattered light measurement is measuring surface roughness.
> Perhaps a technique similar to measuring particle size (e.g. clean room

Well this is in fact a good point, and one I eluded to earlier, in a
way. The goal is to get to the maximal theoretical reflection with a
well polished surface of appropriate geometry. If you satisfy the
usual tests we do, then what is left is basically the quality of
aluminizing/coating, which most of us are not in a position do much
about. Some light will traverse the aluminum and even the glass and
would therefore be transmitted light and not scatter. Light loss occurs
by various means in addition to scatter. If you already have a
detector and some kind of standard, maybe the question could
be "Does the photon flux reaching the detector approach the
theoretical expectation?". If not, too thick or thin aluminum could
easily be implicated, having nothing to do with scatter. As it
was explained to me long ago, if the signal is well below expectations,
the quality or average thickness of the aluminum should be suspected.
This can readily be tested by electrical conductivity. Taking it the
other way, if you have an optimal surface as most of us are after,
then scatter from the mirror is essentially a non-issue. Your
photons are already accounted for.

Dominic-Luc Webb