Page 163 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

156 Chapter Seven

Index-matched coatings AR coat to reduce
to absorb internal light external reflections
Thin-edged
apertures Beam-dump

Do edges
scatter?
Test coating reflectivity
using prism
Check field-of-view
of detector
Detector

Figure 7.15 Sensitive light scatter and fluorescence measurements
require obsessive attention to avoiding or absorbing all stray light.
Internal reflections in a small cuvette can be reduced by coating the
external surfaces with an approximately index-matched absorbing
material. One way to evaluate these is to measure reflection coefficient
in a 45° prism.

at the silica/coating interface is to match the complex refractive indices of the
two materials. The power reflectance formulas of Eqs. (7.1) to (7.5) are valid
even when the refractive indicies are complex. However, even if you could match
both real and imaginary parts of n, it would just put off the problem to the next
interface. The best approach is to find a substance which is a good match of the
(real) refractive index, which is also moderately absorbing (not a metal). For
example, a perfect match of real refractive index of a 1-mm-thick coating with
an absorption length of 100mm should work quite well. The 100mm figure
implies that the imaginary part of the refractive index, that is the absorption,
is very small, and hence will not contribute to a large impedance mismatch
between silica and coating. Spray-paints and acrylic coatings from art shops are
as reflective as glass when viewed from the air, but can be extremely effective
when viewed from inside the silica material; a few experiments will be needed
to choose the best materials. It is easier to screen available paints and gels by
applying them to the hypotenuse of a 45/45/90° silica prism rather than a flat
substrate. In the latter case it is too difficult to separate the front- and back-
surface reflections unless the flat is very thick. Some weakly absorbing acrylic
paints can achieve better than 0.1 percent power reflectivity in the visible when
coated onto silica and glass.
To absorb the straight-through free-space beam rough paints containing
black flocks are available and useful, but can be polished and degraded by

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