Page 164 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 157

inappropriate handling. Black velvet is available, even self-adhesive, which is
generally even better. Some of the thick carbon-loaded plastic foams used for
electrostatic protection and packaging of integrated circuits can have very low
reflectivity, of the order of 0.1 percent in air, but 10mm or more may be neces-
sary due to the tenuous porous structure.
Blackening treatments for metal are available and are usually performed elec-
trochemically. If the original metal is highly polished, blackening to reduce
reflections is of limited use (but they are attractive). Some materials can be
effectively textured to give an interface of mechanically-graded refractive index.
For example, silicon can be treated by ion-beam etching to give a dendritic
structure with a depth much greater than the wavelength of visible light. Such
surfaces can have an almost magically invisible appearance due to their low
reflectivity. Stacks of new razor blades (Fig. 7.16a) can function in a similar way,
with the sharp edges limiting scattered energy, and multiple reflections between
blades gradually absorbing penetrating light. This implies that absorbing,
highly polished surfaces may perform better than roughened ones, as they limit
the change in angle after many reflections. The blades can also absorb consid-
erable energy without damage.
Achieving absorption efficiency is often easier if you have lots of space. One
of the best absorbers is a large hollow cavity with a small entrance hole (Fig.
7.16b). The cavity should be diffusely reflecting and blackened using any of the
above techniques, although the main effect in use is the distribution of the input
light over a large internal area, so that only a small fraction escapes through
the hole. Even if the cavity were white and diffusely reflecting, a 1-mm-
3
-5
diameter hole in a 1000cm cavity would only allow about 10 of the input light
out. Blackening will get you another order of magnitude or more. Infrared
black-body sources are often fabricated in this way, with a small hole leading to
a large, heated internal cavity.

(a) Razor blade Stack (b) Black-body

Figure 7.16 Stacks of new razor blades can form an effective, high power beam dump via
multiple reflections and absorptions. A small aperture in a large, diffusely reflecting cavity also
works well.

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