Page 157 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

150 Chapter Seven

Color corrected laser protection
100
90
80
Transmission (%) 60
70

50
40
30
20
10
0
400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700
Wavelength (nm)
Figure 7.9 Almost arbitrary wavelength filtering responses are possible
with the new synthesis tools and manufacturing techniques of “rugate”
filters. These use a deposited film of continuously varied refractive
index, rather than the Hi-Lo stacks of conventional interference filters.
The curve shows a filter for visual use with color-correction and strong
suppression of a small band of wavelengths around 532nm. Figure sup-
plied and reproduced by permission of Advanced Technology Coatings
Ltd., Plymouth U.K.

Company’s original range. These are cheaper than colored glass filters, but
generally do not offer as high suppression. A selection of long-wavelength pass
filters is available from about 395nm (Type “1A”) in the blue to 852nm (“87C”)
in the near IR. In addition, there are a few bandpass filters in the visible wave-
length range. Being plastic, they are easily cut to size for custom applications
and quick tests.
Another way to obtain very high suppression of particular wavelengths, espe-
cially with a long-pass characteristic, is to use pure, simple liquids. For example
ethyl acetate in aqueous solution is transparent in the visible, but absorbs
strongly below about 255nm. Carbon tetrachloride performs similarly. Figure
7.11 shows a dilute solution of acetone (ª50ml/ml in water). The absorption is
a smooth peak centered on 270nm (lower curve), which is useful to absorb the
254nm mercury line. As the concentration is increased (upper curve), this looks
more and more like an edge filter, which shifts to longer wavelength. It remains
very transparent in the visible range. The ultimate absorption is much greater
than 3AU, which is the measurement limit of the spectrometer used for this
spectrum. The convenience of a liquid filter lies mainly in wide-range adjust-
ment of the absorption strength (via concentration), very high absorption at
large thicknesses and the ability to form complex shapes. Transition metal ions
in aqueous solution are also useful, having strong visible colors which can be
used to absorb relatively narrow spectral regions. These metal ions often can

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