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Control of Ambient Light
152 Chapter Seven
3.0
2.5
Absorbance (1cm) 2.0
1.5
1.0
0.5
0.0
200 250 300 350 400 450 500
Wavelength (nm)
Figure 7.11 Absorbance spectrum of dilute and strong solutions of acetone in
water. Many chemicals have useful filter responses, especially edge filters in the
UV. Their great advantages are strong blocking, easy absorbance adjustment, and
shape flexibility.
absorption modulator which is matched to modulate the received power of a
specific absorption. This technique is discussed briefly in Chap. 10.
7.4 Polarizers
Polarization coding can be applied both in passive and active approaches for
ambient light suppression. In the simplest case the signal is polarized before
transmission, and a matched polarizer is used at the receiver. If the disturbing
light is unpolarized, its intensity will be reduced by 50 percent; not great, but
even this may be useful. The suppression can be much greater if the disturb-
ing signal is itself polarized. For example, Fig. 7.12 shows a turbidimeter
designed to see below the surface of an outdoor flowing water stream. The signal
source is a linearly polarized diode laser, arranged with its polarization in the
plane of incidence. Light that refracts into the water is multiply scattered,
which greatly reduces the degree of polarization. Hence the returned scattered
light is almost depolarized, and 50 percent of it passes through a polarizer to
reach the detector.
When used as designed in the open air, these sensitive receivers can easily be
overloaded by intense sunlight reflecting off the water surface. Even with the
best orientation of the turbidimeter to avoid direct sunlight reflection, the
diffuse sky illumination seriously limits performance, even in the United
Kingdom! A solution is to operate the sensor close to Brewster’s angle (Fig.
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