Page 160 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Control of Ambient Light

Control of Ambient Light 153

Unpolarized source
Photoreceiver
Collimated (sky reflection)
laser source highly polarized
on reflection
Unpolarized laser
-1
reflection: passed q = tan (n)
B
Polarizer
Polarized reflection:
absorbed

Water-, glass- Refracted, scattered
surface light

Figure 7.12 Polarizers can be useful to suppress highly polarized interfering
sources, for example from daylight reflections off surfaces close to Brewster’s
angle. This example is from a laser-based water turbidimeter for outdoor use.

-1
7.12), given by q B = tan n, where n is the refractive index of the water. For n
= 1.33 q B is 53°. At this angle the reflected light is strongly polarized perpendi-
cular to the plane of incidence. A polarizer oriented in the plane of incidence
as shown can achieve suppression to better than 0.1 percent (plastic polarizers)
or even 0.01 percent (crystal polarisers), as long as the acceptance angle of the
receiver is less than a few degrees. Good suppression using plastic polarizers is,
however, achieved at the expense of higher absorption losses. For incident and
refracted angles q and y and incident and refracted medium indices n 1, n 3 we
have:
n 1 sinq = n 3 siny (Snell’s law ) (7.1)

The reflected amplitudes are given by (Longhurst 1968):
- )
-sin (qy
As = (7.2)
2
+ )
sin (qy
- )
tan (qy
Ap = (7.3)
2
+ )
tan (qy
And the transmitted amplitudes are:
2 siny cosq
As = (7.4)
3
+ )
sin (qy
2 siny cosq
Ap = (7.5)
3
- )
sin (qy (q y
+ )cos
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