Page 203 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

196 Chapter Nine

Source
array
a
Red
Photodetector
Blue
Red

Blue

Window Absorbing Sample
fouling layer under test
Figure 9.5 Multiple wavelengths, preferably with beams
interspersed to equalize attenuation, can reduce the
effects of contaminating films.

0.25

0.20

Absorbance (1cm path) 0.15

0.10

0.05

0.00
250 300 350 400 450 500 550 600 650
Wavelength (nm)
Figure 9.6 A full spectral measurement of an absorption of limited extent can ease determination
of its magnitude, even in the presence of large uniform absorbance and an increased noise level.

of that off the absorption feature, this ratio of 9/10 (A =-log 10(9/10) = 0.046AU
absorbance units) will be obtained regardless of the multiplicative attenuation
due to the contamination. Alternatively, on an absorbance scale we must deter-
mine the difference by subtracting the on- and off-peak absorbances. The effect
of the fouling is just a vertical offset of the chemical species’ spectrum, and the

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