Page 241 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Multiple Channel Detection

234 Chapter Eleven

0.4
Permang.
Blue 430nm
0.3
Absorbance 0.2
Green 525nm

0.1

0
250 300 350 400 450 500 550 600 650 700
Wavelength (nm)
Figure 11.1 The concentration of chemicals with well-defined absorbing
and loss-less regions can be determined with high resolution by measuring
at two wavelengths on and off the absorption feature and subtracting the
absorbance values.

f 1 R
Color L
filters
+ A
Blue
-
LED l 1 PD1 Ch. A
l + A
Red 2
-
LED PD2 Ch. B
Cuvette
Figure 11.2 Multiple channels can be separated
spatially and with matched color filters.

compensates for the bulk of any additional water matrix absorption. We will
use source modulation and synchronous detection, and a brute-force approach
with duplicated optoelectronics (Fig. 11.2) would do the job. The two source
beams with different colors are arranged side by side in the cuvette holding the
sample. Separation of the two channels is primarily spatial, although in order
to reduce problems of the two beams seeing different parts of the liquid and
different cuvette surfaces, we would like the beams to be as close together as
possible. On the other hand, proximity will increase the risk of one source
illuminating the wrong detector. Therefore we additionally isolate the channels
using individual color filters in front of the detectors. These also serve to reduce
the detection of ambient light. To aid in output-power tracking of the two LEDs,

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