Page 170 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
8

Stability and Tempco Issues

8.1 Introduction

Many optical measurement systems suffer from a lack of optical signal energy.
Either the source is weak, or very distant, or must be detected in a short time,
or suffers from great attenuation on its path to the photodetector. In all these
cases, the main design effort is directed at achieving adequate receiver sensi-
tivity, where optical and electronic noise and interference are the adversaries.
This has been the primary focus of the chapters up to now. Another class of
measurement has plenty of light, such that enormously high, shot-noise limited
signal-to-noise ratios are possible in principle, and the goal is to measure the
optical power associated with the signal with exacting precision over the life of
an instrument. Here the main issue is a lack of stability.

8.2 Chemical Analytical Photometry—
Design Example

Figure 8.1 shows an example of such a measurement, namely of the transmis-
sion or attenuation of a liquid sample in a transparent silica cuvette or capil-
lary. This is typical of many applications in analytical chemistry, where changes
in attenuation can be related to the (low) concentrations of an absorbing chem-
ical, either directly using their own optical absorptions, or via a colorimetric
indicating reagent. Let’s assume that the chemical we wish to measure absorbs
at 430nm in the visible blue range. The solvent is water and high power LEDs
are available emitting near to this peak absorption wavelength. Light from the
LED is collimated and couples through the aqueous sample-filled cuvette to a
well-designed photoreceiver. The cuvette outer walls are antireflection coated
to give almost zero Fresnel reflection losses, and index mismatches at the
silica/water interface (ª0.14) give only small intensity reductions (ª0.25
percent) at each interface.
The water exhibits an absorption of less than 0.01cm -1 at this wavelength,

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