TABLE OF CONTENTS 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 sensitivity, 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.
Figure 8.1 shows an example of such a measurement, namely of the transmission or attenuation of a liquid sample in a transparent silica cuvette or capillary. This is typical of many applications in analytical chemistry, where changes in attenuation can be related to the (low) concentrations of an absorbing chemical, 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.
