Page 148 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
7

Control of Ambient Light

7.1 Introduction

Apart from some optical fiber communications links, most optical detection
systems do not enjoy the luxury of illumination just by the signal of interest;
when the source is off, the detector is dark. The great majority are bathed in a
background of ambient light from other sources (Fig. 7.1). The signal of inter-
est may even be far weaker than the disturbing light. Clearly we need drastic
measures to beat this level of adversary. Mechanical optical screening of either
the photodetector, the complete apparatus, or the lab windows with shields and
opaque curtains is an effective solution to stop light from entering, but much
apparatus under development is operated in an open state. It is very difficult
and even dangerous to debug optoelectronics in total darkness, so a compro-
mise has to be reached. In addition, some optical systems such as TV remote
controls, bar code readers, light-barriers, free-space communications links, and
environmental monitoring systems are required to be open to ambient light.
At the very least we need systems that are unaffected by fairly constant light
sources such as overcast skies and by the slow variations caused by moving
clouds and people. Suppression of the main 100/120Hz signals from incandes-
cent and fluorescent lighting is also mandatory. To combat these problems we
have emphasized the great benefits of operating our optical systems at a fre-
quency well above DC, and we will assume in this chapter that this has been
achieved. Although the signal processing techniques of modulation and syn-
chronous demodulation greatly reduce the degrading effects of such signals,
they do have their limitations, so the demodulator should receive as clean a
signal as possible. High background light levels will inevitably increase noise
and may overload the receivers. Hence we need to control ambient light and
suppress its effects. The question is, what characteristics of the desired signal
of Fig. 7.1 can be used to separate it from the general ambient light background?
There are many approaches, some optical, some electronic; we will look at a few
here.

141
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.