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19.8 Light Detection, Image, and Vision Systems

Stanley S. Ipson

Introduction
Light detectors span a broad spectrum of complexity. The simplest are single sensors whose output
signals are easy to interpret and to interface to other components like microprocessors. In contrast, the
image sensors in video and digital cameras, incorporating arrays of up to several million detectors,
produce output signals which are complicated to interface and require powerful processors to interpret.
Regardless of complexity, the purpose of a light detector is to measure light, and the section ‘‘Basic
Radiometry’’ introduces a number of radiometric terms that are employed in the characterization of
light, light sources, and detectors. However, manufacturers often specify the performance of their devices
using photometric units, which take into account the human visual response to light, and so it is necessary
to understand both radiometric and photometric measures of light. Sources of light are briefly discussed
in section ‘‘Light Sources.” There are several types of light detector in common use and the principles
of operation and characteristics of the most widely used, including pyroelectric, photoresistive, photo-
diode, and phototransistor are summarized in section ‘‘Light Detectors.” Vision systems have optical
components to form an image and an image sensor to convert the light image into an electrical signal.
Image formation is reviewed in section ‘‘Image Formation,” before introducing the most widely used
detectors, based on charge-coupled device (CCD) technology and complementary metal oxide semicon-
ductor (CMOS) technology, in section ‘‘Image Sensors.” The elements required to complete a vision
system are discussed briefly in the final section.

Basic Radiometry
Visible light is electromagnetic energy radiated with very short wavelengths in the range between about
400 and 700 nm. At shorter wavelengths, to about 30 nm, is invisible ultraviolet light and at longer wave-
lengths, up to about 0.3 mm, is invisible infrared radiation. Although electromagnetic radiation displays
wave behavior including interference and diffraction, it can also behave like a stream of particles and is
emitted and absorbed by matter in discrete amounts of energy called photons. The energy ε of a light

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