Page 509 - Instrumentation Reference Book 3E

21 Optical measurements

A. W. S. TARRANT

21.1 Introduction Take, for example, a spectrophotometer, which
consists of a light source; a monochromator, a
A beam of light can be characterized by its spec- sample cell, and a detector. An alteration in the
tral composition, its intensity, its position and position of the lamp will affect the distribution of
direction in space, its phase, and its state of polar- light on the detector and upset its operation,
ization. If something happens to it to alter any of despite the fact that the detector is several units
those quantities, and the alterations can be quan- “down the line.” Optical systems must be thought
tified; then a good deal can usually be found out of as a whole. not as a group of units acting in
about the “something” that caused the alteration. series.
Consequently optical techniques can be used in a It should be noted that the words “optical” and
huge variety of ways, and it would be quite impos- “light” are used in a very loose sense when applied
sible to describe them all here. to instruments. Strictly, these terms should only
This chapter describes a selection of widely refer to radiation within the visible spectrum, i.e.,
used instruments and techniques. Optical instru- the wavelength range 380-770 nm. The tech-
ments can be conveniently thought of in two niques used often serve equally well in the near
categories-those basically involving image for- ultraviolet and near infrared regions of the spec-
mation (for example, microscopes and telescopes) trum, and naturally we use the same terms. It is
and those which involve intensity measurement quite usual to hear people talking about “ultra-
(for example, photometers). Many instruments violet light” when they mean -‘ultraviolet radi-
involve both processes (for example, spectrophoto- ation,” and many “optical” fibers are used with
meters), and it is convenient to regard these infrared radiation.
as falling in the second “intensity measurement”
category. For the purposes of this book we are 21.2 Light sources
almost entirely concerned with instruments in
this second category. “Image-formation” instru- Light sources for use in instruments may be
ments are well described in familiar textbooks, grouped conveniently under two headings, (1)
such as R. S. Longhurst’s Geonzetricul und conventional or incoherent sources and (2) laser
Physical Optics. or coherent sources. Conventional sources are
The development of optical fibers and light
guides has enormously broadened the scope of dealt with in Sections 21.2.1 to 21.7.3 and laser
sources in Section 21.2.4.
optical techniques. Rather than take wires to The principal characteristics of a conventional
some remote instrument to obtain a meaningful light source for use in instruments are:
signal from it. we can often now take an optical
fiber, an obvious advantage where rapid response (1) Spectral power distribution:
times are involved or in hazardous environments. (2) Luminance or radiance;
This is discussed in Part 4. (3) Stability of light output;
In all branches of technology, it is quite easy to (4) Ease of control of light output:
make a fool of oneself if one has no previous (5) Stability of position.
experience of the particular techniques involved.
One purpose of this book is to help the non- Other factors which may have to be taken into
specialist to find out what is and what is not account in the design of an instrument system are
possible. The author would like to pass on one heat dissipation, the nature of auxiliary equip-
tip to people new to optical techniques. When we ment needed, source lifetime, cost, and ease of
consider what happens in an optical system we replacement. By the “radiance“ of a light source
must consider what happens in the whole optical we mean the amount of energy per unit solid
s.vs~er77: putting an optical system together is not angle radiated from a unit area of it. Very often
quite like putting an electronic system together. this quantity is much more important than the