Page 526 - Instrumentation Reference Book 3E

Measurement of optical properties 509
Consequently colorimetric measurements are 21.7 Measurement of optical
used on a large scale in all consumer industries, properties
and the colors of manufactured products can now
be very tightly controlled. It is possible to achieve Transparent materials affect light beams passing
a high degree of precision, so that the minimum through them, notably changing the speed of
color difference that can be measured is slightly propagation; refractive index is, of course. a meas-
smaller than that which the human eye can per- ure of this. In this section we describe techniques
ceive. It should be noted that nearly all surfaces for measuring the material properties which con-
have markedly directional characteristics, and trol such effects.
hence if a sample is measured in two instruments
which have different viewinglilluminating geom-
etry, different results must be expected. 21.7.1 Refractometers
Diagrams of these instruments, such as Figure
21.17, make them look very simple. In fact, the The precise measurement of the refractive index of
spectral sensitivity of the filter/photomultiplier transparent materials is vital to the design of optic-
combination has to be controlled and measured al instruments but is also of great value in chemi-
to a very high accuracy; it certainly is not eco- cal work. Knowledge of the refractive index of a
nomic to try to build a do-it-yourself colorimeter. substance is often useful in both identifying and
It is strongly emphasized that the foregoing establishing the concentration of organic sub-
remarks ,are no substitute for a proper discussion stances, and by far the greatest use of refracto-
of the fascinating subject of colorimetry, and no metry is in chemical laboratories. Britton has used
one should embark on color measurement with- the refractive index of gases to determine concentra-
out reading at least one of the books mentioned tion of trilene in air, but this involves an interfero-
above. metric technique which will not be discussed here.

"1

a

(b) (d)
Figure 21 .I8 (a) Refraction of ray passing from less dense to more dense medium; (b) refraction of ray passing from more
dense to less dense medium; (c) total internal reflection; (d) the critical angle case, where the refracted ray can just emerge.