Page 316 - Instrumentation Reference Book 3E

300 Temperature measurement

in perfect register. The orientation of one layer of tively with the incident ray and the sandwich
molecules lies twisted by a small angle compared looks opaque. In (b), however, the liquid crystal
to the layer below. This helical structure rotates is above the temperature at which the ordered
the plane of polarization of light passing through structure breaks up. The material is no longer
the liquid in a direction perpendicular to the optically active, and the light ray is reflected
layers of molecules. Figure 14.68 illustrates this back in the normal way-the material looks
effect diagrammatically. The optical effect is very transparent.
pronounced. the rotation of polarization being of The temperature at which the ordered
the order of 1000" per millimeter of path length. structure breaks up is a function of the exact
The laminar structure can be enhanced by con- molecular structure. Using polarized light a
fining the cholesteric liquid between two parallel noticeable change in reflected light occurs for
sheets of suitable plastic. The choice of polymer a temperature change of 0.001 "C. In white light
for this plastic is based on two prime require- the effect occurs within a temperature range
ments. First, it is required to be transparent to of 0.1 "C. Both the appearance of the effect
light, and second, it should be slightly chemically and the exact temperature at which it occurs
active so that the liquid crystal molecules adja- can be affected by addition of dyes or other
cent to the surface of the polymer are chemically materials.
bonded to it with their axes having the required
orientation. 14.7.3.4 Tlzennnl imaging
When used for temperature measurement the
liquid crystal is confined between two sheets of In Section 14.6 the measurement of temperature
transparent plastic a few tens of micrometers by infrared and visual radiation was discussed in
apart. The outer surface of one plastic layer is some detail. This technique can be extended to
coated with a reflective layer; see Figure 14.69. In measure surface temperature profiles of objects.
(a) a light ray enters the sandwich and travels to This is known as thermal imaging. The object to
the bottom face where it is reflected back. Since be examined is scanned as for television but at a
the liquid crystal is in its ordered form it is opti- slower rate and in the infrared region instead of
cally active. The reflected ray interferes destruc- the optical part of the spectrum. The signal so

i

Destructive
(a) interference (b)
Figure 14.69 Destructive interference of reflected ray in liquid crystal