Page 158 - Fundamentals of Light Microscopy and Electronic Imaging

PRINCIPLES OF ACTION OF RETARDATION PLATES 141

compensator to be used as a nulling device to undo the phase shift imparted by the spec-
imen through the introduction of an equal but opposite phase shift. The number of
degrees of rotation on the compensator required to null the birefringence of an object
and bring it to extinction is used to calculate . If other variables are known, the retar-
dation value can be used to determine if birefringence is positive or negative, measure
the amount of birefringence—the difference in refractive index (n n ) as experienced
e
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by the O and E rays—or determine the specimen thickness. It is even possible to deduce
the geometry and patterns of molecular organization within an object (e.g., tangential
vs. radial orientation of polarizable bonds of molecules in a spherical body). It is the
compensator that makes the polarizing microscope a quantitative instrument.
Compensators are also used for qualitative purposes to control background illumi-
nation and improve the visibility of birefringent objects. In a properly adjusted polariz-
ing microscope, the image background looks very dark, approaching black. Inoué has
shown that the visibility improves if 5–10 nm retardation is introduced with a compen-
sator, which increases the intensity of the background by a small amount (Inoué and
Spring, 1997). The compensator can also be used to increase or decrease the amount of
phase displacement between the O and E rays to improve the visibility of details in the
object image. Thus, birefringent objects are sometimes seen with greater contrast using
a compensator than they are using the polarizer and analyzer alone.

-Plate Compensator

Retardations of a fraction of a wave to up to several waves can be estimated quickly and
economically using a -plate retarder as a compensator (Fig. 9-5). The plate is also
known as a full-wave plate, first-order red plate, red-I plate (read “red-one plate”), or
color tint plate. The plate is composed of a film of highly aligned linear organic poly-
mers or of a sheet of mica, and thus is birefringent. The axis of polarizable bonds in the
material defining the slow axis (higher refractive index) of the wavefront ellipsoid is
usually marked on the plate holder. When placed between two crossed Polaroid sheets
at a 45° angle and back-illuminated with white light, the plate exhibits a bright 1st-order
red interference color, hence its designation as a first-order red plate. Full-wave plates
introduce vivid interference colors to the image of a birefringent object and are useful
for making rapid quantitative assessments of relative retardation, as well as for deter-
mining the orientation of index ellipsoids. In geology and materials science, full-wave
plates are commonly used to identify birefringent crystalline minerals and determine
specimen thickness (see Color Plate 9-1). For retardations of /3 or less, phase retar-
dations can be measured with an accuracy of 2 nm.
When placed in front of the analyzer so that its slow axis is oriented 45° with
respect to the crossed polars, a red-I plate introduces a relative retardation between O
and E rays of exactly one wavelength for green wavelengths of 551 nm (Pluta, 1988).
Green wavelengths therefore emerge from the retardation plate linearly polarized in the
same orientation as the polarizer and are blocked at the analyzer. O and E waves of all
other wavelengths experience relative phase retardations of less than 1 ; they emerge
from the plate as elliptically polarized waves and are only partially blocked by the ana-
lyzer. The color of white light minus green is bright magenta red, thus accounting for the
color of the red-I plate. (To review the principles governing the formation of interfer-
ence colors, see Chapter 2.) You can confirm that the green wavelength has been
removed by examining the red interference color with a handheld spectroscope. All of