Page 355 - Fundamentals of Light Microscopy and Electronic Imaging

338 GLOSSARY

the orientation of incident waves. The phenomenon reflects the difference between
the absorption curves for chromophores oriented in different directions in the
dichroic object. 123
DIC microscopy. See Differential interference contrast (DIC) microscopy.
DIC prism. See Wollaston prism.
Dielectric constant. A parameter describing the electrical permittivity of a material (the
degree to which a material is permeated by the electric field in which it is immersed).
Substances with low electrical conductivity (and low permittivity) such as glass,
plastic, and water are called insulators or dielectrics. The dielectric constant is
related to the refractive index n such that n . 129
Differential interference contrast (DIC) microscopy. A mode of light microscopy
employing dual-beam interference optics that transforms local gradients in optical
path length in an object into regions of contrast in the object image. Also referred to
as Normarski optics after the name of its inventor, George Nomarski. The specimen
is illuminated by myriad pairs of closely spaced coherent rays that are generated by
a crystalline beam splitter called a Wollaston prism. Members of a ray pair experi-
ence different optical path lengths if they traverse a gradient in refractive index in a
phase object. Optical path differences become translated into amplitude differences
(contrast) upon interference in the image plane. DIC images have a distinctive relief-
like, shadow-cast appearance. 155
Diffracted wave. In phase contrast and other modes of interference microscopy, waves that
become deviated from the path of 0th-order (background) waves at the object. Diffracted
1
waves can be shown to be retarded in phase by ⁄ wavelength from the background
4
wave by vector analysis. Diffracted waves combine with background waves through
interference in the image plane to generate resultant particle (P) waves of altered ampli-
tude that are perceived by the eye. See also Particle wave and Surround wave. 101
Diffraction. The bending or breaking up of light that occurs when waves interact with
objects, much in the way that waves of water bend around the edge of a log or jetty.
Light waves that become scattered upon interacting with an object (diffracted waves)
follow paths that deviate from the direction followed by waves that do not interact
with the specimen (nondiffracted or undeviated waves). 20, 61
Diffraction grating. A transparent or reflective substrate containing an array of parallel
lines having the form of alternating grooves and ridges with spacings close to the
wavelength of light. Light that is reflected by or transmitted through such a grating
becomes strongly diffracted. Depending on the geometry of illumination and wave-
length, a grating can generate color spectra and patterns of diffraction spots. 71, 75
Diffraction plane. One of the aperture planes of the light microscope containing the
focused diffraction image of the object. Under conditions of Koehler illumination,
the diffraction plane is located in or near the back focal plane of the objective lens. 4
Digital image processor. In video imaging, a signal processing device that converts
analogue video signals to digital format for rapid image processing operations such
as frame averaging, background subtraction, and contrast adjustment. 249
Digitizer. See Analogue-to-digital converter (ADC).
Distortion. An aberration of lenses, where the magnification factor describing an image
varies continuously between the central and peripheral portions of the image.
Depending on whether the magnification is greater at the center or at the periphery,
the distortion can be of the barrel or the pincushion type, respectively. 52
Double refraction. In polarization optics, the splitting of light into distinct O and E rays
in a birefringent material. When a birefringent crystal of calcite is placed on a page