Page 350 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 333

gain control is useful in fluorescence imaging where specimens are subject to con-
tinual photobleaching. 240
Azimuth angle. A term used to describe the orientation of an object in a plane such as
the specimen plane of the microscope. On a graph with polar coordinates marked off
in 360°, the angle subtended between a fixed reference (designated 0°) and a vector
rotated about the origin. 20
Back aperture of the objective lens. An aperture plane of the light microscope located
at or near the rear lens element of the objective lens and the site of formation of a dif-
fraction image of the object. Optical devices such as phase plates, DIC prisms, and
aperture masks used in forms of interference microscopy are located at or near this
location. 5, 80
Background subtraction. An operation in electronic imaging whereby an image of the
featureless background near an object is subtracted from the image of the specimen
to remove patterns of irregular illumination and other optical faults such as scratches
and dust. 244
Bandwidth. In video microscopy, the range of frequencies expressed as the number of
cycles per second used to pick up and transmit an image signal. To resolve an object hav-
ing 800 alternating black and white lines in the time of a single horizontal raster scan
(the required resolution for closed-circuit TV signals), the camera electronics must be
fast enough to rise and fall in response to each line at a rate that corresponds to a fre-
quency of 10,000 cycles/s. The bandwidth of such a camera is said to be 10 MHz. 245
Barrier filter. See Emission filter.
Beam splitter. An optical device for separating an incident beam of light into two or
more beams. A prism beam splitter in the trinocular head of a microscope directs the
imaging beam to the eyepieces and to the camera simultaneously. A polarizing beam
splitter made of a crystalline birefringent material is used to produce linearly polar-
ized light. A dichroic mirror beam splitter reflects excitation wavelengths while
transmitting long-wavelength fluorescence emission. 126, 192
Bertrand lens. A built-in telescope lens located behind the back aperture of the objec-
tive lens. When rotated into the optical path, the back aperture and diffraction plane
are seen, and other planes that are conjugate to it, while looking in the oculars of the
microscope. 5
Bias noise. One of the noise components contained in a raw CCD image. Bias noise is
calculated as the square root of the bias signal, and is considered in determining the
S/N ratio of images. 273
Bias retardation. In polarization microscopy, the alteration of optical path differences
between O and E rays, made by adjusting a compensator. Bias retardation is intro-
duced by the operator to change the contrast of an object or is introduced in order to
measure the relative retardation of the O and E wavefronts of birefringent objects.
In DIC microscopy, bias retardation is introduced using a Wollaston prism to opti-
mize image contrast. 161
Bias signal. In digital CCD cameras, the signal resulting from the application of a bias
voltage across the CCD chip, a condition required to store and read the pixels on the
chip. The bias signal must be subtracted from the total image signal in order to obtain
a photometrically accurate image signal. 273
Biaxial crystal. A class of birefringent crystals having two optic axes. Mica is an exam-
ple of this class of crystals. 126
Binning. In CCD microscopy, the command given in software to combine the signal
content of multiple adjacent pixels. Because the total number of photon counts