Page 357 - Fundamentals of Light Microscopy and Electronic Imaging

340 GLOSSARY

Extraordinary ray or E ray. In polarization optics, the member of a ray pair whose
velocity varies with the direction of transmission through a birefringent medium. The
surface wavefront of E waves emanating from an imaginary point source in a bire-
fringent medium can be described as the surface of a three-dimensional ellipsoid. See
also Ordinary ray. 124
Eyepiece or ocular. The second magnifying lens of the microscope used to focus a real
magnified image on the retina of the real intermediate image produced by the objec-
tive. The added magnification provided by the eyepiece increases the angular magni-
fication of the virtual image perceived by the eye. The typical range of eyepiece
magnifications is 5–25 . 56
Eyepiece telescope. See Bertrand lens.
Fast axis. In polarization optics, the long axis of the wavefront ellipsoid, a construction
used to describe the surface of an emergent wavefront from a point source of light in
a birefringent material. The fast axis indicates the direction of low refractive index in
the specimen. See also Refractive index ellipsoid. 128
Fast Fourier transform (FFT). A filtering operation used to selectively diminish or
enhance low or high spatial frequencies (extended vs. fine detailed structures) in the
object image. In generating a transform, image details are separated into sinusoidal
frequency components to create a map of spatial frequencies. The transform can be
covered with a mask to enhance or diminish spatial frequencies of interest. Revers-
ing the procedure produces an image (the inverse transform) in which contrast of spa-
tial details is modified. 296
FFT. See Fast Fourier transform (FFT).
Field diaphragm. A variable diaphragm located in or near the aperture plane of the
light source that is used to reduce the amount of stray light in the object image. Since
the edge of the diaphragm is conjugate with the object plane under conditions of
Koehler illumination, the field diaphragm is used as an aid in centering and focusing
the condenser lens. 10
Field planes. That set of conjugate focal planes representing the field diaphragm, the
object, the real intermediate image, and the retina. 4
Flat-field correction. In image processing, the procedure used to obtain a photometri-
cally accurate image from a raw image. A so-called dark frame containing bias and
thermal counts is subtracted from the raw image and from a “flat” or “background”
image. The dark-subtracted raw image is then divided by the dark-subtracted flat-
field image to produce the corrected image. With operation, all optical faults are
removed. The photometric relation of pixel values to photoelectron count is also lost
during division, although the relative amplitudes of pixel values within an image are
retained. See also Dark frame and Flat-field frame. 289
Flat-field frame. In image processing, a picture of featureless background that is used to
prepare a flat-field-corrected image of the object. A flat-field frame is obtained by pho-
tographing a featureless region, close to and in the same focal plane as the object. 289
Fluorescence. The process by which a suitable molecule, transiently excited by absorp-
tion of external radiation (including light) of the proper energy, releases the energy as a
longer-wavelength photon. This process usually takes less than a nanosecond. 179, 181
Fluorescence filter set. On a fluorescence microscope, an assembly of filters used to
transmit excitation and emission wavelengths in an epi-illuminator. A filter set typically
includes an exciter filter, a dichroic mirror, and an emission (or barrier) filter. 190
Fluorescence microscopy. A mode of light microscopy whereby the wavelengths of
fluo-rescence emission from an excited fluorescent specimen are used to form an
image. 177