Page 366 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 349

Photobleaching. The diminution of fluorescence emission due to the chemical modifi-
cation of a fluorochrome upon continued exposure to excitation wavelengths.
Photon-induced damage resulting in photobleaching is largely due to the generation
of free oxygen radicals that attack and permanently destroy the light-emitting prop-
erties of the fluorochrome. The rate of photobleaching can be reduced by including
anti–free radical reagents such as ascorbate or by reducing the concentration of oxy-
gen in the mounting medium. Bleaching is usually irreversible. 183, 223
Photodiode. A semiconductor device such as a CCD for detecting and measuring light
by means of its conversion into an electric current. Incident photons generate charge
carriers (electrons and electron holes) in the CCD matrix that support the conduction
of a current in the presence of an applied voltage. 261
Photomultiplier tube (PMT). An electrical device for amplifying photon signals. Pho-
tons striking a target at the face of the PMT liberate free electrons, which are accel-
erated onto a dynode that in turn liberates a shower of electrons. By arranging several
dynodes in a series, a great amplification of the original photon is achieved, which is
then transmitted to other signal-processing circuits. PMTs amplify photon signals
but do not form an image as does an image intensifier. 209
Photon limited. In image processing, a signal is said to be photon limited when the pho-
ton noise is greater than the electronic noises associated with the camera (bias, ther-
mal, and read noises). 301
Photon noise or shot noise. In image processing, the noise associated with the photon
signal itself. Since the accumulation of photons per unit area or time is random (Pois-
sonian) by nature, the photon signal is typically described as the mean of a collection
of measurements, and the noise is described as the square root of the photon signal.
Since the signal is based on “photoelectron counts,” the photon noise is calculated in
terms of electrons, not in the digitized signal units (analogue-to-digital units or
ADUs) used by the computer. 273, 301
Photopic vision. The mode of vision based on cone cell photoreceptors in the retina that
provides visual acuity and color perception under bright light conditions. 24
Pinhole aperture. In confocal microscopy, the variable diaphragm in the real interme-
diate image plane that is adjusted to receive the focused spot of fluorescent light that
is emitted by an excited laser-scanned spot in a fluorescent specimen. A photomulti-
plier tube located behind the pinhole generates an electrical signal that is propor-
tional to the number of photons passing through the pinhole. 208
Pixel. A “picture element” in electronic cameras. In confocal microscopes, pixels corre-
spond to the photon signal received and digitized per unit of sampling time during
laser scanning. In CCD cameras, pixels are square or rectangular areas of silicon sub-
strate that are delimited by conducting strips on the CCD surface. 236
Plane parallel. In wave optics, the term applies to waves vibrating in a plane that is par-
allel to some reference plane, but not necessarily in the reference plane itself. See
also Linearly polarized light. 117
PMT. See Photomultiplier tube (PMT).
Polar. The common term applied to a sheet of linear polarizing film (dichroic filter or
Polaroid filter) and particularly to its use as a polarizer or analyzer in producing and
analyzing polarized light. 119
Polarizability. In polarization optics, a property describing the strength of interaction of
light with molecules in a manner that depends on the orientation of atomic bonds.
Light waves interact more strongly with molecules when their E vectors are oriented
parallel to the axis defining light-deformable (polarizable) covalent bonds such as the