Page 356 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 339

of printed words, the effects of double refraction are clearly observed as an overlap-
ping, double image of the text. 124, 126
DR. See Dynamic range.
Dwell time. In confocal microscopy, the length of time that the scanning laser beam
remains at a unit of space corresponding to a single pixel in the image. In a laser
scanning microscope, dwell time is typically 0.1–1.0 s or more. Long dwell times
increase the rate of photobleaching and decrease the viability of living cells. 218
Dynamic range (DR). The term describing the resolution of light intensity, the number
of steps in light amplitude in an image that can be resolved by an imaging device.
This number can range from several hundred to a thousand for video and low-end
CCD cameras to greater than 65,000 for the 16 bit CCD cameras used in astronomy.
For CCD cameras, the DR is defined as the full well capacity of a pixel (the number
of photoelectrons at saturation) divided by the camera’s read noise. 216, 246, 274
Elliptically polarized light. A form of polarized light whereby the E vector of a polar-
ized wave rotates about the axis of propagation of the wave, thus sweeping out a
right- or left-handed spiral. If you could view the wave end-on, the movement of the
E vector would appear to trace the form of an ellipse. 131
Emission filter. In fluorescence microscopy, the final element in a fluorescence filter
cube, which transmits fluorescence emission wavelengths while blocking residual
excitation wavelengths. Commonly called a barrier filter. Emission filters are colored-
glass or interference filters and have the transmission properties of a bandpass or
long-pass filter. 191
Emission spectrum. In fluorescence, the spectrum of wavelengths emitted by an atom
or molecule after excitation by a light or other radiation source. Typically, the emis-
sion spectrum of a dye covers a spectrum of wavelengths longer than the correspon-
ding excitation spectrum. 181
Epi-illumination.A common method of illumination in fluorescence microscopy, where
the illuminator is placed on the same side of the specimen as the objective lens, and
the objective performs a dual role as both a condenser and an objective. A dichroic
mirror is placed in the light path to reflect excitatory light from the lamp toward the
specimen and transmit emitted fluorescent wavelengths to the eye or camera. 190
Equalize contrast function. See Histogram equalization.
Excitation filter. In fluorescence microscopy, the first element in a fluorescence filter
cube and the filter that produces the exciting band of wavelengths from a broadband
light source such as a mercury or xenon arc lamp. Commonly the excitation filter is
a high-quality bandpass interference filter. 191
Excitation spectrum. In fluorescence, the spectrum of wavelengths capable of exciting
an atom or a molecule to exhibit fluorescence. Typically the excitation spectrum cov-
ers a range of wavelengths shorter than the corresponding fluorescence emission
spectrum. 181
Extinction and extinction factor. In polarization optics, the blockage of light transmis-
sion through an analyzer. This condition occurs when the vibrational plane of the E
vector of a linearly polarized beam is oriented perpendicularly with respect to the
transmission axis of the analyzer. If two Polaroid filters are held so that their trans-
mission axes are crossed, extinction is said to occur when the magnitude of light
transmission drops to a sharp minimum. The extinction factor is the ratio of ampli-
tudes of transmitted light obtained when (a) the E vector of a linearly polarized beam
and the transmission axis of the analyzer are parallel (maximum transmission) and
(b) they are crossed (extinction). 120, 123, 159