Page 359 - Fundamentals of Light Microscopy and Electronic Imaging

342 GLOSSARY

Gain. In electronic imaging, the amplification factor applied to an input signal to adjust
the amplitude of the signal output. In video, an input voltage signal might be multi-
plied by a positive gain factor to increase the voltage of the signal output and hence
the brightness of the signal on the TV. In digital CCD microscopy the number of pho-
toelectrons is divided by a gain factor (photoelectrons/ADU) to determine ADUs
assigned to an image pixel. 221, 238, 270
Gamma ( ). The exponent in the mathematical expression relating an output signal to
its corresponding input signal for a camera, monitor, or other display device. In
image processing, changing gamma increases or decreases the contrast between high
and low pixel values according to the function, displayed value [(normalized value

min)/(max min)] . For a video camera, the display signal and input signal are

related through the relationship i/i (I/I ) , where i and I are the output current and
D
D
corresponding light intensity for a given signal; i and I are the dark current and its
D
D
corresponding intensity. In both cases, is the slope of the curve in a log-log plot of
this function. For 1, the relationship between signal strength and display inten-
sity is linear. 240, 287
GFP. See Green fluorescent protein (GFP).
Green fluorescent protein (GFP). A fluorescent 30 kDa protein from the jellyfish
Aequorea victoria, which is commonly used as a fluorescent marker to determine the
location, concentration, and dynamics of a protein of interest in cells and tissues. The
excitation and emission maxima of enhanced GFP occur at 489 and 508 nm, respec-
tively. The DNA sequence of GFP is ligated to the DNA encoding a protein of inter-
est. Cells transfected with the modified DNA subsequently express fluorescent
chimeric proteins in situ. 185
Half bandwidth (HBW). See Full width at half maximum (FWHM).
Halo. In phase contrast microscopy, characteristic contrast patterns of light or dark gra-
dients flanking the edges of objects in a phase contrast image. Halos are caused by
the phase contrast optical design that requires that the image of the condenser annu-
lus and objective phase plate annulus have slightly different dimensions in the back
focal plane of the objective. 108
High-pass filter. In image processing, a filter that transmits high spatial frequencies and
blocks low spatial frequencies. After filtering, edges and fine details are strongly
enhanced, whereas large extended features are made less distinct. 293
Histogram equalization or histogram leveling. In image processing, an operation that
reassigns pixel values so that each step of the gray scale is represented by an approx-
imately equal number of pixels. In equalization, the cumulative histogram distribu-
tion is scaled to map the entire range of the gray scale, the rank order of the original
pixel values being maintained as closely as possible. This operation is used to
increase the contrast of flat images in which the pixel values differ by only a small
amount compared to the potential gray scale range of the image. 295
Histogram stretching. In image processing, the operation whereby a limited range of
gray-level values is displayed on a scale having a greater range of amplitudes, fre-
quently from black to white. The effect of this operation is to increase the contrast of
the selected range of gray levels. 286
HMC. Hoffman modulation contrast. See Modulation contrast microscopy (MCM).
Huygens’ principle. A geometrical method used to show the successive locations occu-
pied by an advancing wavefront. An initial source or wavefront is treated as a point
source or a collection of point sources of light, each of which emits a spherical wave
known as a Huygens’wavelet. The surface of an imaginary envelope encompassing an