Page 370 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY 353

Shot noise. See Photon noise.
Signal-to-noise (S/N) ratio. The ratio of the signal of an object to the noise of the sur-
rounding background, where noise is the square root of the sum of the variances of
contributing noise components. In the case that noise is photon limited and back-
ground noise may be approximated by the square root of the background signal, S/N
gives the number of standard deviations that distinguish the object signal from the
mean signal of the background. 217, 247, 276, 299
Sign of birefringence. In polarization optics, a reference to the sign of birefringence b,
where b n n . For the case that the refractive index describing the trajectory of
o
e
the E ray is greater than that describing the trajectory taken by the O ray, the sign of
birefringence of the material is positive. 127
Silicon-intensifier target (SIT) camera.A video camera tube for imaging under low-light
conditions. The camera tube contains a photocathode that accelerates photoelectrons
onto a silicon diode target plate, which greatly amplifies the signal. A scanning electron
beam neutralizes the target while generating a beam current containing the signal. 250
Simple lens. A lens consisting of a single lens element and distinct from a compound
lens having multiple lens elements. 45
SIT camera. See Silicon-intensifier target (SIT) camera.
Slow axis. In polarization optics, the short axis of the wavefront ellipsoid, a construc-
tion used to describe the surface of an emergent wavefront from a point source of
light in a birefringent material. The slow axis indicates the direction of high refrac-
tive index in the specimen. See also Refractive index ellipsoid. 128
Slow-scan CCD camera. Another designation for a full-frame CCD design. The time
required for the serial readout of the parallel register of the CCD is slow compared to
the time to read an interline CCD of comparable size and dimensions. 259
S/N ratio. See Signal-to-noise (S/N) ratio.
Spatial filter. A filter that selectively manipulates a location in an image such as an
aperture in a field plane of a microscope or a sharpening or blurring filter in image
processing. 208, 250
Spatial frequency. The reciprocal of the distance between two objects (periods/
distance). 245
Spatial frequency filter. A filter that selectively manipulates a location in the diffrac-
tion plane in a microscope (aperture plane masks in modulation contrast microscopy)
or a mask applied to Fourier transforms to manipulate low and high spatial frequency
information in image processing. 171, 296
Spatial resolution. The resolution of component features in an image. In optical sys-
tems, resolution is directly proportional to the wavelength and inversely proportional
to the angular aperture. The practical limits on wavelength and angular aperture
determine the limit of spatial resolution, which is approximately one-half the wave-
length of light. 87, 215, 245, 272
Spectral range. The range of wavelengths, or bandwidth, under consideration. 272
Spectroscope. A device for determining the wavelength of a certain emission line,
bandwidth, or color. A diffraction grating is positioned between the eye and a narrow
slit, and the eye-slit axis is directed at a target light source. The grating produces a
spectrum of the constituent wavelengths admitted by the slit, and the spectrum is
superimposed on a ruling so that you can determine the wavelength of a particular
spectral color by simple inspection. 25
Spherical aberration. A lens aberration typical of lenses with spherical surfaces that
causes paraxial rays incident on the center and periphery of a lens to be focused at