Page 371 - Fundamentals of Light Microscopy and Electronic Imaging

354 GLOSSARY

different locations in the image plane. The degree of aberration increases with the
decreasing focal ratio of the lens. The aberration can be corrected in simple lenses by
creating aspherical surfaces. 52
Stepper motor. A motor whose drive shaft does not rotate continuously, but advances in
discrete intervals or steps. 205
Stokes shift. The distance in nanometers between the peak excitation and peak emission
wavelengths of a fluorescent dye. 182
Subarray readout. An option for image acquisition with a CCD camera whereby a por-
tion of the total available imaging area of the CCD is selected as the active area for
acquiring an image. Selection of the subarray region is made in the image acquisition
software. In subarray readout mode, the acquisition rate is fast, and images take up
less storage space on the hard drive. 269
Super-resolution. In electronic imaging, the increase in spatial resolution made possi-
ble by adjusting the gain and offset of a camera. In confocal microscopy, super-
resolution is obtained by constricting the confocal pinhole to about one-quarter of the
Airy disk diameter. 216
Surround wave or background wave. In phase contrast and other modes of interfer-
ence microscopy, waves that traverse an object but do not interact with it. Surround
waves are not deviated by the object and do not become altered in phase. For pur-
poses of describing diffraction and interference, such waves are called the 0th-order
component. Surround (S) waves combine with diffracted (D) waves through interfer-
ence in the image plane to generate resultant particle (P) waves of altered amplitude
that are perceived by the eye. See also Diffracted wave and Particle wave. 99
System MTF. A function describing the percent modulation (percent reduction in the
peak to trough amplitude difference for a signal) of a signal resulting from transit
through a series of signal-handling devices. For a cascaded series of devices, the sys-
tem MTF for a given frequency f is the product of the values of percent modulation
for each individual component in the system so that % modulation of the system
a% b% c% . . . 252
Thermal noise. In CCD imaging, the noise of the thermal signal in an image caused by
the kinetic vibration of silicon atoms in the matrix of a CCD device. Thermal noise
is considerably reduced by cooling the CCD to 20°C. Low-light-level cameras
used in astronomy are sometimes cooled to the temperature of liquid nitrogen to
effectively eliminate thermal noise. 264
Thin lens. A lens whose thickness is small compared to its focal length. A line through
the center of the lens (a plane representing the two coincident principal planes of the
lens) provides a reasonably accurate reference plane for refraction and object and
lens distance measurements. Lenses are assumed to be thin when demonstrating the
principles of graphical ray tracing. 45
Tube lens or Telan lens. An auxiliary lens in the body of the microscope, which in con-
junction with an infinity focus objective lens forms the real intermediate image. The
Telan lens provides some of the correction for chromatic aberration, which lessens
constraints on the manufacture of the objective lens. 50
Two-photon and multi-photon laser scanning microscopy. In laser scanning confocal
microscopy, a method of fluorochrome excitation based on an infrared laser beam
whose energy density is adjusted to allow frequency doubling or tripling at the point
of beam focus in the specimen. Thus, molecules that simultaneously absorb two or
three photons of 900 nm fluoresce the same as if excited by a single higher-energy
photon of 450 or 300 nm, respectively. The method allows deep penetration into