Page 348 - Fundamentals of Light Microscopy and Electronic Imaging

GLOSSARY

Aberrations of a lens. Faults in lens design that cause optical performance to deviate
from that of an ideal lens. Usually attributed to materials composing the lens and the
spherical curvatures of lens surfaces. Lens aberrations include chromatic and spher-
ical aberration, astigmatism, coma, distortion, and field curvature. 50
Absorbance or optical density. The amount of absorption of light by a substance as
measured in a spectrophotometer and given as the log of the reciprocal of the trans-
mittance, where transmittance is the ratio of the transmitted light intensity to incident
light intensity. 38
Achromat. A lens corrected for chromatic aberration at two wavelengths (red and blue)
and for spherical aberration (green). 53
Acousto-optical tunable filter (AOTF). A device, based on sound waves, to control the
wavelength or intensity of light delivered from a laser or other light source to a spec-
imen. Sound waves induced in a glass block set up a standing wave with alternating
domains of high and low refractive index, allowing the block to act as a diffraction
grating and to deflect an incident beam of light. The period of the grating is changed
by altering the frequency of sound waves delivered to the block. 221
ADC. See Analogue-to-digital converter (ADC).
ADU. See Analogue-to-digital unit (ADU).
AGCC. See Auto-gain control circuit (AGCC).
Airy disk. The central diffraction spot in the focused image of a point source of light.
Diffraction at the front aperture of the lens disturbs the incident wavefront, causing
the diffraction pattern. The Airy disk diameter is determined by the wavelength of
light and the angular diameter of the lens as seen from the image plane. 65
Alexa dyes. Registered trademark for a series of fluorescein and rhodamine derivatives by
Molecular Probes, Inc., noted for their stability and resistance to photobleaching. 185
Aliasing. In microscopy, the false pattern of spacings that is observed when periodic
detail in an object is recorded by an electronic imager, which itself has periodic sam-
pling units (pixels, raster lines), and when the number of samples per specimen
period is less than 2. Aliasing is avoided by adjusting the magnification high enough
so that two or more imaging elements cover a unit period in the object. See also
Nyquist criterion. 248

Numbers after each glossary term indicate corresponding page numbers. 331