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 spherical aberration,
astigmatism, coma, distortion, and field curvature.
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 transmittance, where transmittance is the ratio of the
transmitted light intensity to incident light intensity.
Achromat: A lens corrected for
chromatic aberration at two wavelengths (red and blue) and for spherical
aberration (green).
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.
Amplitude object. Objects that absorb
light as opposed to those that shift the phase of light (phase objects) as the
basis for image formation.
Amplitude of an electromagnetic wave.
The magnitude of the electric field vector of an electromagnetic wave.
Amplitude is distinguished from intensity (irradiance), a measure of the amount
of light energy or photon flux, which in visual perception is proportional to
amplitude squared.
Anisotropic. In describing the optical
properties of an object or propagation medium, having dissimilar properties in
different directions.
Annulus. In phase contrast microscopy,
the transparent ring at the front aperture of the condenser that provides
illumination of the specimen.
Aperture angle. The angle subtended by
the edges of a lens as seen from a point in the specimen plane or in the image plane.
Aperture angle is included in the expression for numerical aperture (NA/ n sin), where n is the
refractive index and is onehalf of the
full aperture angle. See also Numerical aperture (NA).
Aperture plane. In a microscope
adjusted for Koehler illumination, the set of conjugate focal planes located at
the light source, the front aperture of the condenser, the back aperture of the
objective lens, and the iris of the eye. Adjustable aperture diaphragms at
these locations are used to limit stray light and determine the numerical
aperture, and hence the spatial resolution, of the instrument.
Apochromat A lens especially designed
to correct for chromatic aberration at three or four wavelengths (red, green,
blue, UV) and at two or more wavelengths (green, red) for spherical aberration.
The high degree of color correction makes these lenses suitable for
fluorescence microscopy and stained histological specimens in bright-field
microscopy.
Astigmatism. An off-axis aberration of
lenses whereby rays from an off-axis object passing through the horizontal and
vertical diameters of a lens are focused as a short streak at two different
focal planes. The streaks appear as ellipses drawn out in horizontal and vertical
directions at either side of best focus, where the point image is a small disk.
Off-axis astigmatism increases with increasing displacement of the object from
the optic axis. Astigmatism is also caused by asymmetric lens curvature due to
mistakes in manufacture or improper mounting of a lens in its barrel.
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