Collector lens. A focusable lens of the
illuminator capable of collecting light over a wide area and directing it
toward the specimen. In Koehler illumination, the collector lens is used to
focus a magnified real image of the filament or arc of the bulb in the front
aperture of the condenser.
Collimated beam. A beam in which rays
proceed in the same direction and follow trajectories that are parallel to one
another. Collimated light need not be monochromatic, polarized, or coherent.
Colored-glass filter. A slab of glass
containing colloids or metals that absorb certain wavelengths while freely
transmitting others. In microscopy, colored-glass filters are commonly employed
in fluorescence filter sets and as effective blockers of UV and IR light.
Coma. An off-axis aberration of lenses,
whereby rays from an off-axis point passing through the edge of the lens are
focused closer to the optic axis than are rays that pass through the center of
the lens, causing a point object to look like a comet with the tail extending
toward the periphery of the field. Coma is the most prominent off-axis
aberration. For lenses having the same focal length, coma is greater for lenses
with wider apertures.
Compound light microscope. An optical
instrument that forms a magnified image of an object through a two-step series
of magnifications: The objective forms a magnified real image of the object,
and the eyepiece forms a magnified virtual image of the real image made by the
objective. This basic design forms the basis of all modern light microscopes.
Condenser annulus. In phase contrast
and dark-field microscopy, a transparent annulus in an opaque black disk
located in the front aperture of the condenser that serves as the source for
illuminating the specimen.
Condenser lens. A lens assembly located
near the specimen and specimen stage that collects light from the illuminator
and focuses it on the specimen. Proper optical performance requires that the
condenser be highly corrected to minimize chromatic and spherical aberration.

Confocal laser scanning microscope (CLSM).
A mode of light microscopy whereby a focused laser beam scans the specimen in a
raster and the emitted fluorescent light or reflected light signal, sensed by a
photomultiplier tube, is displayed in pixels on a computer monitor. The
dimensions of the pixel display depend on the sampling rate of the electronics
and the dimensions of the raster. A variable pinhole aperture, located in a
plane confocal with the specimen, rejects out-of-focus signals and allows for
optical sectioning.

Conjugate focal planes. In light
microscopy, two sets of field and aperture planes whose precise geometrical
positioning in the microscope is assured by adjusting the focus of the
objective, condenser, and lamp collector lenses as required for Koehler
illumination. The two sets of focal planes are conjugate with each other but
not with the focal planes belonging to the other set; as a consequence, looking
from one focal plane along the optic axis simultaneously reveals the images of
the other conjugate focal planes.

Constructive interference. In wave
optics and image formation, the condition where the summation of the E vectors
of the constituent waves results in an amplitude greater than that of the
constituents. For interference to occur, a component of one wave must vibrate
in the plane of the other.

Contrast. Optical contrast is the
perceived difference in the brightness (intensity or irradiance) between an
object and its surround, and is usually given as the ratio of the light
intensity of an object Io to the light intensity of the object’s background Ib.

Contrast threshold. The minimal
contrast required for visual detection. The contrast threshold is strongly
dependent on the angular size, shape, and brightness of the specimen, the
brightness of the viewing environment, the region of the retina used for
detection, and other factors. For extended objects, the contrast threshold is
usually given as 2–3% in bright light and 30–100% or even greater in dim light. 


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