Thursday, 26 December 2013

Definitions in Microscopy - for MDS Oral Pathology students

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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