Extended Dictionary continued..
Cylindrical Lens:
A lens with at least one surface shaped like a portion of a cylinder. Positive and
negative cylindrical lenses (depending
on the shape of the curved surface) are used in anamorphic systems to accentuate features
in one axis, such as
expanding points of light into lines. Applications include astigmatism correction in the
human eye and astigmatism
production in rangefinders or fire-control equipment.
Damage Threshold:
The maximum energy density to which an optical surface may be subjected without failure.
Decentration:
The failure of one or more lens surfaces to align their centers of curvature with the
geometric axis of a lens system.
Density:
The logarithm of the degree of opacity of a translucent medium; expressed as D = log
(1/Trans).
Depth of Field:
The distance along the optical axis through which an object can be located and clearly
defined when the lens is in focus.
Depth of Focus:
The distance along the optical axis through which an image can be clearly focused.
Deviation:
The angle between the paths of a ray of light before and after passing through one or more
lenses.
Dielectric Coatings:
High-reflectance or low-reflectance coatings made up of alternating layers of material
with higher and lower indices of
refraction than the substrate.
Diffraction:
The sidewise spread of light as it passes the edge of an object or emerges from a small
aperture; causes halos or
blurring of the image.
Diffraction Limited:
Describes an optical system in which the quality of the image is determined only by the
effects of diffraction and not by
lens aberrations.
DIN:
Abbreviation for Deutsche Industrie Norm and describes a large family of product
standards. For film emulsions, the
DIN System is the logarithmic method of determining emulsion speeds developed by the
German standards
organization Deutscher Normenausschuss. The emulsion speed is doubled for each increase of
three in the DIN speed
value.
Dispersion:
The separation of light into its component colors, as a prism disperses white light into a
color band, or a rainbow effect.
Distortion:
Variations in magnification from the center to the edge of an image, making straight lines
seem to curve. Barrel, or
negative, distortion causes a square grid to appear barrel-shaped; pincushion, or
positive, distortion increases in
proportion to the distance from the center of the image.
Diverging:
The bending of light rays away from each other, achieved with a negative (concave) lens.
Edging:
Grinding, or finishing, the edge of an optical element or lens.
Effective Focal Length:
See Equivalent Focal Length.
Electroform:
To form shaped objects by electrodeposition on a mold.
Elliptical:
Aspherical, as in a lens whose surface is a section of an ellipse rather than of a circle.
Entrance Pupil:
The image of the aperture stop as viewed through the object side of the lens.
Equivalent Focal Length (EFL):
The focal length of an infinitely thin lens having the same paraxial imaging properties as
a thick lens or multiple-element
system.
Erect Image:
An image whose spatial orientation is the same as that of the object; both image and
object appear "right side up."
Extinction Ratio:
The ratio of the intensity of a plane-polarized beam that is transmitted through a
polarizer whose polarizing axis is
parallel to the beam's plane, versus the intensity when the polarizer's axis is
perpendicular to the beam's plane.
F-Number:
A measure of the ability of a lens to gather light. Also called its "speed". The
ratio of the focal length of the lens to its
effective aperture.
Fibre Optics:
The transmission of radiant energy through transparent fibers of glass, plastic or fused
silica.
Field Curvature:
An aberration in which the edges of a field seem to be out of focus when the center is
focused clearly.
Field of View:
The maximum visible space seen through an optical instrument or lens.
Filar Eyepiece:
A measuring eyepiece containing a screw-micrometer-driven crosshair commonly used to
measure image size.
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