Dictionary F onwards
The process whereby glass is raised to its melting point and formed by molding with a
highly polished metal surface.
An optical glass with higher dispersion and higher refractive index than crown glass; a
heavy, brilliant glass, softer than crown glass.
See Equivalent Focal Length.
Interference bands, such as Newton's Rings, which are formed when light is reflected from
two adjacent polished surfaces, placed together with an air space between them. Used to determine the fit of a
lens surface to a test glass.
Front Focal Length:
The distance from the front focal point of an optical system to the first surface.
Crystal quartz melted at high temperature to make an amorphous, non-birefringent glass of
low refractive index.
Full Width, Half Maximum. The bandwidth of an optical instrument as measured at the
An instrument for detecting and measuring a small electric current by movements of a
magnetic needle or of a coil in a magnetic field.
Optical characteristics limited to infinitesimally small pencils of light; also called
paraxial or first-order optics.
That branch of optics dealing with the tracing of ray paths through optical systems.
Term commonly associated with scan and all other lenses. Typically, half the angular
subtense of the object. See Input
An achromatic doublet made of a bi-convex crown element cemented to a meniscus flint
element, with the crown facing the long conjugate.
Specialized coating applied to optics to improve transmission or reflection.
The state in which all volume components of a substance are identical in optical
properties and composition.
An ocular consisting of two plano-convex lenses which are formed from similar glass and
separated by a space equal to the sum of their focal lengths. This eyepiece is free of lateral chromatic aberration,
but because the image plane falls between the two elements it is not suitable for applications involving crosshairs.
Anything formed out of heterogenous elements.
The circular image field over which image quality is acceptable; can be defined in terms
of its angular subtense. Alternately known as circle of coverage .
Change in the orientation of an image in one meridian.
The plane perpendicular to the optical axis at the image point.
The flipping of an image's orientation, such as inversion of an image's orientation in one
meridian or the reversion of an image's orientation in two meridians.
Index of Refraction:
The ratio of the speed of light in air to its velocity in another medium; determines how
much light bends as it passes through a lens, e.g., high-index flint glass bends light more than low-index crown glass
The portion of the spectrum whose wavelengths are invisible to the human eye (range = .76
microns and higher).
A filter which controls the spectral composition of transmitted energy by interference.
Such filters, typically constructed of thin alternating layers of metals and dielectrics, are also known as narrowband or
broadband bandpass filters.
An instrument that uses the interference of light waves to measure the accuracy of optical
The distance between the pupils of the eyes when viewing objects at a distance; normal
distance is 62mm.
A mechanical device capable of varying the effective diameter of a lens.
International Standardization Organization of Geneva, Switzerland. Term often applied to
families of product and process standards developed by ISO sponsored technical committees.
A measure of hardness determined by the depth of penetration of a diamond stylus under a
specified load. Similar to a Rockwell hardness test.
A laser which uses a forward biased semiconductor junction as the active medium. Also
known as injection laser diode.
A chromatic aberration resulting in image size variation as a function of wavelength. Also
known as chromatic difference of magnification.
Not really a "ray" but the path of a point of light on a wavefront, indicating
the direction the light is traveling.
Limit of Resolution:
The limit to the performance of a lens imposed by the diffraction pattern resulting from
the finite aperture of the optical system.
Interference filter type which efficiently passes radiation whose wavelengths are longer
than a specific wavelength, but not shorter.
The longitudinal variation of focus (or image position) with wavelength; often referred to
as axial chromatic aberration.
Material used as antireflection coating for lenses because of its low refractive index.
The enlargement of an object by an optical instrument; ratio between the size of the image
and the actual size of the object.
Describes a lens having one convex and one concave surface.
A thin layer of metal applied to a substrate by evaporation to create a mirrored surface.
A term referring to small (less than 2mm in size) lenses, beamsplitters, prisms, cylinders
or other optical components commonly found in endoscopes or microsocopes. Micro-Optics are also used to focus light in
semiconductor laser and fibre optic applications.
An eyepiece located at the near end of the microscope tube. Often a simple Huygens
eyepiece, though other varieties(negative eyepieces, flat field projection eyepieces) are common, depending on
The lens located at the object end of a microscope tube. Many types of objectives are used
in microscopy; simple achromats and color-corrected apochromats are popular choices.
Modulation Transfer Function (MTF):
Describes the modulation of an image as the frequency increases; ratio of modulation
between image and object. Also called sine wave response.
An assembly of single and/or compound lenses optimized to provide certain optical
Coating composed of several layers of material with alternating high-low refractive
indices; various combinations produce a variety of coating properties.
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