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Dictionary F onwards

Fire Polished:
The process whereby glass is raised to its melting point and formed by molding with a highly polished metal surface.
Flint Glass:
An optical glass with higher dispersion and higher refractive index than crown glass; a heavy, brilliant glass, softer than crown glass.
Focal Length:
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.
Fused Quartz:
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 half-power points.
An instrument for detecting and measuring a small electric current by movements of a magnetic needle or of a coil in a magnetic field.
Gaussian Optics:
Optical characteristics limited to infinitesimally small pencils of light; also called paraxial or first-order optics.
Geometric Optics:
That branch of optics dealing with the tracing of ray paths through optical systems.
Half Angle:
Term commonly associated with scan and all other lenses. Typically, half the angular subtense of the object. See Input
Scan Angle.
Harting Doublet:
An achromatic doublet made of a bi-convex crown element cemented to a meniscus flint element, with the crown facing the long conjugate.
High-Efficiency Coating:
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.
Huygenian Eyepiece:
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.
Image Circle:
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 .
Image Inversion:
Change in the orientation of an image in one meridian.
Image Plane:
The plane perpendicular to the optical axis at the image point.
Image Transposition:
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 does.
The portion of the spectrum whose wavelengths are invisible to the human eye (range = .76 microns and higher).
Interference Filter:
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 surfaces.
Interocular Distance:
The distance between the pupils of the eyes when viewing objects at a distance; normal distance is 62mm.
Iris Diaphragm:
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.
Knoop Hardness:
A measure of hardness determined by the depth of penetration of a diamond stylus under a specified load. Similar to a Rockwell hardness test.
Laser Diode:
A laser which uses a forward biased semiconductor junction as the active medium. Also known as injection laser diode.
Lateral Color:
A chromatic aberration resulting in image size variation as a function of wavelength. Also known as chromatic difference of magnification.
Light Ray:
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.
Long Pass:
Interference filter type which efficiently passes radiation whose wavelengths are longer than a specific wavelength, but not shorter.
Longitudinal Color:
The longitudinal variation of focus (or image position) with wavelength; often referred to as axial chromatic aberration.
Magnesium Fluoride:
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.
Metallic Coating:
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.
Microscope Eyepiece:
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 application.
Microscope Objective:
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.
Multi-Element System:
An assembly of single and/or compound lenses optimized to provide certain optical characteristics.
Multi-Layer Coating:
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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