| Glossary of Terms
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Adhesion - The intermolecular attraction between two surfaces, as between a substrate and a coating; it is an important factor in the durability of optical thin films. Adsorption - The process by which a substance, usually a solid, attracts and retains on its surface the molecules of another substance. Aluminizing - The process of applying a film of aluminum to a surface, usually by evaporation in a vacuum. Anamorphic distortion - A type of distortion in which the magnification varies in different orientations, the directions of maximum and minimum magnification being orthogonal. Angle of deviation - The angle through which a ray of light is deviated by a refracting or reflecting surface, or a prism; the angle between an incident ray and the refracted or reflected ray. Angle of incidence - The angle formed between a ray of light striking a surface and the normal to that surface at the point of incidence. Angle of reflection - The angle formed between the normal to a surface and the reflected ray. This angle lies in a common plane with the angle of incidence and is equal to it. Angstrom - Unit of length equal to 10-10m. Not an SI unit. (Å) Annealing - The process of heating and slowly cooling a solid material, like glass or metal, to stabilize its thermal, electrical or optical properties or, as in semiconductor materials, to reverse lattice damage resulting from ion implantation of dopants. Annealing furnace - An oven or furnace that possesses the design requirements and heat control necessary to anneal glass for the optical industry. Anti-reflection coating - A thin layer of material applied to a lens surface to reduce the amount of reflected energy. Ideally the index of refraction of that material should be equal to the square root of the product of the indices of the material on either side of the coating, while the ideal thickness for a single-layer coating is one-quarter of the wavelength at which reflectance is to be minimized (see also multilayer coating). Astigmatism - A lens aberration that results in the tangential and sagittal image planes being separated axially. Astronomical mirror - Generally, a mirror used instead of lenses in astronomical and space applications. It is not limited in size, does not absorb UV or IR radiation, and is usually made aspheric to reduce aberrations. However, a spherical mirror can be used if it is combined with other optics to correct spherical aberration. Autocollimator - A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light. It consists of a lens with an illuminated reticle. The reticle usually carries several parallel lines subtending known angles at the lens. An electronic autocollimator provides a continuous output voltage, the sign and amplitude of which are related to the angular displacement of the mirror. Back-coated mirror - A glass substrate that has its rear surface coated with a reflective coating. Also known as a back-surface mirror. Back focal length - The distance from the last surface of a lens to its image plane. Back-surface mirror - See back-coated mirror. Bakeout - The elimination of gases from the surfaces of a vacuum system by heating the surfaces when the pumping phase is occurring. Bandwidth - The range of frequencies over which a particular instrument is designed to function within specified limits. See also fiber bandwidth. Beamsplitter - An optical device for dividing a beam into two or more separate beams. A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces. The cemented face of one prism is coated, before cementing, with a metallic or dielectric layer having the desired reflecting properties, both in the percentage of reflection and the desired color. Bevel - A chamfer ground on the edge of a lens or prism. Bevels are used to prevent chipping or to achieve a mechanical fit. Bevel face width - The actual width of a bevel rather than its width projected along the lens aperture. Biconcave lens - A lens having each of its outer faces curved inward. Biconvex lens - A lens having each of its outer faces curved outward. Birefringence - The separation of a light beam, as it penetrates a doubly refracting object, into two diverging beams, commonly known as ordinary and extraordinary beams. Blank - A piece of glass, quartz or other transparent material formed roughly by molding or cutting into the approximate shape and size of the finished part. Blanking - The process by which the raster beam in a video tube is cut off during the retracing and sync periods. Blocking pitch - An adhesive used to affix optical elements to an approximately shaped body - usually of cast iron. Blocking tool - An instrument used to support optical parts to be cemented, or to be mounted in plaster. Borosilicated glass - A strong, heat-resistant glass that contains a minimum of 5 percent boric oxide. Brewster's angle - For light incident on a plane boundary between two regions having different refractive indices, the angle of incidence at which the reflectance is zero for light that has its electrical field vector in the plane defined by the direction of propagation and the normal to the surface. For propagation from medium 1 to medium 2, Brewster's angle is given as arctan. Broadband - Indicating a capability to deal with a relatively wide spectral bandwidth. Burnishing - The process of lapping a thin edge of metal over the beveled edge of a lens to maintain it within its cell. Calcium fluoride - An optical material used in place of crown glass to produce lenses with extraordinary correction of chromatic aberrations. Its high coefficient of thermal expansion and its tendency to absorb moisture limit its range of application. Candlepower - The luminous intensity of a source of light expressed in candelas. Center thickness - The lens thickness measured at the optical axis. Chip - 1. A localized fracture at the end of a cleaved optical fiber or on a glass surface. 2. An integrated circuit. Clear aperture - Also known as free aperture or objective aperture. The opening in the mount of an optical system or its components that restricts the extent of the bundle of rays incident on the given surface. It is usually circular and specified by its diameter. Coated optics - Optical elements having refracting or reflecting surfaces that have been coated with one or more layers of dielectric or metallic material. These layers serve to reduce or increase reflection and to protect the surfaces from fumes or abrasion. Magnesium fluoride, silicon oxide and zinc sulphide are a few of the most commonly used coatings. Coefficient of thermal expansion - A numerical representation of the rate at which a material will exhibit dimensional changes as a direct result of changes in temperature. Cold coating - A method of applying antireflection coatings to optics that avoids the elevated temperatures normally used. A cold coating will not be as durable as a normal coating. Cold mirror - A mirror whose coating serves to reflect visible radiation while transmitting the infrared. Component - 1. A constituent part. It may consist of two or more parts cemented together, or with near and approximately matching surfaces. 2. The projection of a vector on a certain coordinate axis or along a particular direction. 3. In a lens system, one or more elements treated as a unit. 4. An optical element within a system. Concave - Describing a hollow curved surface; curved inward. Concave lens - See diverging lens. Concavo-convex lens - A lens with one concave surface and one convex surface; synonymous with meniscus. Concentric - Characterized by having the same center. Concentric circles differ in radius but have a common center point. Concentricity error - The distance between the center of the two concentric circles of an optical fiber that designate the diameter of the cladding and the center of the two concentric circles that designate the diameter of the core. Converging lens - Also known as convergent lens or convex lens. A lens that converges an incident bundle of rays to a focus. Convex - Denoting a spherically shaped surface; curved outward. Convex lens - See converging lens. Corner-cube prism - See corner reflector. Corner reflector - Also known as a corner-cube prism. A prism having three mutually perpendicular surfaces and a hypotenuse face. Light entering through the hypotenuse is reflected by each of the three surfaces in turn and will emerge through the hypotenuse face parallel to the entering beam. The prism thus returns entering beams to the source. Also may be constructed from three first-surface mirrors. Crown glass - One of the two principal types of optical gall, the other being flint glass. Crown glass is harder than flint glass, and has lower index of refraction and lower dispersion. Both types are used in the production of compound lenses. A glass with an Abbe v-value larger than 50 or 55, depending on its index. Curvature - The measure of departure from a flat surface, as applied to lenses; the reciprocal of radius. Applies to any surface, including lenses, mirrors and image surfaces. Curvilinear distortion - A lens aberration in which the focal length varies radially outward from the center of the field. It has the effect of making a straight tangential line in the object appear curved in the image, either convex outward (barrel distortion) or concave outward (pin-cushion distortion). Straight radial lines remain straight in the image. Deblocking - The removal of optical elements from a block. Degreaser - A tank with a boiling solvent at the bottom and a ring of cold piping higher up that condenses the liquid and returns it to the bottom. Objects such as lenses or small metal parts to be cleaned are held in racks below the cold region so that the condensing liquid passes over the parts and removes the grease. Density - See optical density. Diamond cutting tool - A tool made by imbedding small particles of diamond in the working edge. In the optical field, the most commonly used diamond tools are glass saws, cup-shaped tools for curve generators, and diamond mills for forming prisms and flat glass plates. Diamond milling is much quicker than the older usage of iron tools with loose emery or carborundum as an abrasive; moreover, it leaves the glass so smooth as to be almost ready for polishing. Diamond tools do not make deep fissures as loose abrasives do. Dichroic mirror - A mirror used to selectively reflect light according to its wavelength and not its plane of vibration. Dielectric - Exhibiting the characteristic of materials that are electrical insulators or in which an electric field can be sustained with a minimum dispersion of power. They exhibit nonlinear properties, such as anisotropy of conductivity or polarization, or saturation phenomena. Dielectric coating - A high-reflectance coating consisting of alternating layers of quarter-wave film of a higher refractive index and lower refractive index than the substrate. Such coatings can be made very specific to a reflected wavelength or, by varying the layers' thicknesses or film indexes, spread over a wide wavelength interval. Diffuser - A device used to scatter or disperse light emitted from a source, usually by the process of diffuse transmission. Diffusion pump - A vacuum pump in which heated oil or another substance is forced through jets as a vapor that collides with gas molecules and carries them away from the chamber being evacuated. Dig - A vacuum pump in which heated oil or another substance is forced through jets as a vapor that collides with gas molecules and carries them away from the chamber being evacuated. Diopter (D) - A unit of optical measurement that expresses the refractive power of a lens or prism. In a lens or lens system, it is the reciprocal of the focal length in meters. For example, if a lens has a focal length of 25 cm, the lens would have a power of 4 D. The lens with the shorter focal length has the greater power in diopters. Distortion - A general term referring to the situation in which an image is not a true-to-scale reproduction of an object. The term also is used to connote the temporal alteration of the signal's waveform shape. There are many types of distortion. See also anamorphic distortion; curvilinear distortion; keystone distortion; panoramic distortion; perspective distortion; radial distortion; stereoscopic distortion; tangential distortion; wide-angle distortion. Diverging lens - Also known as divergent lens, negative lens, concave lens or dispersive lens. A lens that causes parallel light rays to spread out. The lens surfaces may be planoconcave, double concave, or concavo-convex. The edge of a diverging lens is always thicker than the center. Double-concave lens - A diverging lens with both surfaces concave. Double-convex lens - A converging lens with both surfaces convex. Dove prism - A form of prism invented by H.W. Dove. It resembles half of a common right-angle prism in which a ray entering parallel to the hypotenuse face is reflected internally at that face and emerges parallel to its incident direction. One of the incident rays emerges along a continuation of its incident direction, and if the prism is rotated about that ray through some angle, the image rotates through twice that angle. A Dove prism must be used in parallel light. Edge - The flat or angled surface, usually fine-ground, that limits the aperture of a lens or prism surface. Edging - The finishing of the edge of an optical element by grinding. Effective aperture - 1. That portion of the aperture that functions to collect energy and deliver it to the final system detector. 2. For an obscured or noncircular aperture, the equivalent nonobscured, circular aperture. Electron beam (EB) - A stream of electrons emitted by a single source that move in the same direction and at the same speed. Electron-beam evaporation - A method of thin-film deposition in which electrons boiled off a heated cathode are used to melt the coating material. If a multipocket crucible is used, many materials can be processed at the same time. Electron-beam gun - 1. A device generally used in a cathode-ray or camera tube to emit a stream of electrons moving at uniform velocity in a straight line. It consists of an emitting cathode and an anode, with an aperture for passage of some of the electrons. 2. A device used in electron-beam evaporation, comprising a heated cathode to generate the electrons, a high-voltage system to accelerate the electrons, a water-cooled crucible containing the material to be evaporated, and electromagnets to deflect the electrons into the crucible. Epoxy - Common name for a variety of adhesives used for lens bonding, fiber optic splicing and other photonics applications. The term is actually a prefix denoting the presence of an epoxide group in a molecule. Fizeau interferometer - A type of interferometer noted for producing narrow multiple-beam interference fringes. As a result, when compared with the Twyman-Green, the Fizeau interferometer has fewer optical components, does not have the large beamsplitter and can be adjusted to a greater accuracy. Flat - See optical flat. Flat blank - A piece of glass having a crude Fluorescence - The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation. Fluoride glass - Optical glass containing zirconium fluoride that results in special characteristics such as improved transmission. Focal length (f) - The effective focal length (EFL) is the distance from the principal point to the focal point. The back focal length (BFL) is the distance from the vertex of the last lens to the second focal point. The front focal length (FFL) is the distance from the first lens surface to the first focal point. Free aperture - See clear aperture. Fringe - An interference band such as Front-surface mirror - An optical reflector with the reflective coating applied to the front surface of the substrate. This eliminates the ghost image formed with second-surface mirrors. Fused quartz - Fused silica - Glass consisting of almost pure silicon dioxide (SiO2). Also called vitreous silica. Frequently used in optical fibers and windows. Generating - A rapid roughing process for the quick removal of glass, the first step in manufacture of a curved lens surface. It is accomplished with coarse emery or a diamond-impregnated tool. Glass - A noncrystalline, inorganic mixture of various metallic oxides fused by heating with glassifiers such as silica, or boric or phosphoric oxides. Common window or bottle glass is a mixture of soda, lime and sand, melted and cast, rolled or blown to shape. Most glasses are transparent in the visible spectrum and up to about 2.5 µm in the infrared, but some are opaque such as natural obsidian; these are, nevertheless, useful as mirror blanks. Traces of some elements such as cobalt, copper and gold are capable of producing a strong coloration in glass. Laser glass contains a small amount of didymium oxide. Opal glass is opaque and white, with the property of diffusing light. Some opal glass has a thin layer of opal material flashed onto the surface of ordinary glass. Tempered glass has a high degree of internal strain, caused by rapid cooling, which gives it increased mechanical strength. Glass annealing furnace - A furnace, generally electrically heated, with a control system capable of following a cam by which the temperature can be made to go through a definite cycle over a period of days, or even weeks, if the glass is massive. Glass has a softening temperature above which it can be easily bent or molded to shape, and an annealing temperature below which the properties of the glass remain fixed down to room temperature. Between these two points is the annealing range in which it is essential to cool the glass very slowly to remove all strain and to drive the refractive index up to its stable maximum value. Gram (g) - Unit of mass in the SI system. Grinding - The process in the manufacture of an optical system that gives it the required geometric shape. Grinding and polishing machinery - Machinery used to grind and finish a component, such as a lens or prism, to a desired precision. Usually such machines carry a cup-shaped or flat tool (lap) into close contact with the part. An abrasive or polishing material is placed between the tool and the part, and the lap is moved in a nonrepetitive series of movements. Driving mechanisms for aspheric shaping are complicated; for spherical and flat parts, they are relatively simple. Grinding tool - A tool of cast iron or another suitable medium used with a slurry of silicon carbide, aluminum oxide or emery for grinding optical surfaces. Ground glass - A plate of glass in which a face has been frosted by grinding or etching. It diffuses light by scattering in directions close to the incoming beam, but the light falls off rapidly at larger angles out from this direction. Homogeneous - That property of a substance that determines that all components of volume are the same in composition and optical properties. Homogeneous orientation - The parallel orientation of the molecular axes of the nematic molecules in a nematic crystal, relative to the electrode plates. Hot mirror - A mirror with a coating that reflects infrared radiation and transmits visible light. Inclusion - A mirror with a coating that reflects infrared radiation and transmits visible light. Index of refraction - The ratio of the velocity of light in a vacuum to the velocity of light in a refractive material for a given wavelength. Indium tin oxide (ITO) - A material widely used as a transparent conductive coating. Interferogram - A photographic or electronic recording of an optical interference pattern. Interferometry - The study and utilization of interference phenomena, based on the wave properties of light. Internal transmittance - The ratio of the radiant power transmitted to the second surface of a medium to the corresponding radiant power that has just passed through the first surface. Internal transmittance does not denote the effects of surface losses and interreflection between the two surfaces. Jig - A device to hold and locate a workpiece as it guides, controls or limits a cutting tool. Keystone distortion - A type of geometrical distortion that brings about a trapezoidal display of a nominally rectangular picture. Usually produced when a picture is projected abnormally to the screen. Lens - A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object. Material dispersion - That dispersion attributable to the wavelength dependence of the refractive index of material used to form a waveguide. Measuring wedge - A wedge in a rangefinder or heightfinder used to displace the image produced by one telescope so that it coincides with that produced by the other telescope, thus affording a measurement of the parallactic angle between the line of sight of the two telescopes. There are two principal types of measuring wedges. The sliding wedge is mounted on slides parallel to the optical axis, between the objective lens of one telescope and its focal plane. It forms an image displacement equal to the product of the deviation of the wedge and its distance from the focal plane. Compensating wedges rotate simultaneously through equal angles in opposite directions. The pair of equal compensating wedges, mounted in front of one telescope, is equal to a wedge having a variable angle of deviation in the plane of triangulation, but none at right angles to it. Meniscus lens - A lens that has one convex surface and the other concave. Metallic coating - A thin layer of metal deposited on the surface of a substrate. The film may serve as a reflector, beamsplitter, neutral density filter or electromagnetic interference filter. Meter (m) - 1. The basic unit of length in the metric system, equal to 39.37 in. or 3.28 ft. 2. Any device or instrument used for measuring. The term often appears in conjunction with the quantity it measures, as in voltmeter, radiometer, wattmeter, etc. Micron (µm) - A unit of length in the metric system equal to one millionth of a meter (10-6 m). Also called micrometer. Millidiopter - A unit of metric measure equal to 0.001 D. The power of a lens in millidiopters is the reciprocal of its focal length in kilometers. Millimeter - A unit of metric measurement, equal to 0.001 m. 25.4 mm equal 1 inch. Milling - An automatic surface-generating process involving the abrasion of glass by a diamond-charged wheel. Mirror - A smooth, highly polished surface, for reflecting light, that may be plane or curved. The actual reflecting surface is usually a thin coating of silver or aluminum on glass. Mirror blank - The material on which the reflecting coating is applied. It may be glass, quartz, Cer-Vit or metal. The thickness must be adequate to prevent distortion of the surface under gravity, and a low thermal expansion combined with a high thermal conductivity is desired to minimize temperature variations of figure. Mirror coating - See metallic coating; reflective coating. Molded blank - A blank whose basic surface curves are attained by heating and forming a given weight of raw glass; a rough glass blank resembling the finished lens in size and shape. After molding, a precision lens blank must be fine-annealed to eliminate strain and to restore the refractive index to its stable maximum value. Molded lens - A lens with one or both sides fire-polished to its final surface. Lenses for condensers and viewfinders often are made in this way, an aspheric surface being as easy to mold as a sphere. Multilayer coating - A blank whose basic surface curves are attained by heating and forming a given weight of raw glass; a rough glass blank resembling the finished lens in size and shape. After molding, a precision lens blank must be fine-annealed to eliminate strain and to restore the refractive index to its stable maximum value. Nanometer (nm) - A unit of length in the metric system equal to 10-9 meters. It formerly was called a millimicron. Normal - An axis that forms right angles with a surface or with other lines. The normal is used to determine incident, reflective and refractive angles. Normal incidence - Light striking a surface at an angle perpendicular to the surface. Optical axis (OA) - The line passing through both the centers of curvatures of the optical surfaces of a lens; the optical centerline for all the centers of a lens system. Not to be confused with optic axis. Optical blank - A casting consisting of an optical material molded into the desired geometry for grinding, polishing or, in the case of optical waveguides, drawing to the final optical-mechanical specifications. Optical density (OD) - A measure of the transmittance through an optical medium. Optical density equals the log to the base 10 of the reciprocal of the transmittance. Optical disc - A rigid medium, generally a polycarbonate substrate coated with a reflective aluminum layer, that stores information (such as audio, video or data) as digital bits in the form of variations in the reflective surface, and that modulates a laser beam to decode the stored data. Optical disc formats include read-only (ROM) and write-once/read-many (WORM); erasable discs include phase-change, magneto-optical and dye-polymer media. Optical flat - A piece of glass, pyrex or quartz having one or both surfaces carefully ground and polished plano, generally flat to less than a tenth of a wavelength. Optical glass - A glass that, during manufacture, is carefully controlled with respect to composition, melting, heat treatment, and other processing, to render its optical characteristics, such as its index of refraction, dispersion, transmittance, spectral transmittance, homogeneity, freedom from birefringence, permanence, etc., sufficient for its optical application. Optical surface - A reflecting or refracting surface contained within an optical system. Optics - That branch of physical science concerned with vision and certain phenomena of electromagnetic radiation in the wavelength range extending from the vacuum ultraviolet at about 40 nm to the far-infrared at 1 mm. Now being replaced by the more inclusive term photonics. Overcoat - A layer of material applied to a coated surface to protect it from physical or chemical action. Panoramic distortion - The image distortion produced by a panoramic camera with a swinging lens or a swinging mirror in front of a fixed lens. The magnification is greatest in the center and diminishes to each side in the direction of the swing. Peak wavelength - The wavelength at which the radiant intensity of a source is maximum. Penta prism - A five-sided prism containing two reflecting surfaces at 45° to each other, and two refracting faces perpendicular to the entering and emerging beams. The deviation angle of 90° is independent of any rotation of the prism about an axis parallel to the line of intersection of the two reflecting faces. It is commonly used as an end-mirror in a rangefinder and as an "optical square'' in surveying and alignment machinery. Perpendicular - See normal. Perspective distortion - The distortion that is the result of viewing a print from a point other than the center of perspective. The center of perspective is that viewpoint at which the angular subtenses of points in the picture are identical with the angular subtenses of the original points in the scene, at the camera lens. Viewing a picture from too far away results in an excessive magnification of near points compared with distant points. Pitch - In positioning, rotation about an axis normal to the line of sight. Also known as attitude. Plane - A surface that has no curvature; a surface that is perfectly flat. Plane-parallel plate - See window. Polarization - With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam can vary at random (unpolarized beam), can remain constant (plane-polarized beam), or can have two coherent plane-polarized elements whose polarization directions make a right angle. In the latter case, depending on the amplitude of the two waves and their relative phase, the combined electric vector traces out an ellipse and the wave is said to be elliptically polarized. Elliptical and plane polarizations can be converted into each other by means of birefringent optical systems. Polishing - The optical process, following grinding, that puts a highly finished, smooth and apparently amorphous surface on a lens or a mirror. Polishing jig - In fiber optics, a device used to polish a biconic plug to a specified length and surface finish. Also called a polishing disc. Porro prism - A 45-90-45° reflecting prism whose surfaces form the 90° angle reflecting the light beam through a total angle of 180°. The corners of the prism are rounded to minimize breakage and simplify assembly. Two of these prisms are used in succession in a prism binocular. Power - With respect to a lens, the reciprocal of its focal length. The term power, as applied to a telescope or microscope, often is used as an abbreviation for magnifying power. Pressing - A blank having basic surface curves attained by forming heat-softened glass that is pressed in a mold. Prism - A transparent optical element having at least two polished plane faces inclined relative to each other, from which light is reflected or through which light is refracted. Prism base - The thick edge of a refracting prism. Protective bevel - The removal of a sharp edge on an optical element by grinding, to prevent accidental chipping of that edge during subsequent handling. Protective coating - A film applied to a coated or uncoated optical surface primarily for protecting this surface from mechanical abrasion, from chemical corrosion, or from both. For example, a thin layer of silicon monoxide may be added to protect an aluminized surface. Radial distortion - An alteration in magnification from the center of the field to any point in the field, measured in a radial direction from the center of the field. Some radial distortion is inherent in most optical systems, but can be reduced by proper design. Raw glass - A term that describes any state of glass before its manufacture as an element. Reflection - Return of radiation by a surface, without change in wavelength. The reflection may be specular, from a smooth surface; diffuse, from a rough surface or from within the specimen; or mixed, a combination of the two. Reflective coating - Thin-film coating, single or multilayer, that is applied to a substrate to increase its reflectance over a specified range of wavelengths. Metal film, transparent or opaque, is the oldest form of reflective coating. Reflectivity - The ratio of the intensity of the total radiation reflected from a surface to the total incident on that surface. Refractometer - An instrument used to measure the refractive index of solids and liquids. Several types exist, the most common being the Abbe refractometer. Rhomboid prism - A reflecting prism that is rhomboidal in shape. It has two parallel transmitting faces, and two parallel reflecting faces; the latter are oblique to the former (usually but not necessarily at 45°). This prism is capable of offsetting the optical axis without changing the aspect of the image. If the rhomboid prism is rotated around an axis normal to the entrance surface, the offset emergent axis can be moved parallel to itself in a circular arc; hence pairs of these prisms often are used to provide interpupillary adjustment of the eyepiece of binocular instruments such as stereoscopic rangefinders and heightfinders. Right-angle prism - A type of 45-90-45° prism used to bend a beam of light through a right angle with the surfaces forming the 90° angle acting as transmitting faces. Runout - In a linear stage, any deviation from the desired translation across a flat, straight line. Sag - 1. In the geometric sense, an abbreviation for the term "sagitta,'' the height of a curve measured from the chord.
and it is exact for a parabola. 2. Conforming a sheet of glass to a ceramic or metal form by heating the glass to its softening point and allowing it to settle. Scratch - A defect on a polished optical surface whose length is many times its width. Block reek is a chainlike scratch formed in polishing. A runner cut is a curved scratch caused by grinding. A sleek is a hairline scratch. A crush or rub is a surface scratch or scratches usually caused by mishandling. Scratch-resistant coating (SRC) - Thin layers intended to prevent damage to plastic optics. Scribing - The process of perforating a silicon or ceramic substrate with a series of tiny holes along which it will break. Nd:YAG or CO2 lasers are now routinely used. Sleek - A polishing scratch having no visible conchoidal breaking of the edges. Slurry - The name of the mixture of liquid and grinding or polishing compounds used in processing optical materials. Spectrophotometer - An instrument for measuring spectral transmittance or reflectance. Spindle - A loose term for a single polishing machine. In a lens factory it is the minimum unit of production. Stereoscopic distortion - An exaggerated depth appearance in stereo photographs caused by the lenses in the camera being farther apart than the eyes of the viewer. Stone - An opaque inclusion in glass that contains undissolved or crystalline material. Also known as a seed. Stress birefringence - Birefringence of materials that is induced or altered by stress fields. Striae - An imperfection in optical glass consisting of a distinct streak of transparent material having a slightly different refractive index from the body of glass. It is caused by improper mixing of ingredients during manufacture. Substrate blank - Materials (usually ceramics or other semiconductors) that are molded into approximate forms to use as supports for large mirrors or as blocks for microcircuits. Surface - 1. In optics, one of the exterior faces of an optical element. 2. The process of grinding or generating the face of an optical element. Surface error - The departure of an optical surface from its required tolerance or figure. Surface quality - The specification of allowable flaws in a surface by comparison to reference standards of quality. Two graded sets of surface quality standards are employed. The first denotes defects of a long nature such as scratches, and the second indicates essentially round defects such as digs. Surface quality standards - The standards of MIL-O-13830 set by the Surface reflection - Also known as Fresnel reflection. That portion of the incident radiation that is reflected from the surface of a refractive material. It is a function of the refractive index of the material. At normal incidence: Surface wave - A wave that is guided by the interface between two different media or by a refractive index gradient in the medium. The field components of the wave may exist (in principle) throughout space, even to infinity, but become negligibly small within a finite distance from the interface. All guided modes, but not radiation modes, in an optical waveguide belong to a class known in electromagnetic theory as surface waves. Tangential distortion - An image defect, usually caused by errors of centration, that results in the displacement of image points perpendicular to a radius from the center of the field. Test glass - A transparent block shaped accurately to reverse curvatures of the components it is used to test. By contacting an accurately finished negative lens shape with a newly made positive, the conformity of the latter to the ideal can be easily observed. Interference fringes between the two surfaces may be observed readily. Thin film - A thin layer of a substance deposited on an insulating base in a vacuum by a microelectronic process. Thin films are most commonly used for antireflection, achromatic beamsplitters, color filters, narrow passband filters, semitransparent mirrors, heat control filters, high reflectivity mirrors, polarizers and reflection filters. Tolerancing - The determination of the degree to which a manufactured component can deviate from its ideal specifications of material and geometry without impairing its performance. Transfer blocking - A process used to control thickness and parallelism precisely during the production of plane-parallel plates. Elements are cemented to a blocking tool and their upper surfaces are polished. A second blocking tool is then cemented to the upper polished surface and the original blocking is released. As a result, the optical elements are transferred from the first to the second blocking tool without disrupting their particular positions. The second, or unfinished, surface is then polished. Transmission (T) - In optics, the conduction of radiant energy through a medium. Often denotes the percentage of energy passing through an element or system relative to the amount that entered. See transmission efficiency. Transmittance - The ratio of the radiant power transmitted by an object to the incident radiant power. Ultrasonic cleaning equipment - Ultrasound used in the cleaning of metal and optical parts by virtue of its vibration rates. Large acoustic forces break off particles and contaminants from surfaces. V-coat - A multilayer antireflective thin-film coating, so called because its reflectance rises steeply at wavelengths above and below a narrow band at zero. The v-coat is particularly useful in high-power laser applications. Wave - 1. An undulation or vibration; a form of movement by which all radiant energy of the electromagnetic spectrum is estimated to travel. 2. A type of surface defect, usually due to improper polishing. Wavefront - In considering a field of electromagnetic energy emanating from a source, the wavefront is a surface connecting all field points that are equidistant from the source. Wavelength (WL) - Electromagnetic energy is transmitted in the form of a sinusoidal wave. The wavelength is the physical distance covered by one cycle of this wave; it is inversely proportional to frequency. Wedge - An optical element having plane-inclined surfaces. Usually the faces are inclined toward one another at very small angles. Wedges divert light toward their thicker portions, and may be circular, oblong or square. See also absorbing wedge; calibrated wedge; neutral density wedge. Wedge tolerance - A method of specifying the allowable edge-thickness difference or decentering of a lens. Wide-angle distortion - A common aberration in lenses covering large fields of view; it results in images of objects near the edge of the field being compressed in the radial direction. Window - 1. A piece of glass with plane parallel surfaces used to admit light into an optical system and to exclude dirt and moisture. 2. A particular region of the electromagnetic spectrum that has been singled out for some purpose; for example, the region near 850 nm at which optical fibers operate with lowered losses often is referred to as the first window. Working aperture - The maximum aperture of a lens at which it will still give a sharp image, even though its physical aperture may be larger. Zerodur - Schott Glass Technologies' trade name for a glass-ceramic material with a very low coefficient of thermal expansion. |
