A
Abbe Number:
     The ratio of refractivity to dispersion in an optical medium. Specifically: (nd-1)/(nF-nC), where n  is the index of refraction
     for the   Fraunhofer d, F and C lines, respectively.
Aberration:
     An optical defect resulting from design or fabrication error, e.g., coma, distortion, curvature of field that prevents the lens
     from achieving precise focus.
Achromatic:
     Free of color. Describes an optical system in which chromatic aberration has been corrected at a minimum of two
     wavelengths.
Airy Disc:
     A pattern of illumination caused by diffraction at the edge of a circular aperture, consisting of a central core of light
     surrounded by concentric rings of gradually decreasing intensity.
Anamorphic:
     Distorted, as in an optical system with different magnification levels or with focal lengths perpendicular to the optical axis.
Angle of Incidence:
     The angle between a ray of light striking a surface and the normal (a line perpendicular to the surface at that point).
Anti reflection Coating:
     A thin layer of material that, when applied to a lens, increases its transmittance and reduces its reflectance.
Aperture:
     A hole through which light may pass. The aperture stop is that hole in an optical system limiting the bundle of light able to
     traverse the system.
Aspherical:
     Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one or more  surfaces are aspherical.
Astigmatism:
     An aberration in a lens in which the tangential and sagittal (horizontal and vertical) lines are focused at two different points
     along the optical axis.

B
Back Focal Length:
     The distance between the last surface of a lens to its back focal plane.
Bandpass:
     For a filter or thin-film coating, the wavelength range over which transmission is allowed and possibly  maximized.
     Transmission above or below the bandpass  range is restricted by design through absorption and/or reflection.
Bandwidth:
     Usually described in terms of transmission level, the bandwidth is the spectral range over which an interference filter
     transmits.
Beamsplitter:
     An optical device which divides an incident beam into two or more separate and distinct beams. A beamsplitter may be as
     simple as an uncoated plano-plano piece of glass inserted in a beam at an angle to divert a portion of the beam in a different
     direction. More complex beamsplitters employ coated and cemented right- angle prisms to separate colors.
Bending:
     The ratio of the curvatures of a lens's two refracting surfaces.
Bi-Concave:
     Having two outer surfaces that curve inward.
Bi-Convex:
     Having two outer surfaces that curve outward.
Birefringence:
     The separation of a beam of light into two beams (ordinary and extraordinary) as it passes through a doubly refracting
     material or object.
Blocking:
     Blocking refers to the filter transmittance outside the bandpass region, and can be thought of as the degree to which
     undesired wavelengths are prevented from being transmitted. Filters with deep out-of-band blocking significantly enhance
     the signal-to-noise ratio of the system.
Blur Circle:
     The image of a point-source object formed by an optical system on its focal surface. The precision of the lens and its state
     of focus determine the size of the blur.
BNC:
     Term commonly associated with a type of connector prevalent in industry for joining cables to receivers, transmitters or
     other cables.
Borosilicate Glass:
     An optical glass containing boric oxide, along with silica and other ingredients, having relatively high -value and low index of
     refraction. Some varieties, such as Corning Pyrex® and Schott Tempax®, are resistant to thermal shock.
Broadband Coating:
     A multi-layer antireflection coating with reduced reflection over a broad spectral band.

C
Catadioptric:
     An optical system containing both reflective and refractive elements.
Cavity:
     A periodic structure of thin films comprised of two quarter-wave stack reflectors separated by a dielectric spacer. Cavities
     are the building blocks of bandpass filters.
C/CS-Mounts:
     Two different but standard video camera mount configurations. The more prevalent of the two types is the standard
     C-mount, which provides a 17.52mm flange focal distance (FFD). The CS-mount typically provides a 12.5mm FFD.
     C-mount lenses can be used in CS-mount configurations with a 5mm adapter.
CCD:
     Charge Coupled Device. A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor (MOS),
     surface storage and information transfer technologies.
CCTV Lens
     Closed Circuit Television Lens. Term commonly applied to a family of compound lenses which offer exceptionally high
     resolution, short focal length and color imaging at fast lens apertures, such as required in the television industry.
Center Wavelength:
     For filters and coatings, the average of the wavelength values at the half-power points of the transmission band.
Centration:
     The degree to which the optical axis of a lens and the mechanical axis of its mounting coincide.
Chromatic Aberration:
     An optical defect in a lens resulting in different wavelengths of light focusing at different distances from the lens, which can
     be seen as halos around the image.
Chromatic Dispersion:
     The dispersion of white light into its constituent colors. The refractive index of blue light is higher than that of red light,
     resulting in a change of image size from one color to the other.
Circle of Least Confusion:
     The smallest cross-section of a focused beam of light; the point of best focus for the image.
Clear Aperture:
     The opening in the mount of an optical system that controls the amount of light incident on a given surface; the entrance
     pupil of the lens.
Coefficient of Thermal Expansion:
     A material property defined as the ratio of the change in length per original length (or change in volume per original volume)
     to the incremental change in temperature from a reference. Typically an order of magnitude higher for metals relative to
     glasses.
Collimate:
     To adjust two or more optical axes with respect to each other; to make rays of light parallel.
Collimated Beam:
     A beam of light in which all of the rays are parallel to each other.
Color:
     The aspect of a light source that can be described in terms of hue, brightness and saturation; the specific property of
     objects seen as red, yellow or blue as opposed to black, white or gray.
Coma:
     An aberration that occurs in a lens when rays emanating from points not on the optical axis do not converge, causing the
     image of a point to appear comet-shaped.
Compound Lens:
     Two or more optical glass elements, sometimes cemented together, shaped to cancel out aberrations present in either lens
     alone.
Cone Angle:
     The central angle of a cone of rays converging to or diverging from a point. See Numerical Aperture.
Conjugate:
     Two optical elements joined as a pair.
Continuous Wave Irradiation:
     Emission of radiant energy (light) in a continuous, rather than pulsed, wave.
Contrast:
     The difference in light intensity in an object or image; defined as (Imax - Imin)/(Imax + Imin), where Imax and Imin are the
     maximum and minimum intensities.
Converging:
     The bending of light rays toward each other, achieved with a positive (convex) lens.
Cosmetics:
     Describes the surface defects of a lens that are not optically critical and do not impair its function. Usually described in
     terms of scratch and dig.
Critical Angle:
     Maximum angle of incidence formed by a ray of light as it passes from a dense to a less dense medium, e.g., from glass to
     air, where the critical angle is about 42 degrees. When the critical angle is exceeded, all the light reflects back to the denser
     of the two media.
Crown Glass:
     A silicate glass containing oxides of sodium and potassium, used in compound lenses and spectacles; harder than flint glass,
     with low index and low dispersion.
Crystal Quartz:
     Crystalline form of silicon dioxide; very hard with a low expansion coefficient. Transmits light through the range of 180nm
     (ultraviolet) to 4.5 micrometers (infrared).
Curvature:
     Departure from flatness of a surface. Defined as the reciprocal of the radius of curvature.
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.

D
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.

E
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
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.
Fiber 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.
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.
Fringes:
     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.
FWHM:
     Full Width, Half Maximum. The bandwidth of an optical instrument as measured at the half-power points.

G
Galvanometer:
     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.

H
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.
Homogeneity:
     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.
Hybrid:
     Anything formed out of heterogenous elements.

I
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.
Infrared:
     The portion of the spectrum whose wavelengths are invisible to the human eye (range = .76 microns and higher).
Input Scan Angle ():
     Input scan angle, also known as "half angle," is half the total angular field of the scan lens. Twice this angle in radians,
     when multiplied by the calibrated focal length of the scan lens, results in the scan length.
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.
Interferometer:
     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.
 

K
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.

L
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.

M
Magnesium Fluoride:
     Material used as antireflection coating for lenses because of its low refractive index.
Magnification:
     The enlargement of an object by an optical instrument; ratio between the size of the image and the actual size of the object.
Meniscus:
     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.
Micro-Optics:
     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
     fiber 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.

N
Nanosecond:
     A billionth part of a second.
Narrowband Coating:
     A coating designed to provide transmittance (or reflectance) over a very restricted band of wavelengths.
Neutral Density:
     As applied to thin films, a coating which appears gray to the eye and has a flat absorption curve throughout the visible
     spectrum. Neutral density filters decrease the intensity of light without changing the relative spectral distribution of energy.
Newton's Rings:
     Used to measure the fit of a lens surface against the surface of a test glass. The rings result when two adjacent polished
     surfaces are placed together with an air space between them and the light beams they reflect interfere.
Nodal Points:
     The two points at which the nodal planes appear to intersect with the optical axis, i.e., when a ray is directed at the first
      nodal point in an optical system, it  appears to emerge from a second nodal point on the optical axis with no deviation in its
      angle.
Numerical Aperture:
     Describes the angle in a cone of light emitted by the condenser and accepted by the objective of a microscope; the index of
     refraction of the medium in which the image lies multiplied by the sine of the half angle of the cone of light.

O
Object-to-Image Distance:
     Also known as the total conjugate distance or track length. Can be finite or infinite depending on application.
Objective:
     The optical element which receives light from the object and forms the first or primary image in telescopes, microscopes
     and other optical systems.
Oblique Ray:
     A ray of light that is neither perpendicular nor parallel, but inclined.
Optical Axis:
     A line passing through the centers of curvature of a lens or series of lenses in an optical system.
Optical Flat:
     A piece of glass with one or both surfaces polished flat. Also known as a test plate, test glass or reference flat.
Optical Interference:
     The interaction of two light waves on the total intensity of light.
Orthogonal:
     Mutually perpendicular.

P
Paraxial Image Plane:
     Image plane located by using first-order geometric optics. See Gaussian Optics.
Peak Wavelength:
     For bandpass filters, the discrete wavelength which has the maximum transmission value in the passband region.
Percent Position Error:
     Scan linearity can be defined as a percentage of the expected (design) scan height using the percent position error
     technique. The difference between the expected (design) and observed scan height at any scan angle divided by the
     expected design height yields this measure of linearity once converted to a percentage.
Percent Velocity Error:
     The percent velocity error technique involves calculating the percentage difference between the velocity of an image
     spot at a specific field angle and the velocity at the optical axis of the lens. See percent position error.
Pinhole:
     Generally a small, sharp-edge hole without a lens which can function as an aperture or eye lens.
Plane of Incidence:
     The plane that is defined by the incident and reflected rays.
Planetary Polisher:
     A polishing machine used in the production of plano parallel elements where both surfaces are polished simultaneously.
Plano Elements:
     Lenses or mirrors with perfectly flat surfaces.
Plano-Concave:
     A lens with one flat (plano) surface and the other curved inward.
Plano-Convex:
     A lens with one flat (plano) surface and the other curved outward.
Poisson's Ratio:
     The ratio of the transverse contraction of a bar of material to the elongation per unit length.
Polarization of Light:
     The process of affecting light so that its waves vibrate in one plane only; reflection, double refraction, selective
     absorption and scattering are all ways to polarize light.
Polarized Light:
     Light that vibrates in only one plane.
Polychromatic Aberrations:
     The separation of an image into planes of distinct color, caused by the variation of the index of refraction of glass,
      and the focal length of a lens, with the wavelength of light; in a given plane, all colors but one are unfocused.
Polyhedral:
     In the shape of a solid formed by plane faces, as in a prism.
Potentiomenter:
     An instrument used to measure electromotive forces.
Power Loss:
     In applications involving diffraction limited lenses, power loss in the spot is an exponential function of the square of the
     truncation ratio (Dt/Db): PL= e-2*(Dt/Db)².
Primary Reflections:
     The principal, intended reflections at optical surfaces, as differentiated from secondary, usually unintended or unwanted
     reflections occurring in an optical system.
Principal Planes:
     Imaginary planes at right angles to the optical axis of a lens; the intersection of a plane and the axis is a principal point.
Prism:
     A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or
     through which it is refracted.
Pulse Modulation:
     The process of periodically or intermittently varying the amplitude of a pulse of light.

Q
Q-Switched:
     In an optical resonator, the higher the reflectivity of its surfaces, the higher the Q. A Q-switch rapidly changes
     the Q in the optical resonator of a laser to prevent lasing until a high level of optical gain and energy storage has
     been reached in the lasing medium; a giant pulse is generated when the Q is rapidly increased.
Quarter Wave Optical Thickness:
     Common thin-film term. The QWOT (Quarter Wave Optical Thickness) is the wavelength such that the optical
     thickness (index 'n'* physical thickness 'd') of a coating evaporant layer is ¼ wavelength. (n*d=/4)

R
Radiant Flux:
     The measurement of the time rate of flow of radiant energy, expressed in watts.
Real Image:
     Light rays reproduce an object, called an image, by gathering a beam of light diverging from an object point and
     transforming it into a beam converging toward or diverging from another point. If the beam is converging, it produces
     a real image.
Reference Flat:
     An optical flat used as a test glass.
Reflection:
     The return of light from a surface with no change in wavelength.
Refraction:
     The change in direction of a ray of light as it passes through two media through which light travels at different speeds.
Refractive Index:
     The ratio between the speed of light through air to the speed of light through another medium; the ratio determines how
     much a ray of light will bend as it passes through a given medium.
Resolution:
     The ability of a lens to image the points, lines and surfaces of an object so they are perceived as discrete entities.
Reticle:
     An optical element containing a pattern placed at the image plane of a system. The reticle facilitates system alignment
     or the measurement of target characteristics.
Reverted Image:
     An image in which left and right seem to be reversed.
Ring-Type Auto Iris:
     A type of auto iris which utilizes a rotating magnet to move the iris vanes.
Rockwell Hardness:
     Resistance of a substance to penetration by a pyramidal stylus pressed in under a specific load; See also Knoop hardness.
RS 232 Interface:
     A term for electronic data exchange cables common in industry.

S
Sag Formula:
     "Sag" is an abbreviation for "sagitta," the Latin word for "arrow," and refers to the height of a curve from the chord
     to the highest point.
Sagittal Focus:
     The focus of rays lying in the sagittal plane, which is the plane perpendicular to the meridional plane (See Tangential
      Meridian).
Scan Length (SL):
     The image height of the scan lens. Mathematically, scan length can be defined as the product of the calibrated focal
     length (f) and twice the field angle (2* in radians) in object space: SL = 2*(Field Coverage) = f*2().
Scan Lens:
     A multi-component objective which is the heart of a graphic arts image recording, printing or engraving system. The
     balancing of wide angular field, flat image plane and linear relationship between input scan angle and image height make
     scan lenses ideal for writing characters on film, laser engraving figures or recording characters and figures from texts.
     Scan lenses are sometimes also called F- lenses because their image height is proportional to the scan angle () and not the
     tangent of the angle.
Scan Linearity:
     The degree to which the performance of a particular scan lens design follows the equation
      Scan Length = 2*field coverage = f*2(). The scan linearity of a system can be defined using both the percent position
      error and percent velocity error techniques.
Seidel Aberrations:
     The group of primary aberrations in lenses, including coma, astigmatism, curvature of field, distortion, spherical and
     chromatic.
Short Pass:
     Interference filter type which efficiently passes radiation whose wavelengths are shorter than a specific wavelength, but not
     longer.
Shutter:
     A device for controlling the amount of time a light-sensitive medium is exposed to light.
Single-Layer Coating:
     A thin coating that reduces or eliminates reflections at an air-glass surface, such as MgF2.
Slit:
     An aperture, typically rectangular in shape, whose length is large compared to its width. Apertures, generally small
     compared to the light source, may have fixed or adjustable shapes through which radiation enters or exits an instrument.
Snell's Law of Refraction:
     Describes the way a ray of light changes direction at a surface between two media that have different indices of refraction.
     The angle of incidence of the ray is measured from the normal, or perpendicular, to the surface. A ray moving from a
     low-index medium to a high-index medium bends toward the normal; from high-index to low, it bends away from the
     normal.
Solid State Synchronization System:
     As applicable to programmable optical shutters, a system of infrared emitting diodes, infrared sensitive detecting transistor
     and interrupting vane and associated circuitry. The system provides feedback signals associated with the open/closed status
     of the shutter to users.
Spatial Filtering:
     Enhancing an image by increasing or decreasing its spatial frequencies.
Specific Gravity:
     The dimensionless ratio of the mass of an object to the mass of an equal volume of water at 4°C or other specified
     temperature.
Spectrometry:
     Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.
Spectrophotometry:
     Measuring the reflection or transmission of light for each component wavelength in the spectrum of a specimen.
Spherical Aberration:
     An optical defect caused when rays of light passing through the curved surface of a lens near its edge converge at a point
     closer to the lens than those passing through its center (negative aberration); when the outer zone has a longer focal length
     than the center, the aberration is said to be positive.
Spot Size:
     See Blur Circle.
Striae:
     An imperfection in optical glass characterized by streaks of transparent material of a different refractive index than
     the body.
Substrate:
     The underlying material to which an optical coating is applied.
Surface Contour:
     The outline or profile of a surface.

T
Tangential Meridian:
     In an optical system of revolution, the tangential, or meridional, plane is defined as the plane containing the optical axis
     and the specified object point.
Telecentric Lens:
     A multi-component system whose aperture stop is located at the front focus so that the chief rays are parallel to the
     optical axis in image space. For a telecentric lens, the exit pupil is at infinity.
Telephoto Lens:
     A multi-component lens arranged so that the overall length of the compound system is less than or equal to the effective
     focal length.
Test Glass:
     See Optical Flat.
Total Internal Reflection:
     When the angle of incidence of light striking the boundary surface of a substance exceeds the critical angle, the result
     is total internal reflection.
Transmission:
     To admit the passage of light through a medium; light not reflected back to its source is transmitted through the medium.
Transverse Ray Error:
     Errors, or departures from ideal, measured in a direction perpendicular to the optical axis.
Truncation Ratio:
     The dimensionless ratio of the Gaussian beam diameter at the 1/e² intensity point to the limiting aperture of the lens.
TTL:
     Type of connector commonly used for joining cables to receivers, transmitters or other cables.

U
Ultraviolet:
     The range of the electromagnetic spectrum from 10 to 400 nanometers.

V
V-Coating:
     A narrowband coating for specific laser wavelengths.
Vignetting:
     The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear
     apertures ofelements within an optical system.
Virtual Image:
     Light rays reproduce an object, called an image, by gathering a beam of light diverging from a point source and
     transforming it into a beam converging toward or diverging from another point; if the beam is diverging, it produces
     a virtual image.
Visible Spectrum:
     That part of the electromagnetic spectrum that the human eye can perceive, between the ultraviolet and the infrared
     (range = .38 microns to .76 microns).

W
Wave Phenomena:
     The characteristics of radiant energy.
Wavefront Deformation:
     Departure of a wavefront from ideal (usually spherical or planar) caused by surface errors or design limitations.
Wavelength:
     The distance light travels in one wave cycle. Electromagnetic energy travels in waves.
Wedge:
     An optical element with its faces inclined toward each other at very small angles, diverting light toward the thicker
     parts of the element.

Y
Young's Modulus:
     Modulus of elasticity; the amount of stress required to produce a unit change in length (strain). Expressed in pounds
     per square inch (PSI) or dynes per square cm.

-
  -Value:
     The -value is the reciprocal of the dispersive power. See Abbe Number.