View glossary for definition of technical terms related to precision optics and lens assemblies.


The ?-value or “nu-value” is the reciprocal of the dispersive power. See Abbe Number.

3-D Printing

The action of making a physical three-dimensional object from a 3-D digital model by laying down many thin layers of material in succession.


An optical defect resulting from design or fabrication error, e.g., coma, distortion, curvature of field that prevents the lens from achieving precise focus.


Free of color. Describes an optical system in which chromatic aberration has been corrected at a minimum of two wavelengths.


An optical system with object and image points at infinity. Literally, “without a focal length.”

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.


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).

Antireflection Coating

A thin layer of material that, when applied to a lens, increases its transmittance and reduces its reflectance.


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.


An apochromat, or apochromatic lens, is a photographic or other lens that has better color correction than the much more common achromat lenses. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane.


Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one or more surfaces are not spherical.


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.


The decrease in magnitude of a wave, or a signal, as it travels through a medium or an electric system. It is measured in decibels (dB).

Augmented Reality (AR)

A technology that superimposes computer-generated sensory input (sound, video, graphics, GPS) into a live direct or indirect view of physical, real world environment.

Auto iris (AI)

An automatic method of varying the size of a lens aperture in response to changes in scene illumination.

Back Focal Length

The distance between the last surfaces of a lens to its back focal plane.


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.


Usually described in terms of transmission level, the bandwidth is the spectral range over which an interference filter transmits.


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.


The ratio of the curvatures of a lens’s two refracting surfaces.


Having two outer surfaces that curve inward.


Having two outer surfaces that curve outward.


The separation of a beam of light into two beams (ordinary and extraordinary) as it passes through a doubly refracting material or object.


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.


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, as a result of their low coefficient of expansion.

Broadband Coating

A multi-layer antireflection coating with reduced reflection over a broad spectral band.


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.


An optical system containing both reflective and refractive elements.


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.


Charge Coupled Device. A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor (MOS), surface storage and information transfer technologies.


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.


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.


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.


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.


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 Points

The two points on the principal axis of a mirror or lens so positioned that light emitted from either point will be focused at the other.

Continuous Wave Irradiation

Emission of radiant energy (light) in a continuous, rather than pulsed, wave.


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.


The bending of light rays toward each other, achieved with a positive (convex) lens.


Describes the surface defects of a lens that are not necessarily optically critical and do not necessarily 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 more 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; they are 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).


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.

Damage Threshold

The maximum energy density to which an optical surface may be subjected without failure.


The failure of one or more lens surfaces to align their centers of curvature with the geometric axis of a lens system.

Decibel (dB)

A logarithmic ratio of two signals or values, usually refers to power, but also voltage and current. When power is calculated the logarithm is multiplied by 10, while for current and voltage by 20.


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.


The angle between the paths of a ray of light before and after passing through one or more lenses.


A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colors), or one in which light rays having different polarizations are absorbed by different amounts.

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.


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.


A device used to scatter or disperse light emitted from a source, usually by the process of diffuse transmission.


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.


The separation of light into its component colors, as a prism disperses white light into a color band, or a rainbow effect.


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.


The bending of light rays away from each other, achieved with a negative (concave) lens.


Grinding, or finishing, the edge of an optical element or lens.

Effective Focal Length

See Equivalent Focal Length.


To form shaped objects by electrodeposition on a mold.


Aspherical, as in a lens whose surface is a section of an ellipse rather than of a circle.

Enlarging Lens

A lens used chiefly for the creation of a magnified image of a photographic negative onto photographic paper.

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.


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.


A luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength.

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.


A crystalline semiconductor material that transmits in the IR.


Geographic Information System, which is a framework for gathering, managing and analyzing geographical information. Often used in visualization mapping that is captured by LiDAR or photographic aerial and land-based capture systems.

Gray Scale

A pattern of varying levels of grays spanning from black to white. Often used in process-control sensitometry.

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 biconvex crown element cemented to a meniscus flint element, with the crown facing the long conjugate.


A unit that measures the number of certain oscillation per second.

High-Efficiency Coating

Specialized coating applied to optics to improve transmission or reflection.


Having to do with holography, which is a method of producing a three-dimensional image of an object by recording the pattern of interference formed by a split laser beam and then illuminating the pattern.


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.


This term literally means “to lag behind.” It is quite often used to describe the residual effect that remains after the primary effect has been removed, or the lag that exists between the responding parameter and the changing parameter; can be seen in stress-strain and magnetizing force magnetic field relationships.

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 (IR)

The portion of the spectrum whose wavelengths are invisible to the human eye (range = .76 microns and higher).

Input Scan Angle (q)

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.


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 aperture 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.

Kinematic Mount

A mount for an optic element or optics assembly, designed so that all six degrees of freedom are singly constrained. This assures that movement will be prevented, while stress will not be introduced into the optics.

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 Emitting Diode. A semiconductor that produces light when a certain low voltage is applied to it in one direction.


When relating to a video signal it refers to the video level in Volts. In CCTV optics, it refers to the auto iris level setting of the electronics that processes the video signal in order to open or close the iris.


“Light, Detection and Ranging” and uses ultraviolet, visible, or near infrared light beams that are transmitted out and the returning light beam is read by receiver electronics. The resulting reflection of light characteristics can be used to create maps, define object shapes, and create collision avoidance systems.

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.


A pixel (short for picture element, using the common abbreviation “pix” for “picture”) is a single point in a graphic image. A Mega-Pixel camera contains one million pixels.


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 microscopes. 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.


Minimum object distance. This is a feature of a fixed or a zoom lens which indicates the closest distance an object can be from the lens’ image plane, expressed in meters. Zoom lenses have MOD of around 1 m, while fixed lenses usually much less, depending upon the focal length.

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.

Moire Contouring

Topography technique that involves positioning a grating close to an object and observing its shadow on the object through the grating. The resultant moire fringes correspond to a contour line system of the object under certain conditions.

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.


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. The distance from the first nodal point to the second, is known as the nodal point separation or NPS.

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.

Object-to-Image Distance

Also known as the total conjugate distance or track length. Can be finite or infinite depending on application.


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.


The acronym for Optics, Photonics, and Imaging

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.


Where laser energy delivered into biological tissues will be absorbed and converted into heat, leading to ultrasonic emission.


The application of electronic devices or circuits that comprise both electrical and optical functions, which are used to convert electrical signals to visible or infrared energy, or vise-versa. I.e., a thin-film semiconductor device.


The study of the interaction between light and mechanical vibrations of mesoscopic or macroscopic objects.


Mutually perpendicular.


The change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer. Simply put, it is the apparent shift of an object against a background due to a change in observer position.

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.

Photoacoustic Imaging

see Optoacoustics


The science of light (photon) generation, detection, and manipulation through emission, transmission, modulation, signal processing, switching, amplification and detecting/sensing.


Photographic and Imaging Manufacturers Association


Generally a small, sharp-edge hole without a lens which can function as an aperture or eye lens.


Derived from picture element. Usually refers to the CCD chip unit picture cell. It consists of a photosensor plus its associated control gates.

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.


A lens with one flat (plano) surface and the other curved inward.


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.


A mixture of photons and phonons, and operate in the range of frequencies from 300 gigahertz to approximately 10 terahertz. Differs from photonics in that the fundamental information carrier is a polariton.

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.


A device that converts an unpolarized or mixedpolarization beam of electromagnetic waves (e.g., light) into a beam with a single polarization state (usually, a single linear polarization).

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.


In the shape of a solid formed by plane faces, as in a prism.


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

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.


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.


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 (l) such that the optical thickness (index ‘n’* physical thickness ‘d’) of a coating evaporant layer is l/4: n*d = l/4. 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.


The return of light from a surface with no change in wavelength.


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.

Relay Lens

A lens or lens system used to transfer a real image from one point within an optical system to another, with or without magnifying it.


The ability of a lens to image the points, lines and surfaces of an object so they are perceived as discrete entities.

Retardation Plates

Birefringent optical elements with two optic axes, one fast and one slow. The incoming beam is separated into mutually orthogonal polarized beams that are recombined with a phase difference that is a function of the thickness of the material and the wavelength of the light. If the phase difference is exactly 1?4 cycle, the plate is called a quarter-wave plate. If the phase difference is exactly 1?2 cycle, the plate is called a half-wave plate.


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 termination for electronic data exchange cables common in industry.

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).

Sapphire (optical)

A material that differs from ruby only in the slight impurity that gives the material its color. It is useful because of its high refractive index, low dispersion, and exceptional hardness.

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*q in radians) in object space: SL = 2*(Field Coverage) = f*2(q).

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-Theta lenses because their image height is proportional to the scan angle (theta) 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(q). 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.


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.


Varying as a sine function.

Sinusoidal Test Pattern

A pattern of light and dark bands which vary as a sine function. Used for determination of resolution.


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.


Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.


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.


The underlying material to which an optical coating is applied.

Surface Contour

The outline or profile of a surface. 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 Included Angle (TIA)

Twice the value of the deviated angle measured as the optic is rotated about its mechanical center.

Total Internal Reflection (TIR)

When the angle of incidence of light striking the boundary surface of a substance exceeds the critical angle, the result is total internal reflection.


A linear movement.


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.


Type of connector commonly used for joining cables to receivers, transmitters or other cables.


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


A narrowband coating for specific laser wavelengths.


The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures of elements 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.

Virtual Reality

Using computer technology to create a simulated environment from which the user(s) are immersed by multiple senses and are able to interact with the views.

Visible Spectrum

That part of the electromagnetic spectrum that the human eye can perceive, between the ultraviolet and the infrared (range = .4 microns to .76 microns).

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.


The distance light travels in one wave cycle. Electromagnetic energy travels in waves.


An optical element with faces inclined toward each other at small angles, diverting light toward the thicker parts of the element.

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.

Zinc Selenide

Polycrystalline material transmitting in the IR and used in laser windows and FLIR (forward-looking infrared) systems.

Zoom lens

A lens in which the focal length can be varied within a pre-defined range. The lens has a focusing control and a choice of iris functions.

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