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Unit of luminous intensity. 1 candela is equal to 1 lumen per solid angle.


A property that measures the ability of a conductor to hold electrical charge, measured in farads (F). Junction capacitance is related to the rise time of the photodiode. The smaller the capacitance, the shorter the rise time, and vice versa.

Cathode Ray Tube

Video display based on vacuum tube with electron gun and annular anodes at one end and a cathode and phosphor screen at the other. Charged plates or electromagnets deflect the electron beam such that the image is scanned line by line across the face of the tube. Signal response is nonlinear with input, gamma usually between 2 and 3.

See also Oscilloscope


With interference filters, a spacer layer between two stacks of dielectric layers. The number of cavities determines the overall shape of the transmittance curve. Typically, interference filters use three cavities, resulting in steep slopes, improved blocking near the bandwidth, and relatively flat tops.

See also Filter, Bandwidth, Interference Filter, Blocking

CDAR™ (Clear Display Anti-Reflection) Coating

A highly transmissive anti-reflection (AR) coating with transmission of 96 - 99% between 400 - 700nm.

CDRH Laser Class

A laser class standard administered by the Center for Devices and Radiological Health, a division of the US FDA (Food and Drug Administration). It is an alternative to the IEC laser class standard used throughout Europe.

See also IEC Laser Class

CE Certified

A European proof of conformity that allows manufacturers and exporters to circulate products freely within the European Union. The letters "CE", French for "Conformite Europeenne," indicate that the manufacturer has satisfied all assessment procedures specified by law for its product. Products requiring the CE Mark in Europe include, but are not limited to: high voltage devices, toys, construction products, equipment that generates electromagnetic interference, personal protection Equipment, and Medical Devices.

See also IEC Laser Class

Center Thickness (CT)

An optical measurement specified as the distance from a primary principal plane location to the end of an element. A PCX or PCV lens has a single principle plane, yielding a single CT; a DCX or DCV has two principle planes, yielding two CT values.

See also Principal Plane


Also known by centration or decenter, it is specified in terms of beam deviation. The amount of decenter is the physical displacement of the mechanical axis from the optical axis.

Central Wavelength (CWL)

Typically used to denote the peak transmitting wavelength of a filter, it is the midpoint determined by the passband wavelengths where the transmittance is 50% of the peak (denoted by the Full-Width at Half Maximum).

See also Filter, Full Width at Half Maximum (FWHM), Bandwidth

Charge-Coupled Device (CCD)

Silicon chip containing an array of photosensitive sites. The imaging area consists of rows and columns of photosensitive pixels that accumulate and store electric charge. The term CCD refers to the way the photoelectron packets are moved around on the chip, the shift registers. Each column is separated by a shift register, to which pixel charge is transferred. Charge is read out from each pixel of the device through the shift register sequentially to an amplifier. The raw analog signal is either used directly, or quantized by an A/D converter.

See also Complementary Metal Oxide Semiconductor (CMOS), Pixel, Pixel Clock, Pixel Depth

Chief Ray

In geometric optics, the chief ray defines the size of the paraxial object, location of pupils, and size of the paraxial image. It begins at the edge of the object and goes through the center of the entrance pupil, exit pupil, and the aperture stop.

See also Paraxial Rays, Marginal Ray

Chromatic Focal Shift

A measure of axial chromatic aberration, quantifying the change in position along the optical axis where different wavelengths will focus.

See also Longitudinal Chromatic Aberration (LCA), Transverse Chromatic Aberration (TCA), Aberration, Pellicle Beamsplitter, Dispersion

Circular Polarizer

A polarizing component that uses a linear polarizer in combination with a retarder (waveplate). The retarder is used to introduce a wave shift between the orthogonal components of the polarized light. In this way, two equal components of light oscillating perpendicular to each other with a relative phase difference add as vectors to yield a rotating linear polarization state. If the amplitude components of the orthogonal states are not equal, the vector addition of the two components will produce an elliptical polarization. For this reason, the linearly polarized light is input into the retarder at 45° to the fast axis to ensure that both components are of equal amplitude.

See also Polarization, Linear Polarizer, Retarder (Waveplate)


A low refractive index material that surrounds the core of an optical fiber. The difference in index between the core and the cladding contains light traveling through the core, while also serving to protect the fiber against surface contaminant scattering.

See also Bend Radius, Buffer Material, Optical Fiber, Index of Refraction (n)

Clear Aperture (CA)

The unobscured portion of an optic or mechanical component which has a limited area through which light can pass. CA takes into account mounting rings, etc. on optics which reduce the usable area.


A camera and lens standard with a 17.526mm flange distance and mounting threads of 1" x 32 TPI.

See also CS-Mount, T-Mount, F-Mount, Back Flange Distance, S-Mount

Coefficient of Thermal Expansion (CTE)

A measure of the rate of change of the material's size due to a change in temperature.

Coherent Light

With respect to lasers, the amplifying laser medium produces coherent light because the process of stimulated emission produces additional photons with the same phase as the originator. Noise within the laser output is the result of spontaneous emissions which do not have the same characteristics as the stimulated emission. During oscillation, the spontaneous emission component will also be amplified and appear in the output as random noise.

See also Laser

Cold Mirror

A mirror-like filter which transmits infrared light while reflecting visible light. See also Hot Mirror.

See also Filter, Hot Mirror, Dielectric Coating

Collimated Light

A ray bundle, or beam of light, in which every ray is parallel to every other ray of light, which effectively originates from an infinitely small point source located at infinity. Note that light is never perfectly collimated.

See also Off-Set Angle, Collimation, Autocollimator


Converting a diverging or converging beam of light into a parallel beam of light.

See also Autocollimator, Collimated Light

Color Sensor

A camera sensor that uses a mosaic filter to separate incoming light into a series of colors. Each color is then directed by a different set of pixels. In the filtering process some spatial resolution is lost because the sensor is not using every pixel for every color. See also Bayer Filter.

See also Monochrome Sensor

Color Temperature

Colorimetric parameter associated with light sources (compared with blackbodies) that relates to apparent visual color. Expressed in degrees Kelvin.


A method used to measure color and to define the results of the measurement. It includes many factors such as an object's physical characteristics, the surrounding colors, the light source, and the sensor or viewer.


An optical aberration that yields a variation in magnification (and therefore image size) across the FOV, causing an asymmetric blur for features in the image.

See also Aberration

Complementary Metal Oxide Semiconductor (CMOS)

Complementary metal-oxide semiconductor sensor. A type of imaging sensor in which the charge-to-voltage conversion is done at each individual pixel and the signal is read out by a multiplexing to an A/D converter. Can yield a less uniform output and very low power dissipation compared to CCD, but at much higher speeds due to lack of shift registers.

See also Binning, Charge-Coupled Device (CCD), Pixel

Compound Parabolic Concentrator (CPC)

An optical component which is designed to collect and concentrate distant light sources efficiently. CPCs are used in solar energy collection, wireless communication, biomedical and defense research among other applications requiring condensing a divergent light source.

Computer Controlled Precision Polishing

Polishing technique for aspheric lenses which automatically adjusts the tool dwell parameters to polish away high spots where more polishing is needed.


For rotary stages, the maximum variance between a perfect circle and the path that the stage follows.

Condenser Lens

A positive lens or set of lenses designed to collect light and project it evenly across an area. Commonly found in projector systems as part of the condenser lens portion.

Confocal Microscopy

Microscopy technique using spatial filtering to eliminate out-of-focus light in specimens that are thicker than the plane of focus. Allows for controllable depth of field and built-up 3D representations by z-sectioning of an object.

Conjugate Distance

The distance along the optical axis of a lens from the principle plane of that lens to either the image plane (for an image conjugate) or the object plane (for an object conjugate). Lenses designed to focus at infinity have an infinite conjugate distance for the object and a finite conjugate distance for the image.

See also Principal Plane, Finite/Finite Conjugate, Infinite/Finite Conjugate, Infinite/Infinite Conjugate

Continuously Variable Apodizing Filter

A type of neutral density filter with a gradient of optical densities which vary radially from the center. There are two configurations: high optical density in the center that decreases continuously to an uncoated edge, and high optical density on the edge that continuously decreases to an uncoated center.

See also Optical Density (OD), Neutral Density Filter (ND)


Comparison of shades of gray that define an object and its background.

See also Depth of Field (DOF), Modulation Transfer Function (MTF), Resolution

Contrast, Optical

The quantified difference between adjacent features of a specific size for an object, as imaged through a lens.

Counterbore (C'bore)

A cylindrically enlarged end of a hole. Typically used to accept the head of a socket head cap screw, making the screw head accessible but not obstructive.


Loss of optical energy between two optical conductors.

See also Optical Fiber

Crown Glass

One of two types of optical glass used in the manufacturing of achromatic lenses. Crown glass is harder than flint glass and has a lower index of refraction and lower dispersion.

See also Achromatic Lens, Flint Glass


A camera and lens standard with a 12.526mm flange distance and mounting threads of 1" x 32 TPI. CS-mount lenses will not work with C-mount cameras.

See also T-Mount, F-Mount, C-Mount, Back Flange Distance, S-Mount

Cube Beamsplitter

A type of beamsplitter constructed of two cemented right angle prisms, one with a broadband multi-layer dielectric coating on the hypotenuse. 50% of incident light is transmitted, and 50% is reflected. Outside surfaces have an anti-reflection coating to reduce back reflections. No beam displacement occurs between the original and separated beams. The reflected and transmitted beams travel through the same amount of glass, so although the optical path length of each arm is increased, both paths are increased by the same amount. Their cubic shape makes cube beamsplitters easy to mount, thus suffering less from deformation due to mechanical stress.

See also Beamsplitter, Plate Beamsplitter, Anti-Reflection (AR) Coating, Dielectric Coating

Cut-Off Wavelength

In filter terminology, the wavelength at which the transmission decreases to 50% throughput in a shortpass filter. In fiber optics, the shortest wavelength at which a fiber transmits in single mode. Below the cut off several modes will propagate and the fiber is no longer single-, but multimode. In detector technology, the wavelength at which the detector response falls to a set percentage (usually 20 or 50 percent).

See also Filter, Cut-On Wavelength

Cut-On Wavelength

In filter terminology, the wavelength at which the transmission increases to 50% throughput in a longpass filter.

See also Filter, Cut-Off Wavelength

Cylinder Lens

A type of lens that focuses light in only one dimension. Its profile is similar to a PCX lens (though PCV profiles also exist) and it can transform a point of light into a line image.

See also Plano-Convex (PCX) Lens, Plano-Concave (PCV) Lens