Optical Dictionary


In optics, defects of a lens system that cause its image to deviate from the rules of paraxial imagery.


A lens consisting of two or more elements, usually of crown and flint glass, that has been corrected for chromatic aberration with respect to two selected wavelengths. Also known as achromatic lens.

Anti-reflection coating

A thin layer of material applied to a lens surface to reduce the amount of reflected energy.


Not spherical; an optical element having one or more surfaces that are not spherical. The spherical surface of a lens may be slightly altered so as to reduce spherical aberration.


A lens aberration that results in the tangential and sagittal image planes being separated axially.

Back focal

The distance from the last surface of a lens to its image plane.


An optical device for dividing a beam into two or more separate beams.

Broadband coating

Coatings that deal with a relatively wide spectral bandwidth.


The amount of deviation of the optical axis of a  lens from its mechanical axis.

Cold mirror

Filters that transmit wavelengths in the infrared spectral region (>700 nm) and reflect visible wavelengths.

Dielectric coating

Coating consisting of alternating layers of films of higher refractive index and lower refractive index.

Diffraction limited

.The property of an optical system whereby only the effects of diffraction determine the quality of the image it produces.

Effective focal

The distance from the principal point to the focal point.

F number

The ratio of the equivalent focal length of a lens to the diameter of its entrance pupil.


Full width at half maximum.

Infrared IR

Wavelength above 700 nm, invisible to the eyes.


The intense beams of light that are monochromatic, coherent, and highly collimated.

Laser diode

A light-emitting diode designed to use stimulated emission to form a coherent light output.


The ratio of the size of the image of an object to that of the object.

Multilayer coating

A coating made up of many layers of material having alternating high and low refractive index.

Neutral density filter

Neutral-density filters attenuate, split, or combine beams in a wide range of irradiance ratios with no significant dependence on wavelength.

Numerical aperature

The sine of the angle made by the marginal ray of a lens with the optical axis.


The optical element that receives light from the object and forms the first or primary image in telescopes and microscopes.

Optical axis

The line passing through both the centers of curvatures of the optical surfaces of a lens.

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.


Characteristic of optical analyses that are limited to infinitesimally small apertures.


Having coincident focal points.


A small sharp edge hole, used as an aperture or eye lens.


An expression of the orientation of the lines of electric flux in an electromagnetic field.


Return of radiation by a surface, without change in wavelength.


The bending of oblique incident rays as they pass from a medium.

Refractive index

The ratio of the velocity of light in a vacuum to the velocity of light in a refractive material for a given wavelength.


The height of a curve measured from the chord.

Spatical filter

The height of a curve measured from the chord.


An imperfection in optical glass consisting of a distinct streak of transparent material having a slightly different refractive index from the body of glass.

Telecentric lens

A lens in which the aperture stop is located at the front focus, resulting in the chief rays being parallel to the optical axis in image space; i.e., the exit pupil is at infinity.


A compound lens so constructed that its overall length is equal to or less than its effective focal length.


Rays internally incident upon an air/glass boundary at angles greater than the critical angle are reflected with 100% efficiency regardless of their initial polarization state.


In optics, the conduction of radiant energy through a medium.


The invisible region of the spectrum below 380 nm.

V Coat

An anti-reflection for a specific wavelength with almost 0 reflection, so called due to the V-shape of the scan curve.


The decrease in illumination away from the optical axis in an optical system caused by clipping of off-axis rays by apertures in the system.

Wavefront deformation

Departure of the wavefront from ideal sphere due to the design limitation or surface quality.


Waveplates, also known as retardation plates, are birefringent optical elements with two optic axes, one fast and one slow. Waveplates produce full-, half- and quarter-wave retardations.


An optical element having plane-inclined surfaces.