Glossary: A


Also known as Abbe constant, it inversely quantifies the amount of dispersion in an optical medium. It is given by a function of the refractive index of a material at the f (486.1nm), d (587.6nm), and c (656.3nm) wavelengths. A material with a high Abbe number means that the different wavelengths will have nearly the same index of refraction in that medium, yielding less separation between wavelengths of light.


Deviation from perfection in an optical system that results as a product of imperfect ray bending to form an image. Aberrations are inherent in all optical systems. Chromatic aberration, spherical aberration, coma, astigmatism, field curvature and distortion are the most commonly referred to aberrations.

Absolute Efficiency

Regarding diffraction gratings, it is the actual percentage of incident monochromatic light diffracted into a specific order. Also referred to as grating efficiency or relative efficiency.

  See also Diffraction Grating


Light energy lost through transformation to another form, such as heat, while passing through a material.

Acceptance Angle

The maximum angle within which an optical fiber, electro-optic detector, or other component can collect light.

Achromatic Lens

Correction for primary chromatic aberration at two wavelengths. The simplest form is an achromatic doublet, consisting of two optical elements, a positive crown glass element and a negative flint glass element, cemented together.

Achromatic Objective

A type of microscope objective that has a flat field in approximately 65% of the center of the image, compared to the 80% of a Semi-Plan objective and the 95% of a Plan objective. They correct for chromatic aberration in the red and blue wavelengths and spherical aberration in the green wavelength.

Achromatic Singlet Lenses

Novel aspheric lenses that correct for chromatic focal shift using a unique geometry rather than multiple substrate materials connected together. This unique design has been shown to have less chromatic focal shift on axis than traditional achromatic doublets by a factor of 3 and plano-convex (PCX) singlet lenses by a factor of almost 40. This performance degrades off axis. Browse product options or learn more here.

Airy Disk

The resultant diffraction pattern created after a point source of light is imaged perfectly through an aperture, consisting of a bright central core surrounded by diffraction rings.

Amici Prism

Also known as a right angle roof prism, it inverts the image and changes the line of sight by 90°, still having left to right correct. It is essentially a right angle prism with a roof.

  See also Prism


An electro-optical device for increasing the incident signal from a photodiode, photmultiplier, or similar current/voltage source.

  See also Photodiode

Analog Signal

A signal that continuously changes over time with respect to a reference level or standard. Analog-to-digital signal conversion involves sampling an analog signal at high frequency and representing each sample level by a number, stored as binary data. Analog cameras output analog video signals unlike digital cameras, in which case the analog-to-digital conversion takes place in the camera rather than in a computer.


Polarizing filter placed over the objective lens in an imaging system, used in conjunction with polarized illumination, allowing only light of a particular polarization to be imaged.

Angle of Incidence

The angle formed between a ray of light and the normal point of incidence to the surface it approaches.

Angle of Reflection

The angle formed between a ray of light and the normal point of incidence to the surface it leaves.

  See also Angle of Incidence

Angle Tolerance

The maximum angle between surfaces measured using an autocollimator assembly.

Angular Field of View (AFOV)

The angle of a lens’s view cone, typically specified as the full angle (in degrees) with respect to the horizontal sensor dimension.  Used to determine FOV at a given working distance.


The chemical oxidation of the surface of aluminum components to prevent corrosion and increase abrasion resistance. Black anodizing is not only cosmetically appealing, but also does not reflect light in optical, imaging and photonics systems.

Anti-Reflection (AR) Coating

A type of coating applied to optics designed to minimize reflections within an optical system and maximize throughout.

  See also Cube Beamsplitter

Aperture (f/#)

Also denoted by f-number (or f/#) and f-stop. In optics, it refers to the ratio of the focal length to diameter; whereas, in imaging, it refers to the ratio of the focal length to exit pupil of the system. A low f/# (fast lens) has high light collecting ability while a high f/# (slow lens) has low light light collecting ability.

Aperture Stop

The limiting aperture that defines how much light is allowed through an optical system. It can be an optical lens surface or an iris, but it is always a physical surface.


Correction for primary chromatic aberration at three wavelengths.

Argon-Ion Laser

A type of gas laser that uses argon ions as the amplification (lasing) medium. Although argon-ion lasers are generally larger and less efficient than Helium Neon (HeNe) lasers, they can be used to produce a wider variety of wavelengths.

Aspect Ratio

The ratio of width to height in a video device. NTSC, PAL, EIA, and CCIR video signals use a 4:3 aspect ratio.

Aspheric Lens

A type of lens that is not spherical in construction. Its non-spherical geometric shape allows it to correct for spherical aberrations, inherently present in spherical optics.

  See also Hybrid Molding


The loss of average optical power, usually given as decibels (dB) per unit distance.

  See also Decibel (dB)


A single instrument combining the functions of a telescope and a collimator to detect small angular displacement of an optical flat by means of its own generated collimated light.

  See also Collimation , Collimated Light

Automatic Gain Control (AGC)

Used in cameras to maintain a constant average grayscale value despite changes in light levels by automatically adjusting the signal gain value.

  See also Gain

Average Extinction

When two polarizers are placed in front of a light source with transmission axes at 90 degrees to each other, the theoretical transmission should reach zero percent of the input. Because real polarizers will not reach this limit, average extinction is cited as a measure of how closely polarizers approach the theoretical limit.

Axial Illumination

Refers to light that strikes an object along the optical axis, typically introduced by a beamsplitter in front of the lens.

Axial Runout

For rotary stages, this is the amount of vertical motion of the stage as it rotates, measured with respect to the center of rotation.


A conical prism, defined by its alpha and apex angles, that focuses a light source to a line consisting of multiple points along the optical axis. A beam generated by an axicon crosses the optical axis and forms a ring of increasing diameter over distance while maintaining a constant ring thickness.

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