A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Abbe-Number

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 different wavelengths will have nearly the same index of refraction in that medium, yielding less separation between wavelengths of light.

See also Index of Refraction (n)

Aberration

Imperfections in a transmitted image compared with that predicted by paraxial limits. Types of aberrations include chromatic, tilt, defocus, spherical, coma, astigmatism, field curvature and distortion.

See also Field Curvature, Spherical Aberration, Chromatic Focal Shift, Distortion, Coma

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.

Absorption

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

Also known as an achromatic doublet, it consists of two optical elements, usually one of crown glass and the other of flint glass, cemented together. It corrects for chromatic aberrations.

See also Crown Glass, Flint Glass, Doublet Lens

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.

See also Wide-Field Eyepiece, Semi-Plan Objective, Plan Objective

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.

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.

See also Digital Signal

Angle Tolerance

The maximum angle between surfaces measured using an autocollimator assembly.

Anodization

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 back reflection and maximize throughput. AR coatings are available in single wavelength, laser-line, and broadband options.

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.

See also Iris Diaphragm

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.

See also Helium Neon (HeNe) Laser

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.

Attenuation

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

See also Decibel (dB)

Autocollimator

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 video signal output when light levels become very low, especially below minimum sensitivity. Applying gain to a signal lowers the signal-to-noise ratio by intensifying noise as well as the signal.

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

Also known as diffuse axial illumination. Refers to light that strikes an object along the optical axis, generally accomplished by using a beamsplitter.

See also Directional Illumination, Diffuse Illumination, Darkfield Illumination, Glancing Illumination

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.