Glossary

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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), Angle of Incidence , Glass Code, Angle of Reflection, Dispersion

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

Amplifiers

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

See also Photodiode

Angle of Incidence

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

See also Total Internal Reflection (TIR), Angle of Reflection

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.

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.

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 Double Gauss Design, Iris Diaphragm, Aperture Stop

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.

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 Collimated Light, Collimation

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 Diffuse Illumination, Directional Illumination, Glancing Illumination, Darkfield 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.

Axicon

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.