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Definitions: V

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v-coat
A multilayer antireflective thin-film coating, so called because its reflectance rises steeply at wavelengths above and below a narrow band at zero. The v-coat is particularly useful in high-power...
V-groove
A V-shaped channel pressed or etched into a substrate, in which, for example, optical fibers may be placed to create an integrated optical component.
V-number -> normalized frequency
Also referred to as the V number in fiber optics; the normalized frequency is a dimensionless quantity, denoted by the symbol V, given by: where a is waveguide core radius, l is wavelength in vacuum,...
V-value -> Abbe constant
A dispersion relation defined in order to value the reciprocal amount of dispersion. It is defined as the refractivity over the difference in index values of the shortest and longest visible...
vacancy
In an ionic crystal, the region in the crystal lattice where the ion, predicted to be present, is absent.
vacuum
In optics, the term vacuum typically refers to a space devoid of matter, including air and other gases. However, in practical terms, achieving a perfect vacuum, where there is absolutely no matter...
vacuum apparatus
Equipment dependent on the effects of a vacuum. The principal applications in optics are in the coating of lenses and substrates and the fabrication of multilayer interference filters.
vacuum breaker
A valve that serves to release air into an evacuated system.
vacuum chamber
A vacuum chamber is a sealed enclosure from which air and other gases are removed to create a low-pressure environment, typically close to or at a complete vacuum. These chambers are often...
vacuum etching
Also known as cathodic etching. Surface etching achieved by bombarding an evacuated surface with gas ions.
vacuum gauge
A gauge designed to measure the degree of vacuum in an evacuated vessel. A simple U-tube containing mercury is adequate for low vacua, but for high vacua other methods must be employed. See McLeod...
vacuum phototube
A phototube that functions within a vacuum and thus eliminates the effects of gaseous ionization on its electrical properties.
vacuum spectrograph
A spectrograph that functions in a vacuum and therefore eliminates any air-absorption of the emission being surveyed.
vacuum spectrography
The technique of producing spectrograms in wavelengths beyond 120 nm by the use of a diffraction grating and a Schumann plate in a vacuum environment.
vacuum ultraviolet detector
A device that serves to detect the presence of vacuum ultraviolet radiation. It may be photographic film, a thermopile, ion chamber, vacuum photodiode, photomultiplier or channel electron multiplier.
vacuum ultraviolet radiation
Radiation whose spectrum runs between 100 and 300 nm. Any work with these wavelengths requires evacuated equipment.
vacuum ultraviolet source
Any source that emits radiation of wavelengths between 100 and 300 nm. Instruments used to study these sources must be evacuated, as vacuum ultraviolet does not travel freely in air.
vacuum ultraviolet spectroscopy
A method of spectral analysis using wavelengths between 100 and 300 nm that utilizes both emission and absorption techniques.
valence band
In a crystalline substance, the spectral range of states of energy that contains the crystal's binding valence electrons.
valence crystal
A crystal that is held together by covalent bonds.
Van der Waals
Van der Waals forces refer to the attractive or repulsive forces between molecules (or parts of molecules) that arise from fluctuations in electron distribution. These forces are relatively weak...
vanadate laser
Lasers based on neodymium-doped yttrium or gadolinium vanadate crystals. These include yttrium vanadate (Nd:YVO4), gadolinium vanadate (Nd:GdVO4), and lutetium vanadate (Nd:LuVO4). Vanadate lasers...
vapor degreasing
A method of cleaning and drying coated optical components. The cooled parts are placed in a container above a boiling solvent, whose vapors condense on the parts. When the parts reach the temperature...
vapor-phase axial deposition
A process by which high-quality fiber optics are made. See axial vapor-phase deposition.
vapor-phase epitaxy -> epitaxy
A well controlled thin films technique for growing films with good crystal structure in ultra high vacuum environments at very low deposition rates. Epitaxy methods are well known for the growing of...
vapor-phase oxidation
See inside vapor-phase oxidation; outside vapor-phase oxidation.
varactor
A semiconductor diode that exhibits change in capacitance with a change in applied voltage; used as a voltage-variable capacitor.
variable-focus condenser
An Abbe condenser in which the upper lens element is fixed. The lower lens may focus the illumination between the elements so that it emerges from the fixed lens as a large-diameter parallel bundle....
variable-focus lens
A lens assembly containing several movable elements to permit changing of the effective focal length (EFL). Unlike a zoom lens, a variable-focus lens may require refocusing with each change of EFL....
variable-speed scanning
A scanning technique in which the optical density of the film being scanned controls the speed of deflection of the scanning beam.
varifocal -> variable-focus lens
A lens assembly containing several movable elements to permit changing of the effective focal length (EFL). Unlike a zoom lens, a variable-focus lens may require refocusing with each change of EFL....
vector correlation
A machine vision technique of image correlation whereby the correlation kernel (template of the desired image) is transformed into a group of vectors, each representing a specific feature of the...
Vegard-Kaplan bands
The bands found by Vegard in the spectrum of the aurora borealis and by Kaplan in the nitrogen afterglow. They are formed by the effect of metastable nitrogen molecules.
veiling brightness
A brightness, superimposed on the image of the retina, that decreases its contrast and that often results in decreased visibility.
veiling glare
Diffuse stray light at the image plane of an optical system that results in reduced contrast and resolution.
velocimeter
See laser Doppler velocimeter; laser velocimeter.
velocity modulation laser spectroscopy
A method of measuring negatively charged phase ions using a color-center or lead-salt diode laser.
venetian-blind effect
Short-distance scattering of light in holography caused by random index inhomogeneities and the developing index that interferes with the incident beam, thus recording a hologram of the scattered...
Verdet constant
A factor of an equation of the Faraday effect, which is the rotation of the plane of light polarization by transparent materials in a magnetic field. Therefore, in the equation: where a...
vergence
The angular relation between two light rays that originated at the same object point. Sometimes used to indicate the angle between the visual axes of the eyes.
vernier acuity
The degree to which a pair of fine lines can be aligned to each other. A normal observer will demonstrate an accuracy of 10 arc seconds and a repeatability of 5 arc seconds.
vernier interferometer
A phase-shift interferometer used to detect the relative angular speeds or positions of two concentric rotors.
vertex
The point of intersection of the optical axis with any centered optical surface.
vertex power
The reciprocal of the back focal distance of a spectacle lens.
vertical air photograph
An aerial photograph produced when the optical axis of the camera is perpendicular to the surface below.
vertical blanking
With respect to a television system, a pulse transmitted at the end of each field to break off the cathode-ray beam as it begins the next field.
vertical external cavity surface-emitting laser
A laser that has a surface-emitting semiconductor gain element coupled to an external mirror designed to complete the laser resonator. VECSELs can be electrically or optically pumped. The extended...
vertical imbalance
The difference in base up or down prism power at corresponding points located on the two lenses of a pair of spectacles.
vertical incident illumination
With respect to microscopy, light that is directed down through the objective onto the specimen and then returned by reflection.
vertical leveling mirror
A two-sided mirror suspended by a pendulum mechanism. An autocollimator set normal to such a mirror surface will, by definition, have its optical axis set horizontal.

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