constringence

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constringence 

A positive number (symbol: V) which specifies any transparent medium. It is equal to
V = nd − 1/nFnC
where nD, nF and nC are the refractive indices for the Fraunhofer spectral lines d (587.6 nm), F (486.1 nm) and C (656.3 nm). A material with a high constringence (e.g. V = 50) produces less chromatic aberration than one with a low constringence (e.g. V = 30). The reciprocal of the constringence is called the dispersive power. Syn. Abbé's number; V-value. See dispersion; crown glass; flint glass; Fraunhofer's lines.
Table C8 Constringence (V) of some transparent media (index of refraction n)
nV
glass
spectacle crown1.52359
dense barium crown1.62060
lanthana crown1.71354
zinc crown1.50861
light flint1.58141
dense flint1.62036
extra dense flint1.70630
titanium oxide1.70131
polymer
PMMA1.49257
CR-391.49859
Polycarbonate1.58630
Polystyrene1.59031
Water (at 37ºC)1.33156
Aqueous and vitreous (at 37ºC)1.33456
References in periodicals archive ?
In Asia and the majority of Europe the market penetrance of this material is subdued by its low V-value, poor chemical resistance, and susceptibility to cracking, particularly on contact with metal.
6, prior to the development of the next generation, MR8, with a higher V-value.
It has a creditable V-value of 33 but can appear to have a yellowish tinge to the lens, is more brittle when drilled, and has a distinctive, unpleasant odour when being edged.
Polycarbonate, therefore, is not a suitable material as it has a low V-value of 30 which would cause problems (during play in terms of the player being unable to distinguish between the coloured balls due to coloured fringing.