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Glass
Index and Abb¨¦ Number values are the most important
criteria in comparing one material to the next. The index
of refraction is actually a ratio of the speed of light in
a vacuum to that of light in a medium (i.e., a specific type
of glass). Since the speed of light in any glass varies with
the wavelength of light, the index of refraction also changes
with wavelength. Typically, a glass is defined at nd, which
is the index at yellow helium or 587.6nm.
Dispersion,
or spectral variations in index of refraction, results in
differences of focal distances for light of different wavelengths.
This means that even though a lens will transmit a particular
wavelength, if it was not designed at that wavelength then
the performance will not be the same as that stated for the
design wavelength. The Abb¨¦ number (vd)
quantifies the amount of dispersion for a particular frequency
range. This defines how much index changes with wavelength
and the smaller the value means the quicker the change; vd=(nd-1)/(nF-nC),
where nF=486.1nm and nC=656.3nm. Glasses
are typically defined as either crowns or flints. Crown glasses
have the following combination of values: nd<1.6
and vd>55
or nd>1.6 and vd>50.
Flints define the rest and are typically referred to as high
index glass.
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