(also known as v-number)
a measure of the materials dispersion (=variation of refractive index with wavelength),
with high values of V indicating low dispersion (low chromatic aberration).
The value Vd is given by
which defines the Abbe number with respect to the yellow Fraunhofer-Line d (or D3) helium line at 587.5618 nm wavelength.
It can also be defined using the green mercury E-line at 546.073 nm:
where F’ and C’ are the blue and red cadmium lines at 480.0 nm and 643.8 nm, respectively.