2 A B C D E F G H I K L M N O P R S T V W


(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

V_d = \frac{n_d-1}{ n_F - n_C }

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:

V_e = \frac{n_e-1}{ n_{F'} - n_{C'}}

where F’ and C’ are the blue and red cadmium lines at 480.0 nm and 643.8 nm, respectively.