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Index Tolerance

Each specific glass manufacturer publishes a listing of the index number for the glasses that they manufacture. Designers frequently place tolerances on these values to insure that the components meet their system specifications. The standard value for index is measured at 587.56 nm(nd). The standard Abbe Vd value (despersion value) is calculated by the following formula:
Example BK-7 (516.642) Nd = 1.51680 (587.6λ)
Nf = 1.52238 (486.1λ)
Nc = 1.51432 (656.3λ)
Vd = Nd - 1 = Nd (1.51680) - 1 = 64.17


Nf - Nc Nf (1.52238) - Nc (1.51432)
Thus, 516 refers to Nd
642 refers to Vd
BK-7 (517.642)