When you choose your right optical lenses, you need to know your exact requirement on transmission rate and reflection rate. And what will effect them? It needs to be traced back to the raw material – optical glass. It is just the values of “VD” and “ND”. They play a key role.
Then what do they mean? How to understand them?
VD stands for “Abbe number” . It is one material’s dispersion measurement. In other words, how much the refractive index of the material varies with wavelength. And use Abbe number to show different refractive index. With higher Abbe number, this material has larger dispersion. That is to say, such a material separates different colors of light more strongly. On the contrary, With lower Abbe number, this material has smaller dispersion. That is to say, such a material separates different colors of light less strongly.
ND stands for ” Refractive number” . It is a measure of how much light is bent when it passes through one kind of material. With higher Refractive index, this material bends light more strongly. On the contrary, With lower Refractive index, this material bends light less strongly.
Such two values are important, optical glass is often characterized by them. since they affect the optical glass’s behaviour in one optical system. Take one optical lens made of glass with high VD value as one example, it will suffer from chromatic aberration, which means that different colors of light will focus at slightly different points, resulting in blurred images. In order to reduce chromatic aberration, the glass with low VD is often used.
Similarly, the refractive index of the glass affects the power of the lens. Lenses made of glass with high refractive index can be made thinner and lighter for the same optical power as lenses made of glass with low refractive index.
Of curse, exact and appropriate choice are decided by actual application and system.
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