A gentleman named Chester Moor Hall made the first achromatic lens in 1729, by combining a concave flint element with a convex crown element. He reasoned that combining two glasses of opposite powers would eliminate the primary color error of a single lens. We know the result wasn't perfect, and resulted in secondary color - a difference of about 0.05 percent in the blue and red focus and the minimum focus in green.
A typical achromat is made with BK7 (crown) glass and F2 (flint) glass. If you made prisms of the two glasses, the F2 prism would spread out the spectrum about 75 percent more than the BK7 prism.
Why does the pair not provide perfect color correction? If you put the two spectra from the prisms side by side, and stretched the smaller BK7 spectrum to match the length of the F2 spectrum you would see the answer - the two do not match perfectly. Dispersion does not vary in a uniform manner, and varies from glass to glass. (I've seen this called the "irrationality of dispersion.")
For perfect color correction, you need glasses where the two spectrums would match exactly along their length when stretched to the same scale.
Clear skies, Alan
A typical achromat is made with BK7 (crown) glass and F2 (flint) glass. If you made prisms of the two glasses, the F2 prism would spread out the spectrum about 75 percent more than the BK7 prism.
Why does the pair not provide perfect color correction? If you put the two spectra from the prisms side by side, and stretched the smaller BK7 spectrum to match the length of the F2 spectrum you would see the answer - the two do not match perfectly. Dispersion does not vary in a uniform manner, and varies from glass to glass. (I've seen this called the "irrationality of dispersion.")
For perfect color correction, you need glasses where the two spectrums would match exactly along their length when stretched to the same scale.
Clear skies, Alan
