Defining Reference Wavelengths: Is This Exercise Really Necessary?

Although we have written about the need for a reference wavelength in optics before, it is perhaps time to revisit it. There is a standard, ISO 7944, that defines two reference wavelengths for optics: the He "d" (yellow) line at 587.6 nm, and the Hg "e" (green) line at 546.1 nm. This standard is nearing another mandatory five-year review, and some parties want to see the Hg "e" line adopted exclusively as the reference wavelength: Some ophthalmic laboratories are rather upset with this idea because the diopter has been defined (historically) using the Na "D" line at 589.3 nm and a glass with an index of 1.53 at that wavelength. This means that if one desires to produce a plano-convex lens with a power of 1 diopter using a glass of index 1.53, the lens would have a 530 mm radius curve to give the required 1 meter focal length for light at 589.3 nm.

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