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Principal refractive index wavelength ambiguity

Because of dispersion, it is usually important to specify the vacuum wavelength of light for which a refractive index is measured. Typically, measurements are done at various well-defined spectral emission lines.

Manufacturers of optical glass in general define principal index of refraction at yellow spectral line of helium (587.56 nm) and alternatively at a green spectral line of mercury (546.07 nm), called d and e lines respectively. Abbe number is defined for both and denoted V and V. The spectral data provided by glass manufacturers is also often more precise for these two wavelengths.

Both, d and e spectral lines are singlets and thus are suitable to perform a very precise measurements, such as spectral goniometric method.

In practical applications, measurements of refractive index are performed on various refractometers, such as Abbe refractometer. Measurement accuracy of such typical commercial devices is in the order of 0.0002. Refractometers usually measure refractive index n, defined for sodium doublet D (589.29 nm), which is actually a midpoint between two adjacent yellow spectral lines of sodium. Yellow spectral lines of helium (d) and sodium (D) are 1.73 nm apart, which can be considered negligible for typical refractometers, but can cause confusion and lead to errors if accuracy is critical.

All three typical principle refractive indices definitions can be found depending on application and region, so a proper subscript should be used to avoid ambiguity.