Sellmeier equation can be used. It is an empirical formula that works well in describing dispersion. Sellmeier coefficients are often quoted instead of the refractive index in tables. 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 Vd and Ve. The spectral data provided by glass manufacturers is also often more precise for these 2 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 nD, defined for sodium doublet D (589.29 nm), which is actually a midpoint between 2 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 3 typical principle refractive indices definitions can be found depending on application and region, so a proper subscript should be used to avoid ambiguity. Complex refractive index When light passes through a medium, some part of it will always be absorbed. This can be conveniently taken into account by defining a complex refractive index, Here, the real part n is the refractive index and indicates the phase velocity, while the imaginary part κ is called the extinction or absorption coefficient—although κ can also refer to the mass attenuation coefficient—and indicates the amount of attenuation when the electromagnetic wave propagates through the material. That κ corresponds to absorption can be seen by inserting this refractive index into the expression for electric field of a plane electromagnetic wave traveling in the x-direction. This can be done by relating the complex wave number to the complex refractive index through , with λ being the vacuum wavelength; this can be inserted into the plane wave