Wavelength calibration of optical spectrometers

In a method of determining a wavelength calibration function for an optical emission spectrometer with cross dispersion which describes a mapping relationship between a detected position (x, y) of a spectral line and the wavelength lambda and order m of the spectral line, an error correction function f(x,y) is determined by observing differences between a detected dispersion of a known emission spectrum covering a substantial part of the spectrometer detection range and a predicted dispersion of the corresponding emission spectrum calculated using an ideal calibration function mapping (x,y) -> O(m, lambda ) which describes the theoretical or approximated performance of the spectrometer. The wavelength calibration function is defined by the mapping (x,y) -> O(m, lambda ) + f(x,y).