The invention relates to a method of calibrating an optical system, the system comprising a flash lamp as a light source (4), a lens system (6), a monochromator comprising a grating (10), a motor (20) for displacing the grating so as to enable scanning essentially monochromatic light over a detection system. The light source provides at least two high intensity peaks at distinct wavelengths. The method comprises scanning a first wavelength region comprising at least one of said at least two high intensity peaks, and measuring the intensities at a selected number of points during the scan. A first of said at least two peaks is coarsely located. A wavelength region around each of said at least two peaks is scanned, measuring the intensities at closer intervals than previously. At least two peaks are located by autocorrelation. The location of said peaks is determined in terms of a distance from a reference point, said distance corresponding to said displacement of the grating.