An optical wavelength analyser including: an entrance slit (4) for receiving a light beam (3) including signals with various wavelengths and passing the beam at least partly; a diffractor (6, 7, 9) for receiving the passed beam and diffracting the signals dependent on their wavelength; a detector (8) including adjacent detector elements (32, 33, 35, 36, 38, 39) for receiving the diffracted signals and generating their output signals; a processor (21) for determining the wavelengths from the output signals, in which the received light beam has a spatially uniform intensity; the diffractor diffracts each signal on a different detector element subset, consisting of at least a first element (32, 33, 35, 36, 38, 39.) for receiving at least a first signal with a first signal level; the processor determines each signal's wavelength dependent on the first signal level and a calibration value.