Incoming light is spectrally analyzed by diffracting the incoming light with a grating. At least a part of the incoming light is split off so that this part contains mainly one polarization component of the incoming light. It is ensured that this split-off part and a remaining part of the incoming light reach the grating with their polarized component mainly parallel to a main direction of polarization which is diffracted with maximal efficiency by the grating. For this purpose, at least the split-off part is diffracted after being passed through a polarization rotating element.