Abstract: Known optical noise sources are equipped with a light source and an interference filter, optically coupled thereto and based on path length difference, for generating an optical composition signal. If the spectrum of a photodiode to be illuminated with the optical composition signal is to be as flat as possible, said path length difference must be considerably greater than the coherence length of the light source which in present practice leads to disadvantageously large path length differences. By feeding, according to the invention, a noise signal to the light source, the coherence length of the light source decreases, as a result of which it is sufficient to use considerably smaller path length differences.