A process for purifying N-vinyl–caprolactam comprises converting the N-vinyl–caprolactam which is to be purified and has a purity of at least 95% by weight into a melt, partially crystallizing the melt and separating the crystals from the mother liquor. Another embodiment is a unique desulfurization system. The light rays reflected from the member surface pass through the element such that the illuminated member surface can be observed. Said flow rectifier rectifies the as yet non-directional flow of the water in the inlet compartment and guides it in a controlled manner to the outlet compartment from where it emerges through the slotted outlet opening. In this way, interference during the signal scan from the reference source system is reduced.