A self-cleaning optical system for the transmission or reflection of light to and/or from a process region comprises a liquid bath and a transparent or reflective optical component continuously rotatable about a non-vertical axis so that a lower part of the component continuously rotates through the bath. When the component emerges from the bath, a coating of the liquid remains thereon which forms a substantially uniform film on at least a part of the component through which the light is transmitted or from which the light is reflected.