This disclosure relates to an optical limiter for limiting the radiant flux of an optical source beam, including an optical control port for illumination by an optical control beam originating from the source beam; an optical input port for illumination by an optical transmission beam originating from the source beam; an optical output port for illumination by the transmission beam; and a thermally driven light mill; wherein the light mill is arranged with respect to the input port, the control port and the output port such that: illumination of the control port by the control beam drives the light mill to rotate only when the control beam has a radiant flux equal to or in excess of a predetermined radiant flux threshold; and rotation of the light mill causes an area of the output port illuminated by the transmission beam to change.