Abstract: A method for furnishing a perceptor with apparently continuous illumination over an extended target area, in which at any instant only part of said area is illuminated, but every part thereof is intermittently and repeatedly illuminated by discontinuous flashes. With regard to any one part of said target area, the flashes are repeated at time intervals not less than the decay period of the response elicited in the perceptor. The method is performed by means of a device which comprises a beam-generating arrangement (1) that focuses a beam of radiation (5) upon a rotatably-mounted light-deflector (6), which is rotated under control at a suitably high speed by mechanical means (9, 10, 11 and 12) so as repeatedly to illuminate a target area (15) with a narrow flash to radiation (16).