Abstract: A flicker photometer comprises generating means (1, 8, 4) for generating two beams of light of different colors. Both beams are directed to a viewing means (190), and a subject can vary the intensity of light from one beam relative to the other. The color of the light seen by the subject is caused to alternate by a shutter (18) which is rotated about an axis by a motor, the shutter having a portion which extends non-perpendicularly relative to the axis to facilitate a compact construction of photometer. The performance of the photometer is improved by screens which scatter light before it reaches the subject. A first of the screens reflects light from one beam, whilst a second screen transmits light from the other.

Abstract: A flicker photometer has beam generating means (1, 8 and 4) for generating two beams of light of different colors. Light from the beams is viewed at the viewing means, and the intensity of light of one beam relative to that of the other beam can be adjusted by adjustment means comprising an attenuating filter (20) which is movably mounted in the path of one beam. Movement of the filter relative to the beam alters the degree to which the beam is attenuated, and is achieved using an electric motor (22). The motor provides a relatively easy means of adjusting the filter position, and can be used to provide coarse and fine adjustments in response to suitable signals from an optical encoder. The motor also enables the filter, along with the beam generating means, to be mounted on a movable table for use in enabling the photometer to accommodate both short and long-sighted subjects.