The optical system for testing the homogeneity of light guides comprises a light-proof box (20) in which the light-guide bar (11) is suspended. The ends of the bar protrude from the box and are fitted with a plane mirror reflector (18) and photo-electric detector with filter (13, 19). - The top edge of the bar is ground to provide a matt surface (14) which is scanned by a narrow laser beam (15) through a window in the top plate of the box (21). A flat, horizontal reflecting surface (12) is supported in the base of the box and runs the length of the bar. The surface of the mirror can be covered by adjustable shutter plates (16) arranged at intervals along its length. Once the location of any inhomogeneity in the light guide has been found by adjustment of the compensating shutters, a permanent plate can be provided and the adjustable shutter arrangement used to test another guide.
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