Abstract: An optical fiber arrangement providing uniform illumination of a region in a plane beyond the end of an optical fiber and normal to the fiber axis. An array (12) of four prisms (A,B,C,D) is disposed adjacent to, but spaced from, an end (11) of an optical fiber (5). By appropriate choice of the prism angle (.theta.) and its spacing (d) from the fiber (5), the prism array (12) modifies the angular distribution of light from the fiber (5) in two ways. The shape of the illuminated region is changed from circular to square and the illumination becomes substantially constant over the square region. In an assembly of fibers, each modified in this way, the amount of overlap needed between the outputs of neighbouring fibers can thus be reduced and a more uniform light output can be achieved. Other non-circular regions of illumination, for example of pentagonal and hexagonal shape, can be produced using different optical components and the same benefits obtained.

Abstract: Apparatus and method for indicating stress in an object comprising viewing the temperature variation of part of the surface of the object as it is stressed. An infra-red detector detects small changes of temperature and the stress may be applied by natural loading or by cyclic stressing imposed by a vibrator.