Abstract: Test apparatus and method for testing for faults in, for example, transparent sheet material in which two parallel beams of light separated by a distance "w" are passed through a grating having, for example, light transmitting and absorbing areas separated by a distance "w", so that as the two beams are scanned across the object and the grating the light output through the grating is generally constant. If, however, there are faults which displace one beam with respect to the other, then the spacing between the beams will change so the light output will vary from the norm. Furthermore, if there is a fault which would cause attenuation of the two beams, then the light output will again vary from the norm.

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.

Abstract: Method and apparatus for detecting holes in sheet material such as tin plate where a beam from a laser is scanned across the sheet by a mirror drum, successive scans being arranged to strike the sheet material from upstream and downstream, and the light passing through a hole in the sheet material is received by a perspex light guide and passed to a photodetector.