Abstract: Apparatus for comparing a test surface 23 with a reference surface 8 comprising detector means 16.17, a test channel TC for receiving radiation from the test surface 23 and a reference channel RC for receiving radiation from the reference surface 8. A selector means 12 is provided to pass radiation from the test channel means TC and the reference channel means RC to the detector means. Varying means 5 is provided to vary the radiation passed along the test channel TC and reference channel RC. Cycle means is provided to cycle the selector means 12 between a first state in which more radiation from the test channel TC than the reference channel RC is passed to the detector means and vice versa. Control means is provided to control the radiation varying means 5 to provide a zero or minimum variation of radiation received by the detector means as the selector means 12 is cycled.

Abstract: A method and apparatus for producing an aspherical surface, for example, an aspherical optical surface, on a workpiece, on which the workpiece is rotated about a first axis, a cutting tool is rotated about a second axis, and at a radial distance from the second axis, and the second and first axes meet at a point and at an angle with respect to one another, the second axis being moved with respect to the first axis so that the point moves along the first axis and the angle changes.

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: Optical test apparatus and method for testing various parameters of a lens system such as a T.V. lens system. A single apparatus may be used for testing the modulation transfer function, glare, spectral transmission, relative field illuminance, transverse color, T stop, and back focal distance.The apparatus comprises a pin hole light source and an analyzer means in which the lens may be mounted including a set of screens carrying varying patterns for measuring different parameters. The light transmitted through the screens is measured by a light detector means and the output signal is analyzed to provide a measurement of the parameters.