Apparatus for measuring refractive index

Apparatus for the measurement of optical properties of a quasi cylindrically symmetric transparent object, which apparatus comprises source means for providing a source of electromagnetic radiation, illumination forming means for forming an illuminating beam of electromagnetic radiation for illuminating the transparent object, region selecting means for selecting a localised radial region within the transparent object illuminated by the illuminating beam, region scanning means for scanning the localised radial region through a range of radial positions within the transparent object, reflection detecting means for detecting a reflected beam of electromagnetic radiation reflected out of the illuminating beam at the localised radial region, beam measuring means for measuring the component of the position of the point where the reflected beam leaves the transparent object along a direction parallel to an axis of rotational symmetry of the transparent object, supporting means which causes the illuminating beam, the reflected beam and an axis of rotational symmetry of the transparent object to lie in a measurement plane whose thickness is generally small compared with the radius of the transparent object, angle altering means for altering the angle of incidence of the illuminating beam on the transparent object within the measurement plane, control means for controlling the components of the apparatus such that the measurements are made at the required time and in the required sequence, and computing means for computing optical properties of the transparent object.