The optical sensing unit comprises a filament lamps (1) focussed by a plano-convex lens (2) with the emergent beam reflected by a semi-transparent mirror (3). The reflected beam passes through an iris (4) to a (front) biconvex lens (5) which focusses it onto the surface of the strip or grating (7) to meet at a point (6). The image reflection is re-transmitted through the lens (5) iris (4) and mirror (3) to a second (back) biconvex lens (9) to a receiver (10). In the centre of the iris is a small field lens (13). The receiver is typically a photo-electric device. The aperture angle of the light beam in the radial direction is smaller than in the tangential direction.
