GB416509A - Means for controlling undesired reflections in optical apparatus - Google Patents

Abstract

416,509. Optical apparatus. HILGER, Ltd., A., and DOWELL, J. H., 98, King's Road, Camden Road, London. March 17, 1933, No. 8147. [Class 97 (i).] In order to prevent disturbances due to undesired reflection of light from the boundary or partition surfaces of optical elements or apparatus, any such surface is divided into a number of steps inclined at an appreciable angle to the direction of the light path and having sharp angles towards the said path, so that stray light impinging on the surface is reflected in a direction substantially different from that of the useful light. Figs. 1 and 2 show a Nicol prism in which each boundary surface is formed as a series of steps 14 sloping inwardly in the direction of the light path and having sharp edges 16 towards the said path. Stray light is reflected by these steps as shown by the arrows 12, 13, the direction of the useful light being shown by the arrow 11. Figs. 5 and 6 show a reflecting prism as used in binocular telescopes ; each side face is divided into two sets of steps parallel to the entrance and exit surface and sloping in opposite directions, the two sets being separated by an inclined face 19. Alternatively, in such a prism, the two sets of steps may be parallel to the two reflecting surfaces. In a two-colour system using a split lens and a dividing plate between the part lenses, the inner surfaces of the lenses may be stepped as above described, and the plate may also be stepped, the steps on the plate being inclined outwards in the direction of travel of the light and fitting corresponding steps of the lens. The interior walls of optical instruments may be treated in a similar manner. Specifications 346,406, 349,107, and 394,385 are referred to.