GB660352A - Lenses for light projectors and projectors incorporating the same - Google Patents

Abstract

660,352. Lenses for optical projection. HOLOPHANE, Ltd. (Soc. Anon. Holophane). Jan. 8, 1948, No. 636/48. Class 97 (i). [Also in Group XI] A converging lens for a light projector is constructed to be stigmatic for two points on the axis on the light source side of the lens. The lens shown is made stigmatic for the point F which is the principal focus and for a point F<SP>1</SP> nearer the lens and with image at M. The lens is made up of annular prisms each of which is individually determined by reference to a family of curves showing deviation angle plotted against angle of incidence for prisms of different apical angles; thus the deviations necessary to redirect the ray FI parallel to the axis and the ray F<SP>1</SP>I in the direction IM are measured in Fig. 2 and a prism is then found in the graph to effect these deviations. The prism is then oriented so that its light entrant face IZ is at right angles to a line IR drawn so that the angle FIR equals the angle of incidence also given by the graph. The lens is given a stepped formation on both faces although it is possible to replace the steps on the incident face or on both faces by continuous surfaces; and the steps at the centre of the lens can be suppressed. The slightly curved contours of each step can be replaced by spherical or flat surfaces giving approximate correction. The projector is preferably constructed with means for definitely locating a light source at either stigmatic point.