Patent GB-670332-A (GB)
Title: Improvements in luminairs and lighting systems using the same
IPC: F21V5/00
Filed: 1948-10-12
Abstract: 670,332. Light refractors; lighting systems. HOLOPHANE, Ltd. (Holophane Co., Inc.). Oct. 12, 1948. No. 26574/48. Classes 75(ii) and 75(iv). A luminair comprises a pair of superposed refractor plates 50, 51, and a point source of light L offset rearwardly from the central normal to the plates, each plate having a series of circular prisms with centres offset forwardly from the normal through the light source, the prisms on the upper plate 50 being offset less than those on the lower plate, the refracting power of each of the prisms of larger radius on the upper plate 50 being less at the forward part of the plate than at the rear part, the upper plate forming a substantially point virtual image at a point on the line joining the source and the centre of the innermost prism further from the plate than the source, and the refracting powers of the prisms on the lower plate 51 being graduated to increase the convergence imposed by the upper plate whereby an emergent beam substantially parallel and inclined forwardly to the normal through the plates is obtained. The circular prisms on the upper plate 50 have their centres disposed on a line 52, 521 extending forwardly from the geometric centre 52 of the plate. The refractive powers of these prisms increase with their diameters but additionally the part of any prism lying outside a dotted circle 61 centred on the base L<SP>1</SP> of the normal from the light source is made less powerful than the complementary part of the same prism lying within the dotted circle 61; thus where a particular prism 70 lies within the circle 61 it has an active face inclined at 53 degrees to the horizontal, whereas outside the circle 61 the inclination of its active face is 47 degrees. To facilitate the manufacture of the mould the region outside the circle 61 is divided into annular sectors 60 a, b, c, by radial lines 63-68 and by arcs 61a ... 61e. The refracting powers of the prisms in each annular sector increase radially but at different rates to make all the rays emanate from the same virtual image. The power of any one prism thus changes as it crosses the radial lines 63-68. The lower refractor plate 50 comprises a series of circular prisms 80, all centred at a point 81 at the front edge of the plate; the refracting power is zero at the point 81 and increases with the diameter of the prism. The refractor plates are square and the prisms are formed on one side only, the other sides being smooth. A concentric reflector is mounted above the light source. To illuminate a surgical operating table a plurality of luminairs are mounted on an annular area of the ceiling vertically above the table S, Fig. 2, the dimensions of the annular area ensuring that light directed on to the table is inclined at angles between 15 degrees and 35 degrees to the vertical. A line of luminairs may be arranged radially as at 20 ... 22 or transversely as at 23 .... 29. The variation in inclination and obliquity required by such arrangements of luminairs is obtained by moving the light source both forwardly and sidewardly within the luminair, the sideward movement providing a change in direction of 90 degrees, as seen in plan view. Specification 670,331 is referred to.