GB833761A - Improvements in electric lamps - Google Patents

Abstract

833,761. Discharge lamps. GENERAL ELECTRIC CO. Oct. 4, 1956 [Oct. 30, 1955], No. 30301/56. Class 39(1). [Also in Group XI] Electric incandescent lamps and discharge lamps, more particularly for projection purposes, comprising a high intensity light source which emits both visable and infra-red rays, have a hermetically sealed glass bulb characterised in that diametrically opposite portions thereof are provided with dichroic filter means of complementary characteristics. Fig. 1 shows an incandescent lamp of the sealed-beam type comprising a filament 4 at the focus of a reflector portion 2 provided with a light-reflecting infra-red-transmitting coating, and a cover glass or lens 3 which is preferably concave and provided on either its inner or outer surface with a dichroic filter 9 which is highly transmissive of visible light and highly reflective of infra-red rays. The coating on the reflector portion 2 may consist of a film 7 of a semi-conductor such as germanium, silicon, antimony sulphide or selenium upon which is applied a single covering film 8 of dielectric, such as zinc sulphide of such thickness as to reduce the infra-red reflectance. The film of dielectric should have an optical thickness equal to one-fourth the wavelength of mean infra-red light, e.g. 900 to 1500 millimicrons and an index of refraction equal to the square root of the index of refraction of the semiconductor 7 in this same wavelength region. The cover glass or lens section 3 may be a plain window section of uniform thickness or may be configurated with lenses or prisms to provide any desired light distribution, and the dichroic filter provided thereon may consist of an extremely thin film of metal such as gold. In a modification, Fig. 2 (not shown), a bulb of spherical shape has a filament located at the centre, and one half of the surface is provided with a light-reflecting heat-transmitting filter coating, whereas the other half is provided with a light-transmitting heat-reflecting filter, the two coatings being divided by a plane extending through the bulb axis and including the terminal contacts.