Patent GB-1198791-A (GB)
Title: Anti-Dazzle Apparatus Initiated by an Auxiliary Electronic Flash
Inventor: WIPPLER GEORGES CONSTANT
IPC: A61F9/06
Filed: 1965-02-11
Abstract: 1,198,791. Photochromic materials and uses thereof. COMPAGNIE de SAINT-GOBAIN. 11 Feb., 1965, No. 6070/65. Heading C4S. An anti-dazzle apparatus (e.g. spectacles) intended to be positioned between the eye and an intense light source such as the sun, a mercury vapour lamp, an electric arc or the flash of a nuclear explosion, comprises three equidistant transparent discs D 1 , D 2 , D 3  provided with photochrome layers and centred on the optical axis X-X<SP>1</SP> of the apparatus, perpendicular to this axis, and an electronic flash tube S of toroidal form enclosing the central disc D 2 , the assembly being mounted in a casing 1 with an internal reflecting surface 2 and formed with apertures housing the two external discs D 1 , D 3 . The central disc D 2  has photochrome layers 5, 6, respectively, on both faces whereas the external discs D 1 , D 3  each have but one photochrome layer 3, 4, respectively, which in the embodiment illustrated, is on the outer surface of the disc. The colouring of the photochrome layers is produced by means of the electronic flash tube S which is released by an additional device (not shown) in dependence on the signal from a photo-electric cell reacting to the variation in illumination caused by the commencement of the phenomenon from which it is desired to be protected. A suitable photochrome substance is a varnish based on 1,3,3,-trimethyl-2-indoline-2<SP>1</SP>-spiro-6<SP>1</SP>- nitrobenzo (b) pyran. The dimensions of the discs D 1 , D 2 , D 3  are such that the field of vision is substantially the same through each of them, the disc D 3  being the one nearest the eye. The reflecting internal surface 2 of the casing 1 is a surface of revolution generated by the rotation of an elliptical arc, extending on either side of its major axis, around an axis of revolution perpendicular to the major axis and passing through that of the two foci furthest from the point of intersection of the arc with the major axis, the optical axis X-X<SP>1</SP> of the system coinciding with this axis of revolution. The mean radius of the electronic flash tube S is equal to the distance between the two foci of the elliptical arc. In order to eliminate the visible light originating from the electronic flash and received at the eye by reflection, the flash tube may be constructed of a glass which only allows passage of the active ultra-violet radiation. The residual ultra-violet radiation can be stopped by providing a supplementary disc opaque to this radiation beyond the photochrome layer 4. It is also advantageous to provide a supplementary disc opaque to ultra-violet radiation beyond the photochrome layer 3 so as to avoid the slow degradation of the photochrome substance by the ambient ultraviolet radiation. The arrangement shown in which the photochrome layers 3, 4 are on the external faces of the discs D 1 , D 3 , respectively, is preferable when using a flash tube of high energy or situated at a small distance from the discs D 1 , D 3  since the calorific radiations harmful to the colouration of the photochrome layers are thus to a large extent absorbed by the discs.