Patent GB-761272-A (GB)
Title: Improvements in and relating to vehicle headlamps
IPC: F21V11/02
Filed: 1954-04-01
Abstract: 761,272. Anti-dazzle devices for vehicle headlamps; incandescent electric lamps. HENCHOZ, M. April 1, 1954 [April 1, 1953], No. 9608/54. Classes 39(2) and 75(2) A vehicle headlamp is provided with an anti-dazzle device comprising a disc 9, Fig. 1, formed of a series of parallel-edged transparent laminar strips 7 having their upper surfaces polished and their lower surfaces corrugated parallel to their lateral edges which form the surface of the disc, the strips 7 being rigidly assembled in a holder 29 disposed in a forward extension 6 of a parabolic reflector 5 having centred at the focus thereof a, coiled filament 4 of an electric lamp bulb (not shown), and means are provided for tilting the disc about a horizontal axis into a position in which none of the rays passing therethrough are upwardly directed. The disc 9 is rotatable about an axis 31 on pivot pins (not shown) fixed to the extension 6 by a pivoted linkage 32, 35, 37 or by a flexible cable (not shown) connected to the holder 29 and operable by the driver of the vehicle. The disc 9 is so tilted that all rays from the filament 4, such as R<SP>1</SP>, which are horizontal after reflection at the reflector 5 are totally reflected at the upper faces of the strips 7, the corrugations in the lower surfaces of the strips providing gaps for air or for an adhesive of refractive index less than that of the strips. Each corrugation may have the section of an equilateral triangle, or the corrugation may be sinusoidal or of sawtooth form. The rays R, emerge from the disc as a beam directed downwardly at an angle double that of the inclination of the strips 7. Rays R<SP>2</SP> and R<SP>3</SP> are reflected upwardly and downwardly, respectively, from the reflector 5, at maximum inclinations determined by the relation between the diameter of the coiled filament 4 and its distance from the edge zone C of an axial aperture in the reflector 5 for the passage of the lamp bulb. On passing through the disc 9 part of the beam of rays R<SP>2</SP> is diffused on striking the corrugated faces of the strips 7, whilst a portion emerges as a beam inclined downwardly at an angle greater than that of the emergent rays R<SP>1</SP>. The angle of inclination of the strips 7 necessary to obtain total reflection of all the rays R<SP>1</SP> is determined by the relation between the thickness and the width of each strip 7, and this angle is so selected as to be greater than that of the downward inclination of the rays R<SP>3</SP> after reflection at the zone C. Some of the rays R<SP>3</SP> then pass through the strips 7 without internal reflection and emerge in a downwardly inclined direction parallel to that at which they enter the strips. These rays constitute the upper part of the emergent light beam. Examples are described of the dimensions of the strips 7 and of the angles between the rays R<SP>1</SP>, R<SP>2</SP> and R<SP>3</SP>. To reduce the effect of diffraction at the outer edges of the strip 7 which may result in upwardly directed rays, the upper outer edge of each strip 7 is bevelled, and the bevel surfaces may be frosted. The lower outer edge may be similarly bevelled. The corrugated lower surfaces of the strips 7 may be provided with dark coatings in order to reduce light diffusion thereat, and the upper polished surfaces of the strips 7 may be coated with thin metal foil or metallized. The coiled filament 4 may, alternatively, be arranged axially of the reflector 5, the end part 4b, Fig. 7, nearest the reflector 5 being associated with a cylindrical reflector 49 for directing light downwardly and the opposite end part 4a having a cylindrical reflector 48 for directing light upwardly, whilst the part between the parts 4a, 4b, and which passes through the focus F of the reflector 5 is uncovered. Alternatively the middle part of the filament may be omitted to form two filaments 4a, 4b between which a transverse filament is arranged at the focus F. The reflector 5 may be in the form of a sector of a paraboloid of revolution. A convex lens may be disposed in front of the light source and may be formed in the envelope of the lamp bulb, so that parallel rays are emitted therefrom.