GB2226120A

GB2226120A - A lamp lens

Info

Publication number: GB2226120A
Application number: GB8827353A
Authority: GB
Inventors: Keith David Handy
Original assignee: Glasdon Ltd; Glasdon Group Ltd
Current assignee: Glasdon Ltd; Glasdon Group Ltd
Priority date: 1988-11-23
Filing date: 1988-11-23
Publication date: 1990-06-20
Anticipated expiration: 2008-11-23
Also published as: GB2226120B; GB8827353D0

Abstract

A lamp lens comprises a hollow rotationally symmetrical plastics moulding with integral light-refracting means in the form of an annular convex lens (1) and internally projecting annular prisms (9), characterised in that said convex lens is externally plain and internally convex at that side of a transverse plane thereof adjacent an open end of said moulding and internally plain and externally convex at the opposite side of said plane, and in that said prisms project inwardly at said opposite side of said plane in a domed region of the moulding, whereby undercuts are avoided so as to facilitate moulding. <IMAGE>

Description

A LAMP LENS This invention relates to a lamp lens and to a lamp incorporating such a lens, particularly but not exclusively for use as a road danger lamp.

It is well known to provide road danger lamps in which light emitted from a central light source is refracted by an upright cylindrical lens over a full 3600 circle around the vertical axis of symmetry of the lamp. Department of Transport regulations and British Standard 3143 covering road danger lamps state that a 3600 lamp of this kind must emit light in all vertical planes passing through said vertical axis in a zone extending at least 50 above and below the horizontal plane and that the profile of the lens viewed horizontally must fall wholly outside a 4Omm square and wholly within a 125mm square.

Previous lenses of this kind have taken a number of forms. A first known form has a number of annular prisms projecting from its outer cylindrical surface in flanking relationship with an annular outwardly convex lens. This design creates external grooves which are prone to harbour dirt. In a second known form of cylindrical lens the projections of the prisms and the lens are provided on the inside cylindrical surface so as to leave a plain cylindrical surface externally for easier cleaning.

However, both these forms of cylindrical lens suffer from the disadvantage that undercuts of the projecting surfaces make it impossible to injection mould using a simple mould comprising a single piece female part and a single-piece male core part moveable relative to each other along the axis of Metr of tbe molding. The moulding process is thus complicated, requiring either division of the female tool part into two halves or moulding of two half shells. In either case there is a line of optical ineffectiveness where the two halves of the mould or moulding come together.

It is an object of the present invention to provide a lens combining an annular convex lens and annular prisms in such a way that the aforesaid disadvantage is overcome.

According to a first aspect of the present invention there is provided a lamp lens comprising a hollow rotationally symmetrical plastics moulding with integral light-refracting means in the form of an annular convex lens and internally projecting annular prisms, characterised in that said convex lens is externally plain (preferably cylindrical) and internally convex at that side of a transverse plane thereof adjacent an open end of said moulding and internally plain (preferably cylindrical) and externally convex at the opposite side of said plane, and in that said prisms project inwardly at said opposite side of said plane in a domed region of the moulding, whereby undercuts are avoided so as to facilitate moulding.

According to a second aspect of the present invention there is provided a lamp comprising a lens as defined above and a light source whereof the centre is at the intersection of the axis of symmetry with said plane, the light refracting means preferably causing light to be emitted principally within a circular zone lying between an angle of elevation of 50 and an angle of depression of 5 with respect to the horizontal when the lamp is positioned with said axis vertical transverse plane when horizontally positioned.

Preferably, the lamp includes a reflector underneath the light source and having a part spherical curvature centred upon said intersection so as to collect light from said source and reflect it back through the light refracting means.

The invention will now be further described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic vertical section'of one embodiment of road danger lamp in accordance with the invention; Fig. 2 is a corresponding enlarged view of one vertical half of the lamp showing how the light beam is emitted, and Fig. 3 is a diagram showing how the lens of the lamp may be moulded.

Referring now to Figs. 1 and 2, the exemplary road danger lamp comprises a lens 1 in the form of a hollow, rotationally symmetrical moulding of light-transmitting, preferably clear, synthetic plastics material containing an incandescent filament bulb 2 associated with a part-spherical reflector 3.

In normal use the lamp will be positioned upon a barrier post or road cone with the axis of symmetry of the lens substantially vertical. As considered in this illustrated position of use the lamp lens 1 has a flanged, open, lower end 4 connected to a flat upper end 5 by a circular section side wall 6 having adjacent the open end, and spaced therefrom by a short cylindrical section 7, an annular convex lens 8 adjoined at its upper side by a plurality of annular prisms 9 of the kind known as Fresnel prisms. The annular convex lens 8 is bisected centrally by a horizontal transverse plane HP in which lies the fiIaent centre of the bulb 2 which centre is also 1iocate o the axis of symmetry AS of the lamp.

Below the horizontal plane HP the annular convex lens 8 has a cylindrical outer surface merging with the corresponding surface of the cylindrical region 7.

The corresponding internal surface of said lens 8 below said plane HP has a convex curvature. Above said plane, the annular convex lens 8 has a cylindrical internal surface and a convex external surface which merges with a domed region 10 in which the prisms 9 project internally of the lens.

The reflector 3 is part-spherical with its centre of curvature at the intersection of the axis of symmetry AS and the horizontal plane HP.

The annular convex lens 8 and the annular prisms 9 together form light-refracting means which, as illustrated in Fig. 2, refract light in a circular, wedge-section zone of principal illumination having an angle of elevation of 50 and an angle of depression of 5 with respect to the horizonal. The reflector 3 serves to reflect downwardly directed light back through the optical centre to the light refracting means 8, 9.

It will be noted that all the refractive surfaces of the lamp lens, i.e. all those surfaces through which light rays are shown to be passing in Fig. 2, are either parallel to the axis of symmetry AS or diverge therefrom in the downward direction.

As illustrated in Fig. 3, this design enables the lens moulding to be injection moulded in a simple mould having one-piece male and female elements 11, 12 separable on the axis of symmetry AS of the moulded lens with the lens being readily removable from the mould because of the elimination of all undercuts.

Claims

1. A lamp lens comprising a hollow rotationally symmetrical plastics moulding with integral light-refracting means in the form of an annular convex lens and internally projecting annular prisms, characterised in that said convex lens is externally plain and internally convex at that side of a transverse plane thereof adjacent an open end of said moulding and internally plain and externally convex at the opposite side of said plane, and in that said prisms project inwardly at said opposite side of said plane in a domed region of the moulding, whereby undercuts are avoided so as to facilitate moulding.

2. A lens as claimed in claim 1, wherein the plane surfaces of the convex lens are cylindrical.

3. A lens as claimed in claim 1 or 2, wherein the domed region of the moulding is of circular section.

4. A lens as claimed in any one of the preceding claims, wherein the prisms are Fresnel prisms.

5. A lamp comprising a lens as claimed in any one of the preceding claims and a light source whereof the centre is at the intersection of the axis of symmetry with said plane.

6. A lamp as claimed in claim 5, wherein the light refracting means causes light to be emitted principally within a circular zone lying between an angle of elevation of 5 and an angle of depression of 50 with respect to the horizontal when the lamp is positioned with said axis vertical.

7. A lamp as claimed in claim 5 or 6, further comprising a reflector underneath the light source and having a part spherical curvature centred upon said intersection so as to collect light from said source and reflect it back through the light refracting means.

8. A lamp or a lamp lens substantially as herein described witb reference to the accompanying drawings.