GB2069123A - Reflector made from plastics material for lamps - Google Patents

Abstract

The invention relates to a reflector made from plastics material for a lamp, for example a motor vehicle headlamp, intended to co- operate with a light source in order to obtain a fixed beam of light. During construction the reflector is provided in its upper region, which is subject to the greatest heat, with a surface shape (Z1) which diverges significantly from the shape (P) which would produce the required form of light beam but which, in conditions of heat corresponding to the normal functioning of the lamp, is deformed so that the interior surface adopts a shape which will produce the required beam. <IMAGE>

Description

SPECIFICATION Reflector made from plastics material for lamps The present invention reiates to reflectors made from plastics materials for lamps for example motor vehicle head lamps.

In the construction of lights for motor vehicles it has been proposed to replace the reflectors which have previously been made from stamped metal by reflectors made from plastics materials. In such reflectors, the optical surface is produced by vacuum metal plating, particularly by aluminising.

One important problem appears to be that the plastics materials which are most suitable for production of a reflector shape by moulding, followed by the production of an optical surface by metal plating, do not generally stand up well to the temperatures at which an automobile light having the usual dimensions and power functions.

Attempts to construct large reflectors for motor vehicle lamps have shown that the temperatures occurring during functioning of the light affect adversely the durability of a metal plated interior surface, the shape of which may be rigidly defined from the optical point of view, generally a parabolic surface.

In order to remedy this defect, various solutions have already been proposed. One such solution involves the use of plastics materials which stand up well to heat. Such plastics materials exist, but the cost of manufacturing them renders them prohibitive for the construction of lamps. In another proposed solution, cheap plastics materials are used incorporating reinforcements to rigidify the reflector. Also, reflectors having a nonuniform thickness have been produced. These have thinner zones which are intended to limit the deformations caused by heating to the nonessential zones of the reflector.

Unfortunately, none of these solutions has proved completely satisfactory.

It is an object of the present invention to provide a reflector which can be produced by simple moulding of widely available plastics materials, and which is capable of forming a beam which does not suffer as a result of deformation caused by the heat generated while the lamp is on.

According to the invention there is provided a reflector lamp made from a plastics material, having, at least in its upper region which is subject to the greatest heat, an interior surface shape which differs significantly from the shape which would produce the required form of light beam, but which in conditions of heat corresponding to the normal functioning of the lamp is deformed by the effect of thermal expansion so that the interior surface adopts a shape capable of producing the required form of light beam. Thus, the interior shape of the "cold" reflector is an imperfect shape whereas after the deformation caused by the increase in temperature when the light is on, this shape becomes the ideal optical shape to cooperate with a light source associated with the reflector to obtain the required fixed beam of light.

In a preferred embodiment, the reflector has over its whole width a paraboloidal surface which is closed at its lower part and its upper part by two substantially flat flanges, the shape of the opening of the reflector being defined by a rim which forms a raised edge or stiffening rib. In this type of reflector, a zone of the paraboloidal surface is preferably deformed downwards slightly from above and about a hinge line and which is preferably constituted by a section through the paraboloidal surface in a horizontal plane.

The deformed zone is preferably, in all cases adjacent the upper flange. Its total width can be between a fraction e.g. one tenth and the total width of the reflector. Its total height can be between a tenth and threequarters of the halfheight of the reflector. The amount by which it is deformed about the hinge may be between 0.2 and 6% (values in radians). The precise choice of the dimensions and the angles of deformation depend upon the precise characteristics of the reflector, on its dimensions, on its thickness, on the characteristics of the light source with which it must co-operate and on the plastics material of which it is made. The final values may be determined both with the aid of theoretical considerations and by empirical tests.In particular, by successive modifications to a mould it is possible empirically to obtain a series of reflectors with progressively increasing deformed zones both as regards their extent and the amount by which they are set back which make it possible to determine the optimum solution.

In another embodiment the reflector is provided with a series of deformed zones arranged in stepped fashion. Thus, a small, very steeply deformed zone may be located inside a mediumsized zone which is less steeply deformed and which is itself located inside a large zone which is even less steeply deformed, all the zones being adjacent the upper flange of the reflector.

It is to be understood that the deformation of at least one zone of the reflector in accordance with the invention "to compensate for temperature" is not related to the discontinuities and irregularities which have been imposed on certain reflectors in the prior art in order to modify the optical properties of the reflected beam of light. In the prior art, the shape of all the zones of the reflectors is regarded as invariable and it is thus supposed that the beam of light remains the same whether the light is "cold" or "hot". In contrast, the essential idea underlying the present invention is to make use of thermally deformable plastics materials in relatively small thicknesses, seeking not to counter the deformation as a result of increasing temperature, but rather to compensate for it a priori by providing optical shapes diverging from the optical shape necessary in use.

The invention may be carried into practice in various ways and two embodiments will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a front perspective view of a parabolic reflector having a rectangular shaped opening, according to a first embodiment of the invention; Figure 2 is a vertical section along the section line I1--II of Figure 1 (or according to any other parallel central transverse vertical section); and Figure 3 shows a second embodiment of the invention in a perspective view similar to that of Figure 1.

The reflector shown in Figures 1 and 2 is a reflector with a rectangular perimeter and a paraboloidai reflecting surface. It is moulded from a plastics material, in this case butylene polyterephthalate (PBT). Its thickness is uniformly 2 mm. The reflector has a paraboloidal surface P about an axis 00 extending over its entire width, but in the upper part and the lower part it has substantially flat flanges J1 and J2 The part P has an optical role as it constitutes the reflecting surface of the reflector, whereas the flanges J, and J2 merely serve for mechanical connection and have no special optical effect; a rim R defines the outline of the opening of the reflector by forming a raised edge or stiffening rib.The paraboloidal surface P has a focal length of 26.5 mm, a width of opening of 190 mm and a height of 120 mm. In use, a glass (not shown) covers the reflector on its front surface and a bulb (not shown) is mounted on its rear aperture S.

With a reflector of this construction, the bulb provided (type CRE or H4) generates considerable heat when the light is on. The point on the upper flange J, designated B may reach a temperature of 1060C, while the point at the extreme end of the part P designated A may reach a temperature of the order of 1000C and the central zone C of the parabola P reaches a temperature of the order of 800C (see Figure 2).

In order to permit the reflector to accommodate variations of temperature between the cold state and the functioning state, an upper quadrangular zone Z, of the zone P is deformed during manufacture by approximately 2% about a hinge line z1-z1.

The hinge line z1-z1 is constituted by a section through the parabola P in a horizontal plane. The zone Z, extends as far as the flange J, its height being one tenth of the height of the parabola P, that is to say 12 mm, and its width being approximately two thirds of the width of the parabola P, that is to say approximately 130 mm.

The zone Z1 is centred with reference to a vertical plane passing through the optical axis 0--0. When referring to a deformation by 2% about the hinge line, this means that all the sections of the parabola P in the sectional planes such as Il-Il (Figure 2) or in parallel vertical planes are deformed by 2% as shown in Figure 2 about their intersection with the hinge line z1-z1.

In Figure 2 the actual shape after deformation is shown in continuous lines, and the strictly paraboloidal shape (i.e. without deformation) is shown in dotted lines.

When such a reflector, is used in co-operation with an appropriate bulb, its subsequent heating is translated into a tilting upwards of the upper flange J1 and the deformed zone Z1. After this tilting the interior surface of the zone Z1 corresponds to a parabolic surface of revolution about the axis 0--0, and the heat thus produces in the zone Z, an effect which is the reverse of the deformation during manufacture.

The zone Z, can be limited to the central part of the reflector, as shown in Figures 1 and 2. It can also extend in a band over the whole width of the upper part of the reflector below the upper flange J,.

Figure 3 shows a second embodiment of the invention. Similar components are designated by the same reference numerals as in Figure 1. In this embodiment, the reflector has three deformed zones Za, Z2 and Z3 arranged as stepped tiers, Z being inside Z2 and Z2 inside Z3, the three zones being adjacent the flange J1. The zone Z, is deformed by 2% about a horizontal hinge line z1-z1. In a similar manner, the zone Z2 is deformed, but only by 1% about a similar horizontal hinge line z2-z2 (not shown), and the zone Z is deformed by 0.5% about a horizontal hinge line z3-z3 (not shown). When such a reflector is in use, the heat generated causes the interior aluminised surfaces of the three zones Z1, Z2 and Z3 to return to their parabolic shape cf revolution about the axis 0--0.

Claims (6)

1. A reflector for a lamp, made from a plastics material, having, at least in its upper region which is subject to the greatest heat, an interior surface shape which differs significantiy from the shape which would produce the required form of light beam, but which in conditions of heat corresponding to the normal functioning of the lamp is deformed by the effect of thermal expansion so that the interior surface adopts a shape capable of producing the required form of light beam.

2. A reflector as claimed in Claim 1, having, in the upper region which is to be subjected to the greatest heat, a zone which is deformed about a hinge line.

3. A reflector as claimed in Claim 2, having a rectangular opening and a central paraboloidal reflecting surface with an upper and lower flange, and a quadrangular zone turned down from above and about a substantially horizontal hinge line, this zone being adjacent the upper flange.

4. A reflector as claimed in Claim 3, in which the quadrangular zone extends over a height of between one tenth and three quarters of the half height of the paraboloidal surface and over a width of between one tenth and the whole width of the parabolic surface, the amount by which it is deformed being between 0.2 and 6%.

5. A reflector as claimed in any of Claims 2 to 4, having several deformed zones.

6. A reflector for a lamp, made from a plastics material constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 1 and 2 or Figure 3 of the accompanying drawings.