GB2069124A

GB2069124A - A parabolic reflector

Info

Publication number: GB2069124A
Application number: GB8103616A
Authority: GB
Original assignee: Sidler GmbH and Co
Current assignee: Sidler GmbH and Co
Priority date: 1980-02-07
Filing date: 1981-02-05
Publication date: 1981-08-19
Also published as: DE3004422C2; FR2475691A1; DE3004422A1; JPH0230001B2; JPS56125705A; BE887427A; FR2475691B1

Abstract

A reflector, in particular for vehicle lamps and headlamps, is made of a transparent material which concentrates and re-emits by curved total reflection prisms (8) the light emitted by a light source. In order to provide a reflector without a mirror- coating which can be produced at low cost as a rotationally symmetrical body, the total reflection prisms (8) are provided on the inner concave side facing the light source and take the form of triangular annular prisms which are rotationally symmetrical relative to the optical axis (6), a first (10) of the three faces of an annular prism being substantially cylindrical relative to the optical axis (6) of the reflector (1) and a second face (11) being formed by the back of the reflector. <IMAGE>

Description

SPECIFICATION A parabolic reflector The invention relates to a reflector, in particular for vehicle lamps and headlamps, which is made of a transparent material and which concentrates and re-emits by curved total reflection prisms the light emitted by a light source.

Generally, reflectors consist of a cast or moulded metal or plastic part, upon the inner surface of which a mirror coating is vapourized after compietion of the forming step. The vapourization is relatively expensive, and the mirror-coating is very delicate, even if provided with a protective coating. To remedy these drawbacks, it has been tried to obtain the desired concentration of the light not by means of a mirror-coating but by means of total reflection (German Laid-Open Patent Application No.

27 14 793). In this known reflector, the inner surface has the same shape as before, but no mirror-coating, and the reflector consists of a transparent material. The outside or back of the said reflector is provided with a radiate arrangement of integrally formed pentaprisms having its centre in the vertex area of the reflector.

In this arrangement, the ridge of the pentaprisms, or the height of the pentaprisms, must follow a given function. In order to obtain pentaprisms of convenient dimensions, it is moreover necessary to subdivide the reflector surface into several annular areas, i.e. to subdivide the individual pentaprisms in the longitudinal direction into several sections staggered in a manner similar to a fresnal lens. This make the necessary production tools even more expensive and complex.

Now, it is the object of the present invention to provide a reflector which does not need a mirrorcoating, but which takes the form of a solid rotation which can be produced with the aid of low-cost tools.

According to the invention, this problem is solved in a reflector of the type mentioned before by an arrangement in which the total-reflection prisms are provided on the inner concave surface facing the light source and take the form of threesquare annular prisms presenting rotation symmetry in relation to the optical axis, one of the three faces of each annular prism is substantially cylindrical relative to the optical axis of the reflector and the second face is formed by the back of the reflector.

A significant advantage of this reflector of the invention is to be seen in the fact that it presents rotation symmetry relative to the optical axis so that it can be produced easily and at low cost. The tool required for giving a reflector of this type the desired shape may for instance substantially consist of two turned parts corresponding to the outer and inner contour of the reflector.

Considering that the first face of the annular prisms is substantially cylindrical relative to the optical axis, easy removal of the part from the mould in the direction of the optical axis is at any time ensured. To facilitate the removal still further, the first face may take the form of a cone surface differing only very little from the cylindrical surface, without thereby changing the function or notably affecting the quality. The fact that no mirror-coating is needed reduces the number of operations required for the production of such a reflector and, thus, its production cost. And in addition, the reflector obtained has an extended surface life cause the latter does no longer depend on the perfect condition of the mirror-coating.

The shape and arrangement of the third face of the annular prisms are selected to ensure that on the one hand light losses are reduced to a minimum and, on the other hand, the light is radiated in the desired direction which is preferably substantially parallel to the axis. In preferred embodiments of the invention, the third faces of the annular prisms are therefore aligned substantially parallel to a ray emitted by the light source so that they form a portion of the surface of a truncated cone having its apex substantially at the centre of the light source (focus). The third face of the annular prism which has substantially the same distance from the vertex of the reflector as the centre of the light source, is a substantially plane surface.The apex of the cone of those faces which are closer to the vertex of the reflector points in the direction of radiation, while the apex of the cone of those third faces of annular prisms which have an axial distance from the reflector vertex greater than the focus points in the direction of the reflector vertex.

In preferred embodiments of the invention, the back face of the reflector takes substantially the form of a paraboloid of revolution, except for the vertex area. The parabola is approximated by straight sections, each associated with one annular prism and forming its second face.

In the design of the reflector described above, the light emitted by the light source enters the annular prism through the first face, is totally reflected by the second face and emerges through the third face in a direction parallel to the axis or substantially parallel to the axis. If a certain deviation from the direction parallel to the axis is desired for the whole light or only in certain areas, it is only necessary to vary slightly the engle of at least one of the three faces relative to the optical axis.

Generally, an opening is provided in the vertex area of the reflector which serves to introduce and secure in position a light source, for instance an incandescent bulb. As a result, this area situated in the "shadow" of the light source does not reflect the light emitted by the light source and is therefore not used. So, it is not necessary to consider this area in the design of the reflector.

The material used for the reflector must be transparent and permit total reflection in the area in which the light beam impinges upon the second face. In the forward area of the refelector, i.e. in the area extending from the open end approximately to the plane of the focus or the centre of the light source, all known transparent mass-produced plastics may be used as they offer and index of refraction of at least approx. 1.5. In the rearward area, i.e. in the portion of the reflector extending between the forward area and the vertex of the reflector, transparent plastics may be used which have an index of refraction in a range preferably above 1.5, as for instance polycarbonate (PC) which has an index of refraction of n = 1.586.Likewise, polystyrene (PS) which has an index of refraction of n = 1.59 is well suited, just a styrene nitrile (SAN) which has an index of refraction of n = 1.567. The same applies also to epoxy resins having indices of refraction of n = 1.55 to 1.61. It goes without saying that glass is suited, too. The selection of the material, just as the selection of the step height and/or the width of the prism ring depend not only on aspects of style, but also on aspects of reliability and on the fulfillment of the precondition that total reflection must be ensured.

In the vertex area of the reflector, increased light losses are encountered. But this is of no importance if this area is anyway used as an opening for the bulb serving as light source.

However, there do exist certain cases where, either because of the limited space or for other reasons, the vertex area of the reflector is closed and serves also as reflection surface. In this case, the light source is introduced into and secured in position in the reflector through a lateral opening or from the front. In order to obtain a desired total reflection also in this vertex area, without the need to provide a mirror-coating, it will suffice, in addition to the realization of the features outlined above, if the vertex angle formed between the second faces arranged opposite each other relative to the optical axis is approximately 900.

Considering that the total reflection prisms take the form of annular prisms, this means that the face of such an annular prism forming the rear face of the reflector represents the surface of a truncated cone whose angle of cone is 900 (the apex of the cone pointing away from the light source). So, in this area the reflector does no longer have the shape of a paraboloid of revolution, but that of a cone. In this surprisingly simple manner it is possible to utilize even the vertex area of the reflector in an advantageous manner.

In a preferred further improvement of the invention, the third faces of the annular prisms exhibit a convex curvature. In this case, the whole vertex area of the reflector may also take the form of one single total reflection prism having the shape of a cone whose angle of cone is 900 and whose basal plane is a convex spherical segment.

This embodiment, which has relatively great step heights in the vertex area as compared to those existing in the rearward and forward reflector portions may however in certain cases lead to an undesirable agglomeration of material. Therefore, the vertex area is generally subdivided also to give substantially uniform step heights.

In a preferred embodiment of the invention, the second faces of the annular prisms in the vertex area, i.e. their back faces, have a convex curvature and the third faces are plane faces, relative to the optical axis of the reflector. This embodiment offers the advantage that a greater proportion of the light is reflected from the vertex area since all the light impinging upon the second face is reflected and no portion thereof gets lost because of the fact that following the first total reflection it impinges at an unfavourable angle upon the third face of a neighbouring annular prism closer to the optical axis. In this case it must be noted that a double total reflection is encountered in the vertex area.

The size of the vertex area depends upon the type of material used for the rear portion of the reflector or else on the index of refraction of the materials used. Preferably, the vertex area comprises a solid angle of approx. up to 600, with the apex of the angle coinciding with the centre of the light source (focus), and the solid angle in which the second face is convex face and the third face is a plane face, is limited to substantially 400.

In order to arrive at geometrically simple designs, the axial sections of at least two of the faces of the annular prisms are straight-lined.

Preferably, the reflector, except for the vertex area, consists of a material having an index of refraction which is not essentially below, but preferably above 1.5. Plastics materials of this type are available on the market as transparent mass-produced plastics and in shapes easy to process.

Further details and improvements of the present invention will be apparent from the following description of the embodiments shown in the drawing when read in conjunction with the claims. In the drawings Figure 1 shows an axial section through a reflector with open vertex area; Figure 2 is an enlarged view of the detail marked II in Figure 1; Figure 3 is an enlarged view of the detail marked Ill in Figure 1; Figure 4 is an enlarged view of one embodiment of the vertex area of a reflector; Figure 5 shows another embodiment of the vertex area of the reflector, and Figure 6 shows a third embodiment of the vertex area of a reflector with a single coneshaped prism.

The reflector 1 of which a semi-section is shown in Figure 1, comprises a forward reflector portion 2, a rearward reflector portion 3 and a vertex area 4. In the middle of the vertex area 4, an optical axis 6 extends through the reflector.

This axis is simultaneously the axis of rotational symmetry of the reflector 1. A focus 7, at which a light source is arranged, is also located on the said optical axis 6. The light emitted by the light source is to emerge from the reflector substantially parallel to the optical axis 6. The reflector 1 consists of a plurality of annular prisms 8 which are axially symmetrical relative to the axis 6 and in contact with each other in the area of a small web 9 which provides the necessary mechanical connection. The annular prisms 8 are total reflection prisms of triangular cross-section having three outer surfaces or faces, namely a first face 10, a second face 11 and a third face 12. The first face 10 of all angular prisms 8 are cylinder surfaces and coaxial to the optical axis 6.Minor deviations from the cylinder surface towards the shape of the surface of a cone may be applied for production convenience (easy removal from the mould). The second faces 11 which together form the rear face of the reflector 1 in the forward and rearward reflector portion, represent together approximately the shape of a paraboloid of revolution which may also be approximated by individual straight portions extending from one web 9 to the next web 9 and, thus, corresponding to the step height of the individual annular prisms 8. The third faces 12 in the forward reflector portion 2 and in the rearward reflector portion 3 have the shape of the surface of a truncated cone, with the apex of the cone coinciding with the centre of the light source (focus 7).At the junction of the forward reflector portion 2 and the rearward reflector portion 3, the third face 12 is a plane face relative to the optical axis 6, it forms a right angle with the optical axis 6, and its point of intersection with the optical axis 6 coincides with the focus 7.

This arrangement of the third faces offers the advantage that practically no portion of the light is lost through these faces due to a shading effect, as the whole amount of light impinges upon the first face 10 at a relatively steep angle. The angle of incidence of the light at the first face 10 and the inclination of the associated second face 11 together with the double refraction encountered when the light enters an annular prism 8 and emerges from the annular prism 8 results in the desired beam concentration, as shown in detail in Figures 2 and 3.

In the reflector 1 of which one half is shown in Figure 1 , the area portion 4 is a hole which serves to introduce a light source, for instance a bulb, the filament of which must be brought to coincide with the focus 7 if the light rays 13 and 14 emitted by the light source are to emerge after the total reflection from the reflector 1 as light beams 1 5 and 1 6 oriented parallel to the optical axis 6. If the light beams 1 5 and 16 are to converge or diverge, it will suffice to being the light source out of coincidence with the focus 7 and to displace it a little in one or the other direction along the optical axis. Convenientiy, the inclinations or angles of cone of the third faces 12 should be adapted, too.

A diverging or converging orientation of the whole or part of the light beams may be obtained also by either giving the faces a curved shape or else varying their orientation relative to the optical axis 6 so that for instance the second faces 11 do no longer follow an approximated parabolic contour.

In certain cases, the light source is not introduced and secured in position through the vertex area 4, but either through a lateral opening or else from the front of the reflector 1. In such cases it is desirable that the vertex area 4 should also act as reflector surface.

In order to avoid excessive light losses, it is necessary in this case to modify the shape of the angular prisms so that the second faces will no longer follow a parabolic contour but include between them a vertex angle of approx. 900.

Figures 4 to 6 shows correspondingly shaped annular prisms 1 8 for the vertex area 4. The second faces 1 1 ' of the annular prisms 18 form in the vertex area 4 a cone curface whose angle of cone is 900.

The first faces 10 have now as before the shape of cylinder surfaces. The third faces 12' have a lens like curvature. As appears from Figure 4, the third faces 12' of the individual annular prisms 18 form sort of an echelon lens or fresnel lens. As represented, the vertex area 4 comprises a solid angle of 50 the apex of which coincides with the focus 7 (Fig. 4).

In the embodiment shown in Figure 4, certain portions of the light emitted by the light source are suppressed. These portions are shaded in the drawing. The curvature of the third faces 12' is such that the orientation of the light is parallel to the axis following the entry into the annular prism 1 8 and radially relative to the optical axis 6 following the total reflection by the faces 1 1'. This means, however, that the light of a beam portion 1 9 which following the total reflection impinges upon the third face 12' of the inwardly adjacent annular prism 18, is lost. Also, the light of a beam portion 20 which impinges upon the first face 10, is lost.While the light losses represented by the beam portion 20 cannot be avoided, the light losses in the beam portion 1 9 can be eliminated by giving the third face 12" in the vertex area 4 the shape of plane faces extending at a right angle relative to the optical axis 6, as shown in Figure 5.

At the same time, the second faces 11" are given a curvature to obtain total reflection in a manner such that the reflected beam is oriented radially relative to the optical axis 6. The annular prism 18 of the vertex area 4 which has the greatest diameter and is next to the rearward portion 3 may however also remain unchanged as compared to Figure 4.

In the arrangements shown in Figures 4 and 5, the light emitted from the focus 7 is reflected to the focus so that the light emerging from the vertex area of the reflector forms a slightly diverging beam.

It is, however, also possible to give the whole vertex area 4 the form of a single conical prism 21 which only lacks the first faces 10, as compared to the arrangement shown in Figure 4. The angle of cone is again 900, and total reflection is encountered at the second face 1 1'. The face 12' may be given different curvatures so that for instance the emerging light beam is oriented parallel to the optical axis 6 or reflected through the focus.

Claims

1. A reflector for vehicle lamps and headlamps, which is made of a transparent material and which concentrates and re-emits by curved total reflection prisms the light emitted by a light source characterised in that the total reflection prisms are provided on the inner concave surface facing the light source and take the form of threesquare annular prisms presenting rotation symmetry in relation to the optical axis, a first of the three faces of an annular prism being substantially cylindrical relative to the optical axis of the reflector and a second face being formed by the back of the reflector.

2. A reflector in accordance with claim 1, characterised in that the third face is oriented substantially parallel relative to a beam emitted by the light source, thus forming a section of the surface of a truncated cone whose cone apex coincides substantially with the centre of the light source.

3. A reflector in accordance with claim 1, characterised in that, except for the vertex area, the back of the reflector has approximately the shape of a paraboloid of revolution.

4. A reflector in accordance with claim 1 or 2, with closed vertex area, characterised in that the second faces in the vertex area form between them a vertex angle of substantially 900.

5. A reflector in accordance with claim 4, characterised in that the second faces of an annular prism form the surface of a cone and that the third faces have a convex curvature.

6. A reflector in accordance with claim 4, characterised in that the second faces of an annular prism have a convex curvature and the third faces are plane faces.

7. A reflector in accordance with any of the preceding claims, characterised in that the vertex area comprises a solid angle of up to approximately 600, the apex of this angle coinciding with the centre of the light source.

8. A reflector in accordance with any of the preceding claims, characterised in that the widths of the first faces of the annular prisms of a reflector differ.

9. A reflector in accordance with any of the preceding claims, characterised in that the axial section of at least two of the faces of the annular prisms are straight-lined.

10. A reflector in accordance with any of the preceding claims, characterised in that, except for the vertex area, it is made of a material having an index of refraction not significantly below, but preferably above 1.5.

11. A reflector for vehicle lamps and headlamps substantially as hereinbefore described with reference to the accompanying drawings.