WO2000016132A1

WO2000016132A1 - Prismatic array for rear view mirror

Info

Publication number: WO2000016132A1
Application number: PCT/GB1999/003052
Authority: WO
Inventors: Peter Milner
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-09-15
Filing date: 1999-09-15
Publication date: 2000-03-23
Anticipated expiration: 2001-03-15
Also published as: GB2342726B; AU5875199A; GB2342726A; GB9819946D0

Abstract

An optical component (11) has at least two faces (13, 14) inclined to one another and meeting at an edge (15) defined by the line of intersection between the faces. The unwanted phenomenon of flare in light transmitted through or reflected from the optical component is reduced by surface treatment of the faces (13, 14) in the vicinity of the edge (15). This treatment includes surface roughening by etching or abrasion, or obscuration by opaque or semi-opaque coatings.

Description

PRISMATIC ARRAY FOR REAR VIEW MIRROR

The present invention relates to an improved optical component, and particularly to an optical component for use in an optical system involving light transmission through or reflection from one or more such components.

In optical systems generally, but especially those having one or more optical components with regularly repeating surface features such as fresnel prisms, there is experienced a problem with the creation of unwanted images of an object, especially a light-emitting object, due to the apparent extension of the image of the object in one direction. The effect of this is to form a linear image of a point source. In certain circumstances this can be entirely unacceptable. The phenomenon is known as "flare" and, in fact, occurs in a number of optical systems not just those having regularly repeating surface features .

One situation in which flare can be observed, and in which it is unacceptable, is in the motoring field where, at night, the headlights of oncoming or following vehicles act as effectively point sources and are to a large extent the only visible part of the vehicle. If, on viewing a following vehicle through a rear view mirror, the phenomenon of flare should be present this degrades the information which a driver can extract from the image and, in particular, renders almost impossible the judgement of distance which, however, is an important factor in determining safe driving.

The flare phenomenon occurs when, upon refraction or reflection, light rays from a so-called point source are deviated by successively greater amounts the greater the angle of incidence (that is the angle with respect to a normal to the incident plane) such that successively more inclined light rays give rise to successive (and therefore overlapping) images of the source along a preferred direction. Each successive image so overlaps the adjacent images that an effectively linear extension of the original light source is seen by an observer.

The present invention seeks to provide an optical component in which the above-mentioned phenomenon of flare is very substantially reduced, allowing such an optical component to be utilised in a wide range of optical systems in which flare has until now been at least a nuisance and at worst entirely unacceptable.

According to one aspect of the present invention there is provided an optical component having at least two faces which are inclined to one another to meet at an edge defined by the line of intersection between the two faces, in which the optical properties of at least one of the faces in the vicinity of the said edge are different from those of at least the major part of the face itself whereby at least partly to reduce the phenomenon of flare in light transmitted through or reflected by the said component .

The present invention may also be considered as an optical component of the type having a repeating pattern of inclined faces each pair of adjacent inclined faces meeting at an edge defining a crest or a trough of the component, in which the region of each face adjacent an intersection is irregular such that the intersection of two adjacent surfaces comprises a generally plateau like region the surface flatness and the boundaries of which are less regular than that of the said inclined surfaces.

It has been realised that a most significant region of an optical component in the generation of flare is an intersection between two inclined surfaces and it has been determined that, by degrading the optical transmissivity or reflectivity of the component in the immediate vicinity of an intersection between two inclined surfaces the phenomenon of flare can be substantially reduced. Such degradation can be by reducing the optical quality of the surface, for example by surface roughening to produce irregular asperities the effect of which is to cause dispersion of the light incident thereon, either by reflection or refraction, or alternatively by causing an obscuration of the surface or surfaces at or adjacent the intersection. Such obscuration may be achieved for example by the introduction of an opaque layer in the vicinity of the intersection line.

The present invention also comprehends an optical component having at least two inclined surfaces which meet along a line of intersection, in which the said line of intersection of the surfaces is irregular whereby to cause diffusion of that part of the light passing through or reflected by the optical component the path of which takes it into proximity with the said line of intersection.

As an alternative, the present invention may be considered to be constituted by an optical component having at least two inclined surfaces which meet at a line of intersection, in which the said surfaces are obscured in the vicinity of the said line of intersection whereby to limit the transmission or reflection of light at or through the said inclined surfaces to the region thereof other than in the vicinity of the said line of intersection.

One way in which the optical properties of two surfaces intersecting at a line may be treated may be the provision of a "plateau" joining the two intersecting surfaces and effectively representing a removal of material at or adjacent the peak or edge at which the surfaces intersect . Such a plateau should not be an entirely smooth surface matching that of the inclined surfaces, but preferably should be a rough surface and more preferably an irregular rough surface through which, nevertheless, light may pass or at which it may be reflected, although not specularly.

A particular embodiment of the present invention as defined above may be constituted by a fresnel prism the optical quality of the crests of which is degraded by surface roughening. The scale of the asperities caused by the surface roughening may be within a wide range from a very fine matte to a coarse grit. In one embodiment the roughening is of the order of magnitude as would be produced by abrading contact with a 400 grit abrasive.

In this respect the present invention may be considered to be constituted by a fresnel prism in which the phenomenon of flare is at least substantially reduced by modifying the shape of the prism in the vicinity of the crests thereof whereby to introduce irregularities. Such irregularities may comprise roughening of the surfaces of the prism, the formation of irregular boundaries, especially at a plateau created by the removal of crests, and/or the reduction in surface transmissivity, up to and including the rendering of at least part of the surface opaque. The process for achieving this surface finish may be part of the manufacturing process or may be undertaken subsequent to manufacture as one or more so- called "post-process".

The optical component may form part of a motor vehicle rear view mirror and the present invention comprehends a rear view mirror incorporating an optical component as herein defined.

Everything discussed hereinabove in respect of the crests of a fresnel prism is equally applicable to the troughs although different techniques will be required for the achievement thereof, especially formation of a "plateau" which, in a trough region, may only be produced at the manufacturing stage .

Various embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic sectional view of a prior art fresnel prism;

Figure 2 is a schematic sectional view similar to that of Figure 1, illustrating a fresnel prism formed in accordance with the principles of the present invention;

Figure 3 is a plan view of a prior art fresnel prism such as that shown in Figure 1; and

Figure 4 is a plan view similar to that of Figure 3 , illustrating a fresnel prism formed as an embodiment of the invention.

Referring first to Figure 1 a section of a fresnel prism, greatly enlarged, is illustrated. This comprises a body 11 of transparent material having a planar surface 12 on one face and a plurality of inclined faces 13, 14 on the other. Each pair of inclined faces 13, 14 meets at a crest 15 and the next adjacent pair of inclined surfaces 14, 13 meets at a trough 16. The inclination of the surfaces 13 which are inclined in one directional sense are generally at a different angle to the plane of the surface 12 from that of the other surfaces 14 although the variation in inclination differs from one prism to another in dependence on the nature of the optical function it is to perform.

Light can be transmitted through such a prism, either from the side of the face 12 such that it passes through the body towards the inclined faces 13, 14 or alternatively, in the opposite direction from the inclined faces 13, 14 towards the face 12.

It has been established that the flare phenomenon discussed above is associated in at least substantial measure with the optics of the intersection edges between the inclined surfaces 13, 14, perhaps acting in a manner similar to interference or diffraction effects, to cause light from a "point" light source to be refracted at greater angles of refraction at greater angles of incidence whereby to form spurious images of the object. As can be seen in Figure 2 this flare phenomenon can be reduced simply by removing the crests 15 from the fresnel prism 11. However, merely removing the crests to leave fresh intersection lines between a plateaux 17 and the inclined surfaces 13, 14, would not solve the problem since it would merely introduce two fresh edges at which the flare phenomenon could be "seeded". In accordance with the principles of the present invention the plateaux 17 are formed in such a way that they are irregular both as far as the plane of the plateau is concerned, and in Figure 2 it will be seen that the three plateaux illustrated, identified 17a, 17b, 17c are shown (exaggerated for the purposes of illustration) to lie at different angles to the plane of the surface 12. Furthermore, as illustrated in Figure 4, the lines 18, 19 which define the boundaries of the plateau 17, or in other words the intersections between the plateau 17 and the inclined surfaces 13, 14 are not straight clean lines, but rather are irregular lines the effect of which is to cause a general diffusion of the light passing through the prism in the immediate vicinity of the plateau 17 as well as passing through the plateau 17. The surfaces of the plateaux 17 are also preferably not smooth surfaces, but rough surfaces having a plurality of asperities (not illustrated) the scale of which is comparable to that caused by a 400 grit abrasive. In the illustrated embodiment the fresnel prism 11 has crests which are separated by 0.75mm and the width of each plateau is between 0.04mm and 0.06mm.

A further enhancement of the flare-reducing performance may be achieved by rendering the plateaux 17 opaque or at least partly opaque by the introduction of an obscuring material onto the surfaces of the plateaux. Such obscuration may also be relieved by acting on only one or both of the surfaces which meet at a crest or a trough to cause obscuration in the vicinity of the crest and/or the trough. Such obscuration may be achieved by coating the or each surface with a coating material which affects the optical properties of the surface, perhaps by etching or, for example, by changing its refractive index or by causing it to diffuse light upon transmission or reflection, or to cause it to absorb incident light.

Although described specifically in relation to fresnel prisms it will be understood that the present invention is applicable to any optical component having intersecting inclined surfaces at which the intersection line or edge can be optically treated, for example degraded from a smooth straight line to a rough irregular one such as to cause an unpredictable variation in the optical properties of the component from point to point along the intersection line.

Claims

1. An optical component having at least two faces which are inclined to one another to meet at an edge defined by the line of intersection between the two faces, in which the optical properties of at least one of the faces in the vicinity of the said edge differs from those of at least the major part of the face itself whereby at least partly to reduce the phenomenon of flare in light transmitted through or reflected by the said component .

2. An optical component as claimed in Claim 1 or Claim

2. in which the difference in optical properties is exhibited by both said surfaces .

3. An optical component as claimed in Claim 1, or Claim 2, in which the difference in optical properties comprises a greater surface roughness in the vicinity of the said edge than in the major area of the surface.

4. An optical component as claimed in Claim 1 or Claim 2, in which the difference in optical properties comprises a departure from the standard of flatness of the said major part of the face.

5. An optical component as claimed in Claim 4, in which the said departure from the standard of flatness comprises an abrasion or rounding of the faces of the component in the vicinity of said edge.

6. An optical component as claimed in any preceding Claim, in which the or each said face is treated in the vicinity of the said edge to vary its transmissivity .

7. An optical component as claimed in Claim 6, in which the said edge of the component is defined by a generally plateau-like region joining the said major parts of the said two faces and having a surface roughness greater than that of the faces.

8. An optical component as claimed in any preceding Claim, having a plurality of pairs of inclined faces each pair of adjacent faces meeting at an edge defining a crest or a trough of the component, in which the region of each crest or trough has optical properties different from those of the said faces whereby at least to reduce the flare phenomenon exhibited by the component.

9. An optical component as claimed in any preceding Claim, in which the difference in optical properties is achieved by obscuration.

10. An optical component as claimed in Claim 9, in which obscuration is achieved by coating that part of a surface to be obscured.

11. A fresnel prism the optical quality of the crests of which is degraded by surface roughening.

12. A fresnel prism in which the phenomenon of flare is at least substantially reduced by modifying the shape of the prism in the vicinity of the crests thereof.

13. A fresnel prism as claimed in Claim 1, in which each crest is formed as a plateau the surface and/or the boundaries of which are irregular.

14. A method of preparing an optical component of the type having a plurality of crests defined between adjacent pairs of inclined faces, comprising treating the surfaces of the said faces in the region of the events whereby to cause the optical properties of the said regions to differ from those of the reminder of the face .

15. A method as claimed in Claim 14, in which the said surface treatment is abrading.

16. A method as claimed in Claim 14, in which the said surface treatment is etching.

17. A rear view mirror incorporating an optical component as claimed in any of Claims 1 to 13.