WO2003066379A1

WO2003066379A1 - Anti glare rear view mirror

Info

Publication number: WO2003066379A1
Application number: PCT/GB2003/000527
Authority: WO
Inventors: Peter James Milner
Original assignee: Bending Light Ltd
Priority date: 2002-02-06
Filing date: 2003-02-06
Publication date: 2003-08-14
Also published as: AU2003208400A1; GB0202678D0

Abstract

A rear view mirror is described which comprises a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected. Behind the first optical element is positioned a second optical element in the form of a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at such an angle to the first element that no light incident on it through the said first element is reflected from any part of its area towards the eye of an observer positioned to receive light reflected from the first element. The optical elements are contained within a housing the interior of which has a surface form such that any part of it visible through the first element either directly or via reflection from the said second element has a relatively reflectivity sufficiently low as to provide a suitable background for viewing in the second position of the second element.

Description

ANTI GLARE REAR VIEW MIRROR

The present invention relates generally to a rear view mirror system, and in particular to an improved rear view mirror system having means for dimming whereby to avoid dazzling the observer.

Dimming rear view mirror systems are known as such. The requirement for dimming rear view mirrors arises because of the intensity differential between the light arriving at the observer's eye from the mirror in normal daylight conditions, and the light arriving during night driving. In the latter conditions vehicles have illuminated headlights or headlamps, and even if following vehicles use dipped headlights in order to prevent glare from dazzling the drivers of oncoming vehicles or affecting the driver of a vehicle ahead, this can nevertheless frequently cause danger or discomfort. The danger arises because of the involuntary reaction of the eye's pupil to bright lights, reducing in size to restrict the quantity of light entering the eye. This property, known as accommodation, does not distinguish between light from a single bright source and light arriving from the entirety of the field of view. When driving at night the light intensity arriving at the eye is generally lower than during daylight hours, with the exception of the headlights of other vehicles, and if the eye becomes accommodated due, for example, to dazzling by oncoming vehicles, or via the rear view mirror, by following vehicles, the driver's ability to perceive the remaining part of the driving environment is detrimentally affected. The shining of a bright light on the retina also causes physical discomfort and a tendency for drivers to screw up their eyes and/or turn their heads away from the source of bright light. One known rear view dimming mirror system which has been widely adopted to assist in dealing with the problem of bright headlamps in following vehicles comprises a generally wedge-shape prism which is mounted within a housing and the rear face of which (that is the face remote from the observer) is silvered in a conventional manner. 5

It is noted in passing that, throughout this description, the front and rear faces of a reflector will be identified with respect to the observer, namely the front face being _v . that facing the observer and the rear face being that directed away from the observer notwithstanding the fact that in a rear view mirror in a motor vehicle the "rear" face as ° above defined is in fact nearer the front of the vehicle and travels in advance of the

"front" face as herein defined. The term "silvering" will also be understood to refer to any coating on a transparent element by which the element is made more reflective.

The coating may be made of any suitable material for this purpose and a number of such materials are known commercially. 5

A prismatic reflector system such as that described above acts to provide the observer with a number of superimposed images, one from the main silvered reflecting surface, one from the front face of the prism, which reflects a small proportion of incident light, and one from another angle resulting from light which has been refracted at the 0 front face of the prism, reflected at the rear face back to the inside of the front face, resulting in reflection of a small proportion of the light at the front face and subsequent further reflection at the rear face before passing out through the front face. Other images are also formed but the three described above are the brightest. Typically, at a glass/air interface, about 4% of the incident light is reflected and 96% of the incident light is transmitted. This small percentage of reflected light is responsible for one or more of the secondary images.

Although these images are superimposed but not in register with one another they do not cause confusion due to the considerable brightness differential between the primary image, that is the one formed by a single reflection at the silvered surface, and the "secondary" images. The primary image is brighter than the secondary images by a factor of about twenty. The prism has a shallow angle of about 4 degrees between its front and rear faces and is mounted in the housing (or the housing is mounted) so that it can be turned through about this angle between first and second predetermined orientations.

Thus, when the prism is in the first orientation with respect to the housing, the housing itself can be adjusted to bring the desired field of view into the line of sight of the observer via the primary reflection. Then, by reorienting the prism through the small angle necessary to move it to its second predetermined orientation, the light diverted by the main silvered surface from the desired field of view is no longer directed at the observer's eyes, but now light from the desired field of view arrives via the reflection at the front surface of the prism. This reflection, as mentioned above, is less by a factor of twenty in brightness than the primary reflection and, in night driving conditions, this allows the observer to look at the image in the rear view mirror, even if this contains bright headlamps, without his eyes being accommodated, which would reduce his forward vision, and without discomfort. If the prism is in the second orientation when the housing is adjusted to bring the primary reflected image into the line of sight of the observer, then upon displacement to the first predetermined orientation of the prism within the housing the second secondary image of the desired field of view is presented to the observer. This is achieved by an appropriate relationship between the angle of inclination of the two prism faces and the angular displacement of the prism between the first and second orientations.

The known dimming rear view mirror systems, although widely accepted, have a number of disadvantages including weight, complexity of structure and cost. There is also a slight degradation of the image by chromatic dispersion of light passing twice through the prism when being reflected at the main silvered surface.

The three above-mentioned superimposed images thus comprise a bright "central" image and two secondary images which are formed by light arriving from above and below the direction of the "central" image so that by turning the mirror through a predetermined angle in one direction or the other about a horizontal axis the bright central image can be replaced by a dimmer image of the same field of view (assuming the observer's position is fixed). When the mirror is oriented with the "central" image as the defined field of view through the rear window of the vehicle, the "upper" and "lower" secondary images, apart from being dimmer by virtue of the path the light travels in order to reach the observer, also have, in normal circumstances, less intensity variation, and therefore fewer contrasts likely to cause confusion when observed. The eye and brain thus have no difficulty in ignoring the dim images arriving from different directions and partially overlapping the central image when the central bright image is formed from a bright source such as the rear window of the vehicle. However, there are circumstances where a bright light source may cause a confusing secondary image, for example if an open-top vehicle or one with an open sunroof is oriented such that the sun is in the field of view of the upper secondary image this may cause difficulties. Moreover, when the reflector system is set to direct

⁵ the desired field of view through the rear window towards the observer via a secondary light path the possibilities for troublesome brighter images arriving from the "central" light path may, because of their greater brightness, cause double images

^ which it is difficult for the observer's brain to decipher thereby resulting in confusion.

° The present invention seeks to provide an improved rear view mirror system in which at least some of the above disadvantages are avoided.

According to one aspect of the present invention a rear view mirror comprises a first optical element of transparent material through which a proportion of the incident ⁵ light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the 0 first element, and a second position in which it lies at such an angle to the said first element that no light incident thereon through the said first element is reflected thereby from any part of its area in a direction such as to reach the eye of the observer when in a position to receive light reflected from the first element, and the said optical elements being housed in a housing the interior of which has a surface form such that any part of it visible directly or via reflection from the said second element has a relatively low reflectivity such as to provide a suitable background for viewing in the second position of the second element rear view mirror of this form does not suffer from the unwanted superimposition of images resulting from light incident from different directions, and therefore is not at risk of sudden variation in the observed image if the relative intensity of the sources varies. It is also light in weight, economical to produce in that it does not require complex shapes or optical components other than plane reflectors and or optically transparent sheets, and the orientation of the rear view mirror to give the observer the primary field of view is unambiguous.

Preferably the first element comprises a plane sheet of transparent plastics material, suitably polycarbonate. The front surface of the first element may be coated with a suitable hard coating to protect its surface against scratches. One or more coating to increase or decrease the reflective properties of one or both surfaces may also be provided.

In a further aspect of the present invention there is provided a composite rear view mirror assembly comprising two Fresnel prisms each having a flat face and a faceted face, held together with their faceted faces towards one another but not touching, the flat face of one of the Fresnel prisms being formed as a reflective surface or having a plane reflector held in parallel with respect thereto, and means for mounting the said assembly such that it can be turned between first and second predetermined orientations with respect to a housing of the mirror. A front face of the other of the two prisms is preferably coated with a hard coating to protect the surface from scratches and the like.

The housing may be selectively orientable to incline the reflecting surfaces at a selected angle with respect to the observer to enable the obperver to choose a desired field of view.

The present invention also comprehends a rear view mirror of the type having means for dimming the image seen by an observer to avoid dazzling the observer, comprising a Fresnel prism, reflector means on or associated with one face of the prism, and a housing within which the prism is mounted and with respect to which the prism is turnable between first and second predetermined orientations inclined to one another by a predetermined angle.

The said reflector means may be constituted by a silvered surface of the prism and the silvered surface may be the plane face of the prism or the faceted face thereof. The reflector means may alternatively comprise a separate mirror element mounted in physical juxtaposition against one face of the Fresnel prism.

In another aspect the present invention provides a rear view mirror comprising a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at an angle to the said first element and the said optical elements being housed in a housing the interior of which has a surface form such that any part of it visible through the first element either directly or via reflection from the said second element has a relatively low reflectivity such as to provide a suitable background for viewing in the second position of the second element.

The present invention also comprehends a rear view mirror comprising a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at an angle to the said first element of not less than 40°.

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

Figure 1 is a side view of a prior art prismatic dimming rear view mirror;

Figure 2 is a side view of a composite mirror formed as a first embodiment of the present invention;

Figure 3 is a side view of a composite Fresnel mirror formed as a further embodiment of the present invention;

Figure 4 is a side view of an alternative form of composite Fresnel dimming 5 mirror formed as an embodiment of the present invention;

Figure 5 is a simplified embodiment of composite Fresnel dimming mirror; Figure 6 and Figure 7 are respective side views of single element Fresnel dimming reflectors formed as embodiments of the present invention.

° Referring first to Figure 1 the prior art dimming rear view mirror essentially comprises a prism generally indicated 11 having a front face 12 and a rear face 13 inclined at about 4 degrees to one another (although this drawing shows the faces at a greater angle for clarity of illustration). The prism 11 is mounted in a body 14 of the mirror assembly, which body may be suspended by a suspension coupling 15, illustrated as a 5 ball coupling allowing the orientation of the housing 14 and thus of the mirror assembly as a whole, to be adjusted in any direction to allow the observer to select a desired field of view.

The rear face 13 of the prism 11 is silvered to provide a primary reflecting surface. An ° observer O viewing the front face 12 of the prism 11 will receive light representing a plurality of superimposed images the brightest three of which are represented in the drawing by ray traces. The primary image, namely that formed by reflection from the surface 13, is represented by the ray trace 16. This is the brightest image and in the orientation illustrated in Figure 1 the observer O will see the field of view represented by the horizontal line which, in use as a motor vehicle rear view mirror, will represent the direction of incident light through the rear window of the motor vehicle. It should be noted the effects of refraction at the face 12 have not been illustrated in the drawing.

A second and dimmer image arrives at the observer O from jeflection at the front face 12 of the prism 11 , and in the orientation illustrated this image, represented by the ray trace 17, will originate from light from a low region within the interior of the passenger compartment. A second dim image, represented by the ray trace 18 arrives after reflection at the rear face 13 and further reflection at the interface defined by the front face 12 where a small proportion of the light is reflected into a path parallel to the ray trace 16 and then reflected, like the light of ray 16, at the rear face 13 towards the observer O. The two secondary images formed by reflection at the front face 12 (ray trace 17) and multiple reflection within the refractor 11 (ray trace 18) are very much less bright than the primary image represented by the ray trace 16, typically by a factor of 20, because each involves a weak reflection at a glass/air interface.

By turning the refractor 11 through a predetermined angle in one direction or the other from the orientation shown the ray 17 or the ray 18 may be brought to a horizontal orientation so that an observer can see the desired field of view through the rear window, but at a lower intensity due to the weaker reflections along the path follows.

Figure 2 shows an embodiment of the invention in which the disadvantages of the prior art are overcome, and in which there are no superimposed images from unwanted directions, and no dispersion of the light arriving at the observer from the main reflector in either its main or secondary position. The embodiment of Figure 2 comprises a first optical element 21 in the form of a flat sheet of polycarbonate typically in the region of 3mm thick, the front (i.e. facing the observer) face of which is hard coated, and which is fixed in a housing 22 which comprises a flat rear wall 23, an arcuate upper wall 24 and two sector-shape end walls 25. The housing 22 is mounted in a known way (not shown) which allows it to be oriented as desired by the observer O in order to be able to see reflected thereby an appropriate field of view, in this case represented by the target T which may be the view through the rear window of the motor vehicle.

A second optical element, in this case a reflector 26, is mounted within the housing 22 immediately behind the first reflector 21. The second reflector 26 is pivotally mounted by a pivot or hinge 27 at the bottom edge of the housing at the bottom of the rear wall 23 so that the second reflector 26 can be turned between the position shown in solid outline in Figure 2, where it lies parallel to and closely adjacent the first reflector 21 , and a second position, shown in broken outline in Figure 2, and identified

26' in which it lies flat against the rear wall 23 at an angle to the first reflector 21.

' This angle must be such that the difference between the path of the reflected light from this reflector in its first or forward position and the path of the reflected light in its second position is such that an observer positioned to receive light from the reflector in the first position receives no reflected light from any part of this reflector in its second position. Of course the pivot or hinge 27 may equally well be positioned at the top edge of the housing. The second reflector 26 as illustrated is a conventional glass mirror, that is a flat glass sheet with its rear face silvered (conventionally known as a second surface reflector because the reflection of light actually takes place at the rear or second surface of the reflector, having passed through the front or first surface). Alternatively, of course, the second reflector 26 may be formed as a first surface reflector, that is with a reflecting surface on its front face.

When the second reflector 26 is in its "forward" position as shown in solid outline in Figure 2 it reflects light through the same angle as the first reflector 21 so that each reflector directs light to the observer O from the same field of view, the two images being superimposed and non-conflicting. Because there are no prisms-involved there are no alternative paths for light from differently inclined surfaces which may reach the observer from other directions. In the main or "daylight" viewing position, therefore, the observer sees clearly a bright image selected by his chosen orientation of the housing 22, typically through the rear window of a motor vehicle.

When the second reflector 26 is moved to its "rear" or second position 26' lying against the back wall 23, the only path for light reflected by it to the eye of the observer O is now from the interior of the housing 22. This can be achieved either by ensuring that the mirror turns through a sufficiently great angle (probably in excess of 40°) or by bringing the upper wall of the housing forward, as shown at 28 in Figure 2, in order to obstruct the arrival of light from high in the sky, in which case, as shown in Figure 2, the mirror 26 can turn through a smaller angle and still have the same effect. The interior surface of the mirror housing, at least that part of it which the driver sees reflected from the silvered reflector in its second, or night, position, is finished in a texture and colour that provides a suitable background for night time viewing. This could be an ultra low reflectivity matt black if no background at all is desired, but it could be a grey or a colour with a greater reflectivity if a visible background is desired. There are several reasons why a visible background may be desirable, including the ability to indicate the presence of a light source such as a headlight while it is out of the direct view of the observer and to reduce the contrast between the image of a light source such as a headlight and the background for more comfortable viewing. The light reflected by the first reflector 21, being the result of the much weaker reflections from the front and rear faces of a transparent sheet, still reach the observer, and consequently for night viewing, when bright headlamps of following cars may cause dazzling, this weaker reflection provides a sufficiently bright image for the observer to establish the location and presence of following vehicles.

Typically, the night dimming factor is in the region often, that is the brightness of the image from the second reflector 26 is ten times brighter than that from the first reflector 21. This is because there is a weak reflection at each of the front and rear faces of the front reflector 21 , amounting to about 4% of the incident light. This configuration has advantages of economy (since no tapered and ground prism is required), lightness, for the same reason, and simplicity of use since there is only a single "bright" image position and a single "dim" image position unlike the prior art device described in Figure 1 where there are two "dim" image positions. This, therefore, constitutes an inherently safer mirror than the prior art type in that unwanted images from bright sources in unexpected directions cannot suddenly create confusing reflections as can occur, as mentioned above, with the prior art mirror.

A second embodiment of the invention is described in relation to Figures 3 and 4, which illustrate an alternative composite mirror having some advantages over the prior art prismatic mirror illustrated in Figure 1. As shown in Figure 3 the mirror structure comprises a pair of Fresnel prisms 31, 32 oriented with their faceted faces towards one another with a small gap 36 between them, and with the front face 37 of the forward prism 31 hard coated on its flat side to provide protection against scratches or scrapes. A second reflector 33 is bonded to the rear face 38 of the rear Fresnelprism 32. As shown in Figure 3 this is a first surface reflector although a second surface reflector may alternatively be used.

In the orientation shown in Figure 3 the observer O can receive light from three directions, namely a bright reflection from a target direction Tl which is reflected at the reflecting surface of the reflector 33, a second target direction T2 reflected from the interface between the Fresnel facets of the two Fresnel prisms 31, 32 and T3, which is reflected first at the reflector 33, then at the interface between the Fresnel facets of the two Fresnel prisms 31, 32 and finally reflected at the rear reflector 33 again. The path followed by this light ray is represented in the drawing by reflection at the points A, B, C in succession. As with the mirror of Figure 1 the effects of refraction upon transmission through a transparent body have not been illustrated as these do not aid an understanding of the reflections which take place. The second and third images T2 and T3 are dimmer than the main image Tl by a factor of about 10 because each reflection at the interface between the two Fresnel prisms is in fact two reflections of about 4% of the incident light, one by the faceted surface 39 of the prism 31 and one by the faceted surface 40 of the prism 32. The second and third images can be brought into the field of view of the observer by turning the mirror assembly in one direction or the other through a fixed angle which can be pre-set by appropriate mechanical arrangements between the prism assembly and a housing 34. As with the prior art solid prism arrangement it is only necessary for the Fresnel prism assembly of Figure 3 to have two indexed positions within the housing 34 so that dimming can take place by moving the prism assembly from one position to the other dependent on which position it was in when the housing was oriented to bring the main reflected image Tl into the field of view of the observer.

A simplified version of the embodiment of Figure 3 is illustrated in Figure 4, where the same reference numerals are used to identify the same components or those which have the same function. It will be appreciated that the facets of Fresnel prisms shown in any of Figures 3 to 7 have been illustrated on an enlarged scale for the purpose of clarity. In practice the facets are likely to be in the region of 1mm or less in width. The embodiment of Figure 4 differs from that of Figure 3 in that, instead of providing a separate reflector element 33, the rear face of the rearward Fresnel prism 32 is itself silvered, this being represented by the darker line 33 in Figure 4. Again the dimming factor between the bright image and the two dimmer images is about 10: 1 for the same reason as explained above and this structure has the advantage over the prior art mirror of Figure 1 of lightness and economy, and the fact that the dimming factor is in the region of 10 to 1 where the dimming factor in the prior art is about 20 to 1.

Figure 5 illustrates an even simpler Fresnel prism rear view mirror in which a single Fresnel prism 51 oriented with its Fresnel facets 52 facing the observer O is backed by a mirror 53 to which it is bonded by a transparent adhesive. The mirror 53 is illustrated as a first surface reflector although, as before, a second surface reflector may be used to the same effect. The brightness differential in this embodiment is of the order of 20: 1 because the secondary reflections are only from one surface rather than two as in the embodiments of Figures 3 and 4.

The embodiments of Figure 6 and 7 are even simpler, comprising a -single Fresnel refractor 61 in the case of the embodiment of Figure 6 and 71 in the case of the

' embodiment of Figure 7. The Figure 6 refractor 61 is oriented with its refractor facets 62 facing forwardly, and with its rear face 63 silvered. The refractor 61 is housed within a housing 64 so that it can be oriented between first and second positions, and the housing 64 is mounted (by means not shown) so that it can be oriented as selected by the observer O. A bright image Ti is formed by reflection from the reflective surface 63 after refraction at the faceted face 62. A first dim image T₂ is formed by reflection from the front faceted face 62 and a second dim image by multiple reflection in a manner similar to that which occurs in the embodiment of Figure 5. The advantage of the Fresnel prism structure of Figure 6 is primarily economic since a single micro-replicated Fresnel prism silvered on one side can be produced more economically than the solid prisms of the prior art mirrors. It also has a weight advantage since the single polycarbonate sheet constituting the reflector is very much lighter than a solid glass prism.

The embodiment of Figure 7 corresponds to that of Figure 6, but with the Fresnel prism 71 oriented oppositely so that its flat face 73 faces forwardly and the faceted face 72 faces away from the observer and carries the silyered coating. It has the advantage over the embodiment of Figure 6 that the flat face is directed towards the observer and exposed in the opening of the housing 74, and is thus, being a flat face, easier to keep clean than the faceted surface 62 of the embodiment of Figure 6.

Claims

1. A rear view mirror comprising a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at such an angle to the said first element that no light incident thereon through the said first element is reflected thereby from any part of its area in a direction such as to reach the eye of the observer through the said first element.

2. A rear view mirror as claimed in Claim 1 , in which the said optical elements are housed in a housing the interior of which has a surface form such that any part of it visible through the first element either directly or via reflection from the said second element has a relatively low reflectivity such as to provide a suitable background for viewing in the second position of the second element.

3. A rear view mirror as claimed in Claim 1 or Claim2, in which the first element comprises a plane sheet of transparent plastics material.

4. A rear view mirror as claimed in Claim 1 or Claim 2, in which the first element is a sheet of polycarbonate.

5. A rear view mirror is claimed in Claim 2 or Claim 3 , in which the front surface of the first element is coated with a protective transparent hard coating.

6. A rear view mirror as claimed in any preceding claims, in which the second element is a plane reflector pivotally mounted at or in the region of one edge.

7. A rear view mirror as claimed in Claim 5, in which the second element is silvered on its rear face.

8. A rear view mirror as claimed in Claim 5, in which the second element is silvered on its front face.

9. A rear view mirror as claimed in any preceding claim, further comprising a third reflector of the same form as the first, located between the first and second reflectors and mounted so as to be rurnable about substantially the same axis as the second reflector.

,

10. A rear view mirror as claimed in any preceding Claim in which the interior surface of the housing is of a colour and texture such that the image reflected by the second element when in its second position has a moderate contrast with a light source such as a headlamp.

11. A rear view mirror as claimed in Claim 9, in which the interior surface of the housing is matt black with a minimum reflectivity.

12. A rear view mirror as claimed in any preceding Claim, in which the interior , surface of the said housing is entirely free from surface features or components at which reflection of incident light may take place.

13. A rear view mirror of the type having means for dimming the image to avoid dazzling by bright headlights of following cars, having two reflecting surfaces inclined at a fixed angle with respect to one another and turnable together between two predetermined positions inclined to one another by the same angle as that between the reflectors themselves such that light from an object it is desired to view is reflected to the observer's eye by a selected one of the two reflective surfaces in dependence on which of the said predetermined positions is selected, in which the mirror comprises 8a Fresnel prism and the two reflectors are respective opposite surfaces thereof.

14. A rear view mirror as claimed in Claim 12, in which the plane surface of the Fresnel prism faces away from the observer and is silvered.

15. A rear view mirror as claimed in Claim 12, in which the faceted surface of the Fresnel prism faces away from the observer and is silvered.

16. A composite rear view mirror assembly comprising two Fresnel prisms each having a flat face and a faceted face, held together with their faceted faces towards one another but not touching, the flat face of one of the Fresnel prisms being formed as a reflective surface or having a plane reflector held in parallel with respect thereto, and means for mounting the said assembly such that it can be turned between first and second predetermined orientations with respect to a housing of the mirror.

5 17. A composite rear view mirror assembly as claimed in Claim 15, in which a front face of the other of the two prisms is coated with hard coating.

18. A composite rear view mirror as claimed in Claim 15 or Claim 16, in which the housing is selectively orientable to incline the reflector surfaces at a selected angle o with respect to the observer to enable the observer to choose a desired field of view.

19. A rear view mirror of the type having means for dimming the image seen by an observer to avoid dazzling the observer, comprising a Fresnel prism, reflector means or associated with one face of the prism, and a housing within which the prism is 5 mounted and with respect to which the prism is turnable between first and second predetermined orientations inclined to one another by a predetermined angle.

20. A rear view mirror as claimed in Claim 18 in which the said reflector means is constituted by a silvered surface of the prism. 0

21. A rear view mirror as claimed in Claim 19, in which the silvered surface is the plane face of the Fresnel prism.

22. A rear view mirror as claimed in Claim 19, in which the silvered surface is the faceted face of the Fresnel prism.

23. A rear view mirror as claimed in Claim 18, in which the reflector means • comprises a separate mirror element mounted in physical juxtaposition against one face of the Fresnel prism.

24. A rear view mirror comprising a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at an

, angle to the said first element and the said optical elements being housed in a housing the interior of which has a surface form such that any part of it visible through the first element either directly or via reflection from the said second element has a relatively low reflectivity such as to provide a suitable background for viewing in the second position of the second element.

25. A rear view mirror comprising a first optical element of transparent material through which a proportion of the incident light is transmitted and from an incident face of which a proportion of the incident light is reflected, a second optical element positioned behind the said first optical element, the said second optical element being a reflector movable between a first position in which it lies substantially parallel to the first element such that light is reflected thereby in the same direction as light reflected from the incident face of the first element, and a second position in which it lies at an angle to the said first element of not less than 40°.