GB2494115A - Projection display with grating arranged to avoid double images - Google Patents

Abstract

Image-bearing light 12 is injected into a waveguide 10 via an input means 14 and is directed along the waveguide by total internal reflection. The light 12 is reflected from each of the four sides of the waveguide 10 along a spiral path 18 (fig.4) and passes through an output transmission grating 16. In contrast to prior art (fig.12: output beams 128 & 132), light 12 passing through the output grating 16 is diffracted only when passing through the grating along a first portion 20 of the path 18, producing a single output beam 28. This is achieved by orienting the fringes 24 of the output grating 16 at an angle so that only one of the two modes (at portions 20 & 22 of the path) is aligned at the correct angle with respect to the grating 16 for diffraction to occur. The light passing along the path 18 passes through the output grating 16 without diffraction along the second portions 22 of the path. A double image is thus avoided and manufacturing tolerances are eased. The waveguide may form part of a head-up display.

Description

A PROJECTION DISPLAY

This invention relates to a projection display for displaying an image to a viewer which is particularly, but not exclusively, suitable for use in a head up display, a helmet mounted display or a head mounted display.

A projection display for displaying an image to a viewer is disclosed in the applicants earlier patent WO 2007/029034, the content of which is hereby incorporated by reference. A simplified view of the projection display is shown in Figure 8. The earlier patent describes a projection display 100 for displaying an image to a viewer 102 to generate a collimated display subtending a large exit pupil at the point of viewer and a large field of view while using a small image-providing light source device 104. The projection display uses a rod-like substantially rectangular cross-section waveguide 106 made of light transmissive material and a plate-like waveguide 108 made from a light transmissive and transparent material.

The image-providing light source device 104 located at a first end of the rod-like waveguide 106 injects image bearing light into the rod-like waveguide.

The image-providing light source device includes an image generating light source 110 in the form of a micro-display to provide a display of information.

Additionally the image-providing light source device includes an optical means 112 located between the image generating light source 10 and the first end of the rod-like waveguide. The optical means 112 collimates light received from the image generating light source 110 and injects the collimated image bearing light into the rod-like waveguide. The optical means 112 is of a small size and is used to collimate the light received from the image generating light source 110. The collimated light has a small exit pupil and is therefore fed into the rod-like waveguide 106 which performs the function of stretching the horizontal pupil.

The output from the rod-like waveguide is fed into the plate-like waveguide 108 which stretches the vertical pupil view and also acts as a combiner for the display. In this manner, the display information provided to the viewer 102 looking through the plate-like waveguide subtends a large exit pupil and a large field of view whilst using a small optical means 112 and a small image generating light source 110 such as a micro display.

The display includes an input means 114 provided adjacent one end of the rod-like waveguide which can multiply the image bearing light received from the optical means internally along the rod-like waveguide. In effect, the light impinging on the input means 114 diffracts such that the incidence angle of the light of the internal surfaces of the rod-like waveguide is greater than the critical angle for the material such as glass from which the waveguide is made. This light is constrained to propagate along the rod-like waveguide reflecting from each of the four sides of the waveguide in turn. Thus the relative field angles of the light incident on the rod-like waveguide are preserved within the waveguide and the information required to regenerate the original image is thus preserved.

An output transmission grating 116 is provided along a longitudinal extent of the waveguide through which grating the image bearing light is outputted from the rod-like waveguide 106. The interaction of the light with the grating 116 is discussed in more detail below. The grating is a low efficiency grating which diffracts only a small amount of light out of the waveguide on each pass through the grating, with the remaining light continuing along its path around the waveguide.

The plate-like waveguide 108 is located with an edge adjacent to and in line with a side of the rod-like waveguide to receive the image bearing light outputted from the rod-like waveguide by interaction with the output grating 116.

The edge of the plate-like waveguide is located adjacent and close to, but not touching, the side of the rod-like waveguide. The waveguide also includes (not shown) an input means to perform the same function as input means 114 with respect to the plate-like waveguide, except that light propagates along the plate-like waveguide by reflecting from opposite parallel sides of the waveguide. Th input means helps to improve the uniformity of the display at an output transmission grating 118 of the plate-like waveguide for diffracting the received image bearing light out of the waveguide towards a viewer 102 as shown by arrows. The grating 118 is also a low efficiency grating, such that as rays propagate around the waveguide 108 at each interaction with the output grating 118 a small proportion of light is diffracted out of the waveguide 108. The non-diffracted light continues to propagate. A large number of parallel rays therefore exit the waveguide 108 through the output grating 118 towards the viewer 102, which originated at discrete points on the micro-display forming the image generating light source 110.

It will now be described in more detail the interaction of the image bearing light with the output transmission grating 116 of the rod-like waveguide 106.

Figure 9 is a simplified section through the rod-like waveguide taken along the longitudinal axis. Figure 10 is a simplified section taken perpendicularly to the longitudinal axis. Figure 11 is a simplified of the output transmission grating showing alignment of the fringes. The waveguide has a first, second, third and fourth longitudinal sides 120, 121, 122, 123. Sides 120 and 122 are opposite and parallel and sides 121 and 123 are opposite and parallel. In this example, the second side is provided with a reflective coating. Image bearing light is injected into the waveguide and propagates along a path 124 by total internal reflection from each of the four sides of the waveguide in turn as shown particularly in Fig 10.

The transmission grating 116 is designed having fringes 117 which in this example are perpendicular to the longitudinal axis and the sides 121, 123 of the rod-like waveguide 106. Accordingly, when travelling along path 124, the image bearing light makes equal and opposite passes through output transmission grating 116. Each pass has an equal but opposite angle of incidence with respect to the grating fringes. At first portions 126 of the path the image bearing light is diffracted by the output grating forming diffracted beams 128. At second portions 130 of the path the image bearing light is diffracted by the output grating, reflected from side 122 and form diffracted beams 132. The diffracted beams 128 and 132 are aligned (propagate in a parallel direction) and combine in the plate-like waveguide to form a unique field position within the display image.

However, due to manufacturing tolerances the output diffraction grating 116 may not be orientated correctly in the waveguide 106 as shown in exaggerated form in Figure 12 or the fringes may not be correctly aligned as also exaggerated in Figure 13. In Figure 12, the grating is shown angled with respect to a longitudinal centre line of the waveguide. Alternatively, or additionally, the grating may be rotated about the longitudinal centre line of the waveguide. In Figure 13, the fringes are angled relative to the longitudinal axis and the sides 121, 122 of the waveguide 116.

In Figure 12, when travelling along path 124, the image bearing light no longer makes equal and opposite passes through output transmission grating 116. The passes have different angles of incidence with respect to the grating fringes. At first portions 126 of the path the image bearing light is diffracted by the output grating forming diffracted beams 128. At second portions 130 of the path the image bearing light is diffracted by the output grating forming diffracted beams 132. The diffracted beams 128 and 132 are not aligned (propagate in a dissimilar, non-parallel directions). The two diffracted beams 128, 132 represent two different points in the field of the display image. Hence, the display is split into two angularly displaced images and double imaging occurs.

This can be shown mathematically as follows: p1 direction cosine for component parallel to fringe gratings of first pass input to grating 132 direction cosine for component parallel to fringe gratings of second pass input to grating Thus 1312--13ji w grating misalignment A wavelength of light d grating period Pmi direction cosine for component parallel to fringe gratings of first pass output from grating 13m2 direction cosine for component parallel to fringe gratings of second pass output from grating 13m1 = -1i sinw.A/d rn2 = -3i2 -sinw.AId rn2 = + 13M -sinw.AJd = -13m2 -2.sinw.AId As an example, in a particular example of the waveguide if the output grating is misaligned by 1 mR with respect to the axis of the rod then the outputs are misaligned by 3.9mR. For the two passes to be aligned 13m-i = -Pi2 which is only true when w = 0 and thus the grating is perfectly aligned to the axis of the rod.

Equally, in Figures 13 and 14, the grating itself is correctly located in the rod-like waveguide 106 but the fringes of the grating are misaligned by an angle 0. The misalignment may be as little as 0.5 degrees whilst having a perceivable viewing effect. Figure 14 is a simplified section through the rod-like waveguide taken along the longitudinal axis. Figure 15 is a simplified section taken perpendicularly to the longitudinal axis. Figure 13 is a simplified of the output transmission grating showing misalignment of the fringes. In Figures 14 and 15, when travelling along path 124, the image bearing light makes equal and opposite passes through output transmission grating 116. However, the passes have different angles of incidence with respect to the grating fringes. At first portions 126 of the path the image bearing light is diffracted by the output grating forming diffracted beams 128. At second portions 130 of the path the image bearing light is diffracted by the output grating forming diffracted beams 132.

The diffracted beams 128 and 132 are not aligned (propagate in a dissimilar, non-parallel directions). The two diffracted beams 128, 132 represent two different points in the field of the display image. Hence, the display is split into two angularly displaced images and double imaging occurs.

The present invention seeks to provide an improved projection display.

According to the present invention a projection display for displaying an image to a viewer, includes a waveguide made of light transmissive material, an image-providing light source device arranged to inject image bearing light into the waveguide, an input means coupled to or within the waveguide to direct said image bearing light internally along the waveguide, an output transmission grating for outputting image bearing light from the waveguide by diffraction, wherein image bearing light travelling along a path in the waveguide makes multiple passes through the output transmission grating and the image bearing light is diffracted by the output transmission grating only when passing therethrough along first portions of said path.

The image bearing light passing along said path may pass through said output transmission grating without diffraction along second portions of said path.

Light travelling along first portions of said path may pass through the output transmission grating in a first direction from one side to an opposing side of said grating, and light travelling along second portions of said path passes through the output transmission grating in a second direction from said opposing side to said one side of said grating.

The output transmission grating may comprise fringes which are orientated to diffract the image bearing light from said waveguide when passing through the output transmission grating along said first portions of said path and wherein the fringes may be orientated to allow the image bearing light to pass through said output transmission grating without diffraction when passing therethrough along said second portions of said path.

The said output transmission grating may be misaligned in the waveguide due to manufacturing tolerances.

The fringes of the output transmission grating may be misaligned due to manufacturing tolerances.

The fringes of the output transmission grating may be orientated with respect to said path such that misalignment of the grating or misalignment of the fringes within manufacturing tolerances allows light travelling along said path to not be diffracted by the grating on said second portions.

The input means may comprise a diffraction grating and may be arranged such that incident inputted image bearing light is diffracted therefrom with the incidence angle of the diffracted light at internal surfaces of the waveguide being greater that the critical angle for the material from which the waveguide is made.

The input grating may comprise fringes which are oriented at the same angle as the fringes of the output transmission grating of the waveguide.

The period of the fringes of the input grating may be the same period of the fringes of the output transmission grating of the waveguide.

The output transmission grating may be a low efficiency grating.

A projection display may comprise a first said waveguide which is rod-like into which image bearing light is input and propagates by reflecting from each of the four sides of the waveguide in turn and a second said waveguide which is plate-like into which image bearing light is input and propagates by reflecting between opposing parallel sides of the waveguide, the second waveguide being located with an end portion thereof adjacent to a longitudinal side of the rod-like waveguide to receive the image bearing light output therefrom, which plate-like waveguide includes an output transmission grating for diffracting the received image bearing light out of the plate-like waveguide towards a viewer.

The invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a simplified perspective view of a waveguide of a projection display; Figure 2 is a section through one waveguide showing the waveguide in use; Figure 3 is a section through another waveguide showing the waveguide in use; Figure 4 is a section through the waveguide shown in Figure 3 taken perpendicularly to the section of Figure 3; Figure 5 is a schematic view of an output transmission grating of the waveguides shown in Figures 2 and 3; Figure 6 is a perspective view of a projection display according to an embodiment of the present invention; Figure 7 a perspective view of another projection display according to an embodiment of the present invention; Figure 8 is a known projection display; Figure 9 is a section through known waveguide without manufacturing errors; Figure 10 is a section taken perpendicularly to the section of Figure 9; Figure 11 shows the fringes of an output transmission grating in the waveguide shown in Figures 9 and 10; Figure 12 shows a misaligned output transmission grating in a waveguide; Figure 13 shows misaligned fringes in an output transmission grating; Figure 14 shows a section of a waveguide with the output transmission grating of Figure 13 in use; and Figure 15 is a section taken perpendicularly to the section of Figure 14.

An improved waveguide 10 for a projection display is shown in Figure 1.

The waveguide 10 functions to stretch a horizontal pupil in a projected image.

The waveguide is rod-like and is formed from a longitudinal square or rectangular sectioned light transmissive material such as glass or plastics.

Image bearing light 12 is injected into the waveguide at an end of the waveguide. Input means 14, which in the example shown is an input diffraction grating diffracts the light 12 and directs it along the waveguide at an angle which is greater than the critical angle of the material of the waveguide so that total internal reflection occurs. The path or paths of the light diffracted from the input means 14 is reflected from each of the four sides of the waveguide in turn making passes through an output transmission grating 16. Light striking different parts of the diffraction grating 14 propagates along multiple paths along the waveguide, each path extending in a generally spiral configuration formed as the light propagates along the length of the waveguide and is reflected from the sides of waveguide.

As shown in simplified form in Figure 2, image bearing light propagates along the waveguide travelling on a path 18 which passes through the output grating 16. If, due to manufacturing tolerances, the output grating is not perfectly aligned as in this example, the light along first portions 20 of the path makes an angle of incidence with respect to the output grating which is not the same as the angle of incidence made by the light along second portions 22 of the path. In the prior art projector display the difference in angle of incidence causes light outputted from the waveguide to be out of alignment resulting in double imaging.

However, in waveguide 10 the image bearing light is diffracted by the output transmission grating 16 only when passing therethrough along first portions 20 of the path 18. The diffracted beams output from the waveguide are referenced 28.

The image bearing light passing along the path 18 passes through the output transmission grating 16 without diffraction along the second portions 22 of the path and continues to be reflected within the waveguide by total internal reflection.

Alternatively, and as shown in Figures 3 and 4, image bearing light propagates along the waveguide travelling on a path 18 which passes through the output grating 16. The output grating is perfectly aligned in this example however, due to manufacturing tolerances, the fringes of the output grating may be misaligned. Unlike the prior art however, whether or not the fringes are perfectly aligned or misaligned, the light travelling along first portions 20 of the path 18 makes an angle of incidence with respect to the fringes 24 which is not the same as the angle of incidence made by the light along second portions 22 of the path. In the prior art projector display the difference in angle of incidence causes light outputted from the waveguide to be out of alignment resulting in double imaging. However, in waveguide 10 the image bearing light is diffracted by the output transmission grating 16 only when passing therethrough along first portions 20 of the path 18. The diffracted beams output from the waveguide are referenced 28. The image bearing light passing along the path 18 passes through the output transmission grating 16 without diffraction along the second portions 22 of the path and continues to be reflected within the waveguide by total internal reflection.

As shown in Figure 5, the output transmission grating 16 comprises fringes 24 which are greatly enlarged for the sake of explanation. The fringes are orientated to diffract the image bearing light from the waveguide 10 when passing through the output transmission grating 16 along the first portions 20 of the path 18 and to allow the image bearing light to pass through the output transmission grating without diffraction when passing therethrough along the second portions 22 of the path.

In the embodiment, the output grating 16 is arranged so that only one of the passes interacts with the output grating. This is achieved by orientating the fringes of the output grating at an angle of approximately a so that only one of the two modes (at portions 20, 22 of the path) is aligned at the correct angle with respect to the grating for diffraction to occur. In this example, a is equal to 45 degrees to the longitudinal axis and the sides of the waveguide. The angle a is selected so that no diffraction occurs at portion 22 if the fringes are misaligned at a small angle either side of a. In this example, the range may extend between 44 and 46 degrees over which no diffraction would occur at portion 22. If manufacturing tolerances are within the range 44 to 46 degrees, the light is directed out of the waveguide only when it passes through the diffraction grating in one direction at portion 20 and not at portion 22.

Advantageously, the input means 14 is a diffraction grating. The fringes 26 of the input grating are greatly enlarged for the sake of explanation and shown by hashing in Figure 1. The fringes 26 are orientated at the same angle as the fringes 24 of the output grating, and the period of the grating and the angle of orientation are arranged such that the output from the rod is undistorted. In this way, the input and output gratings can have some degree of misalignment without resulting in double imaging or other degradation of the display.

Whilst the present invention has been described in relation to a rod-like waveguide, the invention applies equally to a plate-like waveguide suitable in a projector display for stretching a vertical pupil of the displayed image.

Figure 6 shows the rod-like waveguide 10 and a plate-like waveguide 30 made from a light transmissive and transparent material such as glass or plastics. The output from the rod-like waveguide 10 may be refractively coupled into the edge of plate-like waveguide 30 but in the example shown in Figure 6, the output 28 of the rod-like waveguide 10 is directed at the surface of the plate-like waveguide 30 by input means 32, such as a diffraction grating, on the surface of or within the plate-like waveguide 30 which then directs the input within the waveguide at greater than the critical angle so that total internal reflection occurs.

The plate-like waveguide 30 includes input grating 32 and an output transmission grating 34. The output from the rod-like waveguide is directed through the top face of the plate-like waveguide and is diffracted at the input grating 32 towards the output grating 34, reflecting from the top and bottom faces of the waveguide. The weakly diffracting output grating 34 directs a portion of the light towards a viewer 36 at each pass through the grating, hence generating a display across the whole area of the grating. Alternatively, the output transmission grating 34 may be adapted to have similar functionality to output transmission grating 16 such that light is directed out of the waveguide 30 only when it passes through the diffraction grating 34 in one direction to avoid double imaging.

A projection display 50 for displaying an image to a viewer 36 is shown in Figure 7. The projection display 50 comprises the rod-like waveguide 10 and plate-like waveguide 30 as described above with reference to Figures 1 to 3.

The projection display 50 generates a collimated display subtending a large exit pupil at the point of viewer 36 and a large field of view while using a small image-providing light source device.

The image-providing light source device includes an image generating light source 52 preferably in the form of a micro-display to provide a display of information. Additionally the image-providing light source device includes an optical means 54 located between the image generating light source 52 and one end portion of the rod-like waveguide 10. The optical means 54 is operable to collimate light received from the image generating light source 52 and inject the collimated image bearing light into the rod-like waveguide 10. The optical means 54 preferably is of a small size, typically less than 25 millimeters in diameter, and is used to collimate the light received from the image generating light source 52.

The collimated light produced by the optical means 54 has a small exit pupil and is therefore fed into the rod-like waveguide 10 which performs the function of stretching the horizontal pupil. The output from the rod-like waveguide 10 is fed into the plate-like waveguide 30 which stretches the vertical pupil view and also acts as a combiner for the display. In this manner, the display information provided to the viewer 36 looking through the plate-like waveguide 30 subtends a large exit pupil and a large field of view whilst using a small optical means 54 and a small image generating light source 52 such as a micro display.

The information to be provided to the viewer 36 is in the form of a display of information which is generated by the image generating light source 52 that is illuminated with visible monochromatic laser light. The micro-display forming the image generating light source 52 may be either reflective or transmissive such that in conjunction with the optical means 54 a collimated image of the display is generated for injection into the rod-like waveguide 10. Alternatively, but not illustrated, collimation may be carried out at the waveguide 10.

In this embodiment, the rod-like waveguide 10 includes an input diffraction grating 14 shown schematically and as described in more detail above, but alternatively may comprise means such as an input reflection hologram provided on or in the rod-like waveguide 10 to reflect and multiply the image bearing light received from the optical means 54 internally along multiple paths 18 in the rod-like waveguide 10. This light is constrained within the waveguide 10 to propagate along the rod-like waveguide 10 reflecting from each of the four sides in turn. Thus the relative field angles of the light incident on the rod-like waveguide 10 are preserved within the waveguide and the information required to regenerate the original image is thus preserved.

The rod-like waveguide 10 may also include therein, although not shown in the Figures, a series of semi-reflecting parallel surfaces or holographic layers.

The surfaces or layers are operable to split each impinging ray of the injected image bearing light into two parts and therefore two paths 18 at each interaction to improve display uniformity at the output grating 16 and to multiply the image bearing light. The net result is that for each ray of light inputting the region of the surfaces or layers many rays exit along respective paths 18. The system efficiency may be further improved by a pair of spaced apart matching reflection means such as matching reflection holograms provided therein or thereon. The pair of spaced apart matching reflection holograms are such as to reflect incident rays of image bearing light back along the rod-like waveguide 10 without change in their propagation angles thereby preserving the image information. Light absorption material 56 is provided at the end of the waveguide 10 to absorb any light reaching the end.

The output transmission grating 16 is a low efficiency grating which diffracts a small amount of light out of the waveguide 10 on each interaction with the incident light rays. This grating 16 preferably is only a few percent efficient and light diffracted by the grating is allowed to escape from the waveguide 10. A light reflective coating 58 is provided on a fourth side of the rod-like waveguide opposite to the longitudinal third side 60 to reflect back to the output transmission grating 16 light incident thereon. Thereby increasing the display efficiency.

The plate-like waveguide 30 may be located relative to the rod-like waveguide 10 as shown in Figure 6 or alternatively is located with an edge surface thereof adjacent to and in line with the third side 60 of the waveguide 10.

The waveguide 30 may include a series of semi-reflecting parallel surfaces or holographic layers (not shown) located therein substantially perpendicularly to the edge surface and operable to split each impinging ray of image bearing light received from the output transmission grating 16 of the waveguide 10 into a plurality of parallel rays which are larger than the critical angle for the material from which the waveguide 30 is made and therefore will propagate inside the waveguide 30. This helps to improve the uniformity of the display at the exit grating 34 on or at a first surface of the waveguide for diffracting the received image bearing light out of the waveguide towards the viewer 36. The grating 34 is a low efficiency grating, such that as rays propagate along the waveguide 30 at each interaction with the exit grating 34 a small proportion of light is diffracted out of the waveguide. The non-diffracted light continues to propagate. A large number of parallel rays therefore exit the waveguide through the exit grating 34 towards the viewer 36, which originated at discrete points on the micro-display.

It will be understood that the exit grating 34 not only diffracts light towards the viewer 36 but also diffracts light away from the viewer. Preferably, a narrow band selective reflection coating 62 is provided on a second surface of the waveguide 30 opposite to and parallely spaced from the first surface 64 to reflect light diffracted from the exit grating 34 back to the exit grating to increase display efficiency. Preferably, the semi-reflecting parallel surfaces or holographic layers and/or the formation of the exit grating 34 are such as to co-operate to generate a multiplicity of overlapping display images. To this end the exit grating 34 can be provided not only at the first surface 26, but may be duplicated within the body of the waveguide 30 and additionally at the second surface 64 thereof Although the surfaces of the waveguides 10, 30 have been shown as planar in the illustrated embodiment of the invention these can be made curved if desired.

Additionally, the projection display illustrated according to the invention can form part of a Head Up Display, of a Helmet Mounted Display andlor of a Head Mounted Display particularly for aircraft usage.

Claims (1)

1. <claim-text>CLAIMSA projection display for displaying an image to a viewer, including a waveguide made of light transmissive material, an image-providing light source device arranged to inject image bearing light into the waveguide, an input means coupled to or within the waveguide to direct said image bearing light internally along the waveguide, an output transmission grating for outputting image bearing light from the waveguide by diffraction, wherein image bearing light travelling along a path in the waveguide makes multiple passes through the output transmission grating and the image bearing light is diffracted by the output transmission grating only when passing therethrough along first portions of said path.</claim-text> <claim-text>2. A projection display as claimed in claim 1, wherein image bearing light passing along said path passes through said output transmission grating without diffraction along second portions of said path.</claim-text> <claim-text>3. A projection display as claimed in claim 1 or 2, wherein light travelling along first portions of said path passes through the output transmission grating in a first direction from one side to an opposing side of said grating, and light travelling along second portions of said path passes through the output transmission grating in a second direction from said opposing side to said one side of said grating.</claim-text> <claim-text>4. A projection display as claimed in claim 3, wherein the output transmission grating comprises fringes which are orientated to diffract the image bearing light from said waveguide when passing through the output transmission grating along said first portions of said path and wherein the fringes are orientated to allow the image bearing light to pass through said output transmission grating without diffraction when passing therethrough along said second portions of said path.</claim-text> <claim-text>5. A projection display as claimed in any of the preceding claims, wherein said output transmission grating is misaligned in the waveguide due to manufacturing tolerances.</claim-text> <claim-text>6. A projection display as claimed in any of the preceding claims, wherein the fringes of the output transmission grating are misaligned due to manufacturing tolerances.</claim-text> <claim-text>7. A projection display as claimed in claim 5 or 6, wherein the fringes of the output transmission grating are orientated with respect to said path such that misalignment of the grating or misalignment of the fringes within manufacturing tolerances allows light travelling along said path to not be diffracted by the grating on said second portions.</claim-text> <claim-text>8. A projection display as claimed in claim 4, wherein the input means comprises a diffraction grating and is arranged such that incident inputted image bearing light is diffracted therefrom with the incidence angle of the diffracted light at internal surfaces of the waveguide being greater that the critical angle for the material from which the waveguide is made.</claim-text> <claim-text>9. A projection display as claimed in claim 5, wherein the input grating comprises fringes which are oriented at the same angle as the fringes of the output transmission grating of the waveguide.</claim-text> <claim-text>10. A projection display as claimed in claim 6, wherein the period of the fringes of the input grating is the same period of the fringes of the output transmission grating of the waveguide.</claim-text> <claim-text>11. A projection display as claimed in any preceding claim, wherein the output transmission grating is a low efficiency grating.</claim-text> <claim-text>12. A projection display as claimed in any preceding claim, comprising a first said waveguide which is rod-like into which image bearing light is input and propagates by reflecting from each of the four sides of the waveguide in turn and a second said waveguide which is plate-like into which image bearing light is input and propagates by reflecting between opposing parallel sides of the waveguide, the second waveguide being located with an end portion thereof adjacent to a longitudinal side of the rod-like waveguide to receive the image bearing light output therefrom, which plate-like waveguide includes an output transmission grating for diffracting the received image bearing light out of the plate-like waveguide towards a viewer.</claim-text> <claim-text>13. A projection display for displaying an image to a viewer, substantially as hereinbefore described and/or as illustrated in Figures 1 and 2, 3, or 4 of the accompanying drawings.</claim-text> <claim-text>14. A Head Up Display including a projection display as claimed in any one of claims ito 13.</claim-text> <claim-text>15. A Helmet Mounted Display including a projection display as claimed in any one of claims ito 13.</claim-text> <claim-text>16. A Head Mounted Display including a projection display as claimed in any one of claims ito i3</claim-text>