Head-up display projector unit

Abstract

A head-up display projector unit (1, 15, 16) is described including two or more projectors (21, 22, …, 2K). Each projector (21, 22, …, 2K) is configured to project an image to a corresponding projector head motion box (31, 32, …, 3K) via a combiner (4), such that an overall head motion box (5) and a total field-of-view observable from the overall head motion box (5) are formed by the combination of the fields-of-view and projector head motion boxes (31, 32, …, 3K) of the two or more projectors (21, 22, …, 2K). An optical path from each projector passes through a common optical element (53) before reaching the combiner (4).

Description

[0001]Head-up display projector unit Field The present invention relates to head-up display projector units, and methods of using the head-up display projector units. Background A head-up display projector works by using a combiner, often a flat sheet of glass angled relative to a users’ viewpoint, to present information to a user merged with their visual field. Originally used in aircraft to allow a pilot to be able to see important information without looking down at the cockpit dashboard, head-up displays are of interest in other areas where it would be beneficial for a user to be able to see information without changing the direction and / or focus of their gaze. One application area is in automobiles and similar vehicles. Using a head-up display allows a driver to be presented with information such as speed, navigation directions and so forth without taking their eyes off the road. One barrier to broader adoption of head-up display technologies has been that the volume of a head-up display projector system increases dramatically as the field-of- view (the range of angles spanned by the projected images from an observers’ point of view) and head motion box (the volume within which an observer will see the projected images) increases. Increasing the size of the head motion box by a factor of two will lead to an approximately eight times increase in the volume of the optical system. This scaling issue has been a barrier to adoption of head-up display systems with large fields-of-view and / or large head motion boxes, due to the difficulty of installing the resulting volume of optics into the dashboard of a modern car or similar vehicle. US 2022 / 390,743 A1 describes an image generation system for providing a ghost image free head-up display, the system including a display screen having a front surface and a back surface, a picture generation unit for projecting an image towards the display screen for reflection towards a predetermined eye box, and a field lens. The picture generation unit is configured to project light through the field lens such that light is incident on the front surface of the display screen forming a first virtual image. A portion of the light is transmitted through the display screen and is incident on the back surface of the display screen forming a second virtual image. The first 132652PCT1 and second virtual images have an field lens is configured such that the offset is below a threshold magnitude and the first and second virtual images are substantially overlaid as viewed from the eye box. 132652PCT1 Summary According to a first aspect of the invention, there is provided a head-up display projector unit including two or more projectors. Each projector is configured to project an image to a corresponding projector head motion box via a combiner, such that an overall head motion box and a total field-of-view observable from the overall head motion box are formed by the combination of the fields-of-view and projector head motion boxes of the two or more projectors. An optical path from each projector passes through a common optical element before reaching the combiner. For example each individual projector may create a beam that is not perfectly collimated but slightly diverging. At a finite distance after the exit pupil of each projector, a large common optical element (for example a lens) may be used to perfect the collimation of the rays (or if required, bring the rays at a finite focal plane in front of the user). Allowing the rays to diverge over a short distance in this way may enable the projector's exit pupil to slightly increase, which may allow further reducing, or even removing any perceivable gap in the beams output from two adjacent projectors. The common optical element may include (without being limited to) one or more of lenses, diffractive lenses, Fresnel lenses, holographic lenses, mirrors, catadioptric elements and so forth. The common optical element may perform a different optical function for each projector, such as different prismatic power for each projector. The common optical element may be continuous in curvature and second derivative. The common optical element may not have a discontinuity of curvature or second derivative. The common optical element may not take the form of an array or lenses, microlenses, or an array of another other type of optical element in which the boundaries of repeating elements match the boundaries of corresponding projectors. The common optical element may not include a distinct focus point corresponding to each projector. The common optical element may take the form of a single, continuous lens, having a pair of foci lying along a single optical axis. The common optical element may take the form of a single continuous freeform surface that can be decomposed into a lower-order Zernike polynomials. In other words, the optical 132652PCT1 surfaces of the common optical be smooth and continuous in terms of curvature or second derivative across the boundaries corresponding to different projectors (for example, boundaries may correspond to projections of the area of each projector onto the common optical element). The combiner may be common to all of the two or more projectors. The combiner is separate from the common optical element. In other words, the combiner is not the common optical element (even in cases where the combiner is common to all of the projectors). The combiner may be a single-bounce reflector. In other words, light incident to the combiner reflects once. The combiner may not be a waveguide. The combiner may not be a volume grating. For each of the two or more projectors, the projector head motion box may be the same as the overall head motion box, such that each projector forms a portion of a total field-of-view observable from the overall head motion box. The head-up display projector unit may be configured to project an output image by generating a number of sub-images equal to the number of projectors, each sub- image overlapping with one or more other sub-images and comprising a portion of the output image, wherein every part of the output image is comprised in at least one sub-image, and projecting a respective sub-image via each of the projectors, such that observed from the overall head motion box the output image spans the total field-of-view. In this way, an observer positioned within the overall head motion box may see a single virtual image corresponding to the output image. Each projector may generate a virtual sub-image (corresponding to the respective image) on a common virtual image plane, and the overlapping virtual sub-image collectively form the virtual image corresponding to the output image. The projectors may be configured to form the common virtual image plane in a far- field condition, for example at infinity, beyond 100 m and so forth. The projectors may be configured to form the common virtual image plane at a distance less than 100 m. 132652PCT1 The head-up display projector unit may to adjust the relative intensity with which each projector outputs portions of the respective sub-image which overlap with one or more other sub-images, such that the output image observed from the overall head motion box has substantially uniform brightness. Substantially uniform intensity may mean that a projected brightness ratio of any pair of pixels in the output image, as observed from the overall head motion box, does not differ by more than 10% from a baseline brightness ratio for the same pair of pixels in the output image before projection. This may be quantitatively evaluated by, for example, taking a picture using a camera arranged within the overall head motion box and arranged to capture an image encompassing the total field-of-view. Alternatively, output images may be pre-prepared with relative pixel brightness values configured to compensate for the fact that some pixels will be projected by two (or more) overlapping projectors). Each projector may be configured to project an output image to the respective projector head motion box, such that the output image is observable from any point within the overall head motion box. Each projector may project the same output image. Each projector may be configured to form a virtual image corresponding to the output image on a common virtual image plane. Each projector may project the same field-of-view, which may be the same as the total field-of-view of the overall head box. Projector head motion boxes of adjacent projectors may overlap one another. Projector head motion boxes of adjacent projectors may not overlap one another. The projectors may be configured to form the common virtual image plane in a far- field condition, for example at infinity, beyond 100 m and so forth. The projectors may be configured to form the common virtual image plane at a distance less than 100 m. A maximum spacing between adjacent projectors may be less than or equal to 5 mm. A maximum spacing between adjacent projectors may be less than or equal to 4 mm. A maximum spacing between adjacent projectors may be less than or equal to 3 mm. 132652PCT1 A maximum spacing between adjacent may be less than or equal to 2 mm. A maximum spacing between adjacent projectors may be less than or equal to 1 mm. Alternatively, the constraints described hereinbefore in relation to a minimum spacing between adjacent projectors may instead be applied as a minimum spacing between beams originating from adjacent projectors and / or projector head motion boxes of adjacent projectors. This spacing may be evaluated at a front face of the overall head motion box. The two or more projector head motion boxes forming the overall head motion box may be arranged to form an array spaced along a first direction. The first direction may be perpendicular to a central direction of the field-of-view of the overall head motion box. The first direction may correspond to a horizontal direction (relative to gravity in use). The corresponding projectors may be arranged to form a linear array. The head-up display projector unit may include three or more projectors. The corresponding three or more projectors head motion boxes forming the overall head motion boxes may be arranged to form an array spaced along first and second directions. The first and second directions may both be perpendicular to a central direction of the field-of-view of the overall head motion box. The first and second directions need not be perpendicular to one another. The first direction may correspond to a horizontal direction (relative to gravity in use). The second direction may correspond to a vertical direction (relative to gravity in use). For example, the three or more projector head motion boxes may be arranged in a triangular configuration. Larger arrays may have projector head motion boxes which combine to form the overall head motion box in a square array, a rectangular array, a hexagonal (or “close packed”) array, or in accordance with any two-dimensional Bravais lattice. The corresponding projectors may be arranged to form a two-dimensional array. The projectors may be arranged in a in a square array, a rectangular array, a hexagonal 132652PCT1 (or “close packed”) array, or in any two-dimensional Bravais lattice. Output optics of the projectors may be tessellated. The two or more projectors may include first and second projectors. An output optical path of the first projector may be different to an output optical path of the second projector. Any one, any group, or all of the projectors may have different optics. The projectors may be configured with different optics so as to compensate for projecting via a combiner, such as a vehicle windscreen or other curved combiner, allowing each projector to be corrected for the shape of the portion of the combiner it projects via. Producing corrective optics to compensate for the shape of a small area of the combiner may be relatively less complex and cheaper than attempting to produce corrective optics to compensate for the shape of a combiner across a large area. The two or more projectors may include first and second projectors. Output optic axes of the first and second projectors may not be parallel, in other words they make a non-negligible angle, for example at least 5°. In other words, two or more projectors arranged to form an array (in one or two directions, do not all need to be parallel to one another. This may allow embedding the head-up display projector unit within a curved surface, without enforcing a flat region. For example, the head-up display projector unit may be seamlessly embedded in a curved vehicle dashboard without disrupting the styling. A projector of the two or more projectors may include a display configured to output an image. Any number, including all, of the projectors may include a respective display. Each projector that includes a display may also include one or more first actuators configured to translate and / or rotate the display relative to one or more optical elements of that projector. Each projector that includes a display may include one or more first actuators configured to translate the display laterally relative to the optic axis of that projector. Each projector that includes a display may include one or more first actuators configured to translate the display along the optic axis of that projector. Each projector that includes a display may include one or more first actuators 132652PCT1 configured to rotate the display about axes perpendicular to the optic axis of that projector. A projector of the two or more projectors may include a laser beam scanner assembly configured to output an image by scanning a laser beam spot. Any number, including all, of the projectors may include a respective laser beam scanner assembly. The laser beam may be scanned to form the image using raster scanning. The laser beam may be scanned to form the image using Lissajous scanning. Each laser beam scanner assembly may include a diffuser. The diffuser may include one or more different technologies including, but not limited to, embossed diffusers, holographic optical elements (HOEs), diffractive optical elements (DOEs) and so forth. A projector of the two or more projectors may include one or more second actuators configured to translate and / or rotate at least one optical element of that projector relative to the corresponding optic axis. One or more second actuators may be configured to translate the at least one optical element laterally relative to the optic axis of that projector. One or more second actuators may be configured to translate the at least one optical element along the optic axis of that projector. One or more second actuators may be configured to rotate the at least one optical element about respective axes perpendicular to the optic axis of that projector. Any number, including all, of the projectors may include one or more second actuators. The optical element may be one or more lenses. The optical element may be one or more mirrors. The optical element may be one or more catadioptric arrangements / components / elements, Fresnel lenses, holographic optical elements (HOEs), diffractive optical elements (DOEs) or polarization optics. A projector of the two or more projectors may be configured for variable focus. Any number, including all, of the projectors may be configured for variable focus. A projector of the two or more projectors may include adaptive optics. Any number, including all, of the projectors may include one adaptive optics. 132652PCT1 A projector of the two or more include polarisation optics. Any number, including all, of the projectors may include polarisation optics. Polarisation optics may include, or take the form of, one or more geometric phase lenses. Alternatively, the optical path of each projector may be entirely independent, having only the combiner as a common element. An output beam of each projector has a shape which may be tessellated in two- dimensions. The output optics of a projector may also be referred to as the “pupil”. The output optics may take the form of one or more lenses. The output optics of the projectors may be tessellated in two-dimensions. A projector of the two or more projectors may include catadioptric optics. Any number, including all, of the projectors may include catadioptric optics. The catadioptric optics of each projector may be prismatic in a direction perpendicular to the optics axis of that projector. Two or more projectors may be provided by an array of displays arranged to output light into a common catadioptric system. The catadioptric optics may include, or take the form of, a single piece of plastic shaped to form one or more lenses and one or more mirrors. Mirrors may be formed by total internal reflection and / or by silvering with a metallic layer / film. The head-up display projector unit may be configured to project via a combiner which is a vehicle window or windscreen. The common optical element may include, or take the form of, a negative lens. When the virtual image plane of each of the two or more projectors corresponds to a finite distance, use of a negative lens as the common optical element may reduce (or even eliminate) mismatches between the virtual image planes of the two or more projectors. Such mismatches may cause image degradation, fatigue / eye-strain of a user, and so forth. The negative lens any may include (without being limited to) one 132652PCT1 or more of lenses, diffractive lenses, lenses, holographic lenses, mirrors, catadioptric elements and so forth. A vehicle may include the head-up display projector unit. The head-up display projector unit may be arranged to use a window of the vehicle as the combiner. The window of the vehicle used as the combiner may be the windscreen. The vehicle may be an automobile. The vehicle may be an aeroplane. The vehicle may be a ship or boat. A head-up display unit may include the head-up display projector unit and the combiner. According to a second aspect of the invention, there is provided use of the head-up display projector unit of the first aspect to project an output image to the head motion box. The use of the second aspect may include features corresponding to any features of the head-up display projector unit of the first aspect. Definitions applicable the head- up display projector unit of the first aspect (or features thereof) may be equally applicable to the user of the second aspect (or features thereof). 132652PCT1 Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a head-up display system; Figure 2 is a schematic top-view of the head-up display system shown in Figure 1; Figure 3 schematically illustrates a projector for a head-up display system; Figure 4 schematically illustrates light rays output from the projector shown in Figure 3; Figure 5 schematically illustrates an outgoing angle of light as a function of pixel position for the projector shown in Figure 3; Figures 6A and 6B schematically illustrate a first configuration of a head-up display projector unit; Figure 7 schematically illustrates a pair of sub-images output by adjacent projectors; Figure 8 schematically illustrates a relationship between output image pixels, projector pixels, and outgoing light rays for the first configuration of a head-up display projector unit; Figure 9 schematically illustrates three offset sub-images; Figures 10A and 10B schematically illustrate a second configuration of a head-up display projector unit; Figure 11 schematically illustrates a relationship between output image pixels, projector pixels, and outgoing light rays for the second configuration of a head-up display projector unit; Figure 12 schematically illustrates a curved head-up display projector unit; Figure 13 schematically illustrates a first example a projector; Figure 14 schematically illustrates a second example a projector; Figure 15 schematically illustrates a third example a projector; Figure 16 schematically illustrates a fourth example a projector; Figure 17 schematically illustrates a fifth example a projector; Figure 18 schematically illustrates a sixth example a projector; Figure 19A schematically illustrates a seventh example a projector; Figure 19B schematically illustrate a first array of projectors; Figures 20A and 20B schematically illustrate a second array of projectors; Figure 21 schematically illustrates an eighth example a projector; and Figure 22 schematically illustrates a ninth example a projector; and Detailed Description of Certain Embodiments In the following, like parts are denoted by like reference numerals. 132652PCT1 The present specification describes head-up display projector units which may overcome the volume scaling problems encountered with conventional head-up display systems. Referring to Figure 1, a head-up display system 1 is schematically illustrated. A number of projectors 2 are arranged in an array. In particular, a head-up display projector unit according to the present specification includes two or more projectors 2. Each projector 2 is configured to project an image to a corresponding projector head motion box 3 via a combiner 4. In the example shown in Figure 1, there are three projectors 21, 22and 23, and the corresponding projector head motion boxes 31, 32, 33are coincident with substantially the same size, shape and orientation (described in more detail hereinafter in relation to Figures 6A to 9). The combiner 4 is preferably common to all of the two or more projectors 2, for example the windscreen of an automobile (whether internal combustion or electric). The combiner 4 preferably a single-bounce reflector. In other words, light incident to the combiner 4 reflects once. The individual projectors 2 of the array are configured such that an overall head motion box 5 and a total field-of-view observable from the overall head motion box 5 are formed by the combination of the fields-of-view and projector head motion boxes 3 of the two or more projectors 2. For example in the system shown in Figure 1, the overall head motion box 5 and each of the projector head motion boxes 31, 32, 33are substantially the same, and each projector 2 forms a portion of the total field-of-view. Each projector 2 has its own optics and operates independently from its adjacent projectors 2. For example first projector 21 is operable separately from second 22 and third 23 projectors. When the light emitted from all the projectors 2 is combined in the overall head motion box 5, an observer 6 (within the overall head motion box, also referred to herein as a “user” or in the context of a vehicle a “driver”) experiences a single image on a common virtual image plane. Compared to the size of optical system needed for equivalent field-of-view and / or head motion box size using a conventional head-up display system, the total size of the array of projectors may be reduced. 132652PCT1 The concept may be better understood further details of the example shown in Figure 1. Three optical projectors 21, 22, 23 are placed under a combiner 4 in the form of the windscreen a vehicle and embedded into a dashboard 7 of the vehicle. Each projector 21, 22, 23forms part of the field-of-view observable from within the overall, and in this case common head motion box 5. A first beam 81from the first projector 21will emerge from a different position under the dashboard 7 and therefore illuminate the overall head-motion box 5 from a different angle compared to the second 22 and / or third 23 projectors. The first beam 81 shows the range of angles leaving the first projector 21 from a single point of the first projector 21 output optics 91. It should be understood that each point on the output optics 91of the first projector 21will illuminate the overall head box 5 from a different range of angles, and similarly for the second 22and third 23projectors. Referring also to Figure 2, a schematic top view of the head-up display system 1 is shown. The combiner 4 in the form of the windscreen is not shown in Figure 2 for simplicity When the combiner 4 (in this example the windscreen) acts as a simple (planar) mirror its optical effect can be ignored. However, when the combiner 4 has significant curvature, which is typically the case for modern vehicle windscreens, mitigating factors can be applied to the projector 21, 22, 23 optics to eliminate the effects of the curvature (this will be further discussed hereinafter). Providing corrective optics to each projector 21, 22, 23to adjust for curvature of the combiner 4 which that projector 21, 22, 23illuminates may be relatively less complex (and less prone to aberration) than attempting to compensate for the curvature across the entire illuminated area of the combiner 4 in a single set of output optics. The beam 81 from the first projector will illuminate the respective projector head box 31 (identical in this example to the overall head box 5) at a range of angles defined by a first field-of-view 101. Similarly, the second projector 21produces a second field-of- view 102and the third projector 23 produces a third field of view. The total field-of- view of the head-up display system 1 with be formed by the combination of the projector 21, 22, 23 fields of view 101, 102, 103. 132652PCT1 In practice, due to the finite size of 21, 22, 23output optics 91, 92, 93the angle of the respective field-of-view 101, 102, 103 will not be constant across the beam 81, 82, 83, and the overall field-of-view 101, 102, 103 will be larger than the field-of- view at a central point of the output optics 91, 92, 93. Output of a single projector It will help with understanding of the head-up display projector units and systems described herein to explain in greater detail the operation of an individual projector 2. Referring also to Figure 3, a projector 2 is schematically illustrated. Although the output optics 9 are shown as a single lens in Figure 3 for simplicity of illustration, in practice the output optics 9 may be a combination of multiple lens (Fresnel lenses, diffractive optical elements (DOEs) and so forth), polarization optics, mirrors, catadioptric elements and so forth. The projector 2 include a light emitting element 11 for outputting an image. The light emitting element may take any suitable form, including without limitation a display 12 (Figure 14), a laser beam scanner assembly 13 (Figure 15), or any other suitable element for emitting light corresponding to an image. The light emitting element 11 emits light from an N by M array (both positive integers) of positions P, which shall be referred to as “pixels” even though in some examples the positions P may instead correspond to positions of a laser raster instead of more conventional pixels of a display 12. Let P(n,m) denote the position at the intersection of the nthof N rows and the mthof M columns. The output optics 9 are configured such that the rays r(1,m) emanating from the pixel P(1,m) make the same angle θ(1,m) to the optical axis 14 of the projector 2 across substantially the whole area of the output optics 9. The outgoing angle θ(n,m) corresponding to the pixel P(n,m) is substantially uniform across the output optics 9, but varies for each pixel. For example, the ray r(4,m) from the pixel P(4,m) makes an angle θ(4,m) ≠ θ(1,m). Rays r(4,m) are not drawn spanning the width of output optics 9 to avoid over-cluttering Figure 3. Referring also to Figure 4, a schematic view of a projector 2 outputting the beam 8 is shown. 132652PCT1 Each point on the surface of the 9 produces a beam 8 bounded in the plane of illustration (constant m in this case) by the rays r(1,m), r(N,m) corresponding to the extreme pixel positions P(1,m), P(N,m). The corresponding angles θ(1,m), θ(N,m) bound the field-of-view 10 (though as noted hereinbefore, in non-ideal cases the field-of-view 10 may vary slightly across the output optics 9). It should also be noted that the array of pixel locations P(n,m) need not be square, i.e. N is not required to equal M. The ratio N / M may be selected based on the desired shape and extent of the overall field-of-view, the overall head motion box 5, the number of projectors 2 and so forth. Referring also to Figure 5, variation of the outgoing angle θ(n,m) as a function of pixel P(n,m) is schematically illustrated along a single column (constant m). Whilst for simplicity of illustration the projector 2 has been illustrated with rays r(n,m) parallel, corresponding to a far-field (infinity focus) condition, in some applications the projector 2 may instead be configured to form a virtual image on a virtual image plane at a finite distance from an observer 6 within the overall head motion box 5. Although illustrated for simplicity as a linear relationship, in practice the variation need not be linear. Expanded field-of-view Referring also to Figures 6A and 6B, a first configuration 15 of a head-up display projector unit is shown (also referred to as simply the “first configuration”). The first configuration 15 corresponds to the example shown in Figure 1, in that the projector head motion box 3 is the same as the overall head motion box 5 for all of the projectors 2. The first configuration 15 may include any integer number K ≥ 2 of projectors (preferably K ≥ 3). In this way, each projector 21, 22, …, 2K forms a portion of a total (overall) field-of-view observable from the overall head motion box 5. The combiner 4 is illustrated schematically as a chained (dash-dot) line in Figure 6A, and is omitted from Figure 6B for visual clarity. The dash-dot-dot line shown in Figure 6A schematically represents the relative position for placement of a common optical element 53 (described hereinafter), when the common optical element 53 is used. 132652PCT1 The following explanations will benefit expansion of the previously defined nomenclature. Let Pk(n,m) denote the pixel position at the intersection of the nthrow and mthcolumn of the kthprojector 2k, and rk(n,m) denotes the corresponding ray, which has an outgoing angle θk(n,m), Since each projector 2kis arranged to illuminate the overall head motion box 5 from a different range of angles, it shall be apparent that even if each projector 21, 22, …, 2Kis identical (and preferably they are), the output optics 9 must be configured so that the same pixel coordinate (n,m) internally to each projector 21, 22, …, 2K will correspond to a different outgoing angle θk(n,m). In other words, the mapping illustrated in Figure 5 will have a different function for each 21, 22, …, 2K, such that with p ≠ k, θk(n,m) ≠ θp(n,m). Referring in particular to Figure 6B, rays rk(n,m) having equal outgoing angle θk(n,m) = θref are illustrated across the array of K projectors 21, 22, …, 2K. The pixel Pk(n,m) for which the outgoing angle θk(n,m) is equal to the fixed outgoing angle θref varies for each projector 21, 22, …, 2K. In this way, by using the projectors 21, 22, …, 2Kto output staggered but overlapped portions of an overall (or total) output image Im(i,j) with 1 ≤ i ≤ I and 1 ≤ j ≤ J. This is 7 and 8. Figure 7 schematically illustrates a pair of sub-images SUB output by adjacent projectors 21, 22, and Figure 8 schematically illustrates the relationship between output image pixels Im(i,j), projector pixels P1(n,m), P2(n,m) and outgoing rays r1(n,m), r2(n,m). The head-up display projector unit of the first configuration 15 is configured to project the output image Im(i,j) by generating a number of sub-images SUB1, SUB2, …, SUBK equal to the K of projectors 21, 22, …, 2K. Each sub-image SUBk overlaps with one or more images SUBp≠k corresponding to the adjacent projectors 2p≠k, and includes a portion of the output image Im(i,j). Across the entire set of sub- images SUB1, SUB2, …, SUBK, every part of the image Im(i,j) is included at least once. Each projector 21, 22, …, 2Kthe image SUB1, SUB2, …, SUBK, such that observed from the overall head motion virtual image of the output image Im(i,j) is formed which spans the total field-of-view. 132652PCT1 Referring in particular to the example in Figure 7, a pair of sub-images SUB1, SUB2 are shown. For visual clarity of the illustration, the sub-images SUB1, SUB2 are offset relative to one another in the column direction j. Relative to the output image pixels Im(i,j), the first sub-image SUB1starts at row i=i1and extends for a width Δ to row i = The second sub-image SUB2is overlapped with the first sub-image SUB1, row i2= i1+ δ, where δ < Δ. The region of output image pixels Im(i,j) between rows i2 and i1+Δ are output by both first 21 and second 22 Referring in particular to Figure 8, output of light corresponding to a reference pixel at the row marked i0in Figure 7 is illustrated. The width Δ corresponds to the width N of the projectors 21, 22. In the first projector 21, the reference pixel Im(i0,j) corresponds to projector pixel P1(n1,m), whilst in the second projector 22 pixel Im(i0,j) corresponds to projector pixel P2(n2,m). The output 91, 92 are configured such that these projector pixels P1(n1,m), P2(n2,m) generate output rays r1(n1,m), r2(n,m) at the same angle θ1(n1,m) = θ1(n1,m) = φ(i0,j), where φ(i,j) denotes the common outgoing angle for the output image pixel Im 21, 22, …, 2k. The output optics 91, 92of the adjacent by a distance s. The separation distance s is as possible, for example less than or equal to 5 mm, more preferably less than or equal to 3 mm. Using this approach, an observer 6 positioned within the overall head motion box 5 sees a single virtual image corresponding to the output image Im(i,j). Each projector 21, 22, …, 2Kgenerates a virtual sub-image (corresponding to sub- image SUBk) on a common virtual image plane not shown, virtual sub-images collectively form the virtual image corresponding to the output image Im(i,j) and perceived by the observer 6. illustrated in detail for a pair of adjacent projectors 21, 22, the same principles may be applied to project an output image Im(i,j) across any number K of projectors. For example, referring also to Figure 9, three sub-images SUB1, SUB2, SUB3 for projection to an overall head box 5 are illustrated. 132652PCT1 In the head-up display projector unit configuration 15, the projectors 21, 22, …, 2K have been illustrated to be configured to form the common virtual image plane in a far-field condition (parallel rays), for example at infinity, beyond 100 m and so forth. Alternatively, the projectors may be configured to form the common virtual image plane at a distance less than 100 m, for example corresponding to a typical distance ahead that an observer may be focussing for a given application. Since portions of the output image Im(i,j) will be output by two (or more) projectors 2k, this could lead to apparent variations in intensity when the observer 6 views the overlapping fields-of-view. Consequently, the head-up display projector unit of the first configuration 15 is preferably configured to adjust the relative intensity which each projector 2koutputs portions of the respective sub-image SUBkwhich overlap with one or more other sub-images SUBp≠k, such that the output image output image Im(i,j) observable by an observer 6 within the overall head motion box 5 has substantially uniform brightness. In this context, substantially uniform intensity means that a projected brightness ratio of any pair of pixels Im(ia,ja) / Im(ib,jb) in the output image which is observable from the overall head motion box 5 does not differ by more than 10% from a baseline brightness ratio for the same pair of pixels in the output image before projection. This may be quantitatively evaluated by, for example, taking a picture using a camera arranged within the overall head motion box 5 and arranged to capture an image encompassing the total field-of-view. Alternatively, the output image(s) Im(i,j) may be pre-prepared with relative pixel brightness values configured to for the fact that some pixels will be projected by two (or more) projectors 2k. Figures 6A to 8 presume a flat combiner 4. When the combiner 4 is curved the rays, e.g. r1(n1,m), r2(n2,m), will be compensated for the combiner 4 curvature so that the parallelism is met on exit from the combiner 4 instead. Expanded overall head motion box Referring also to Figure 10A and 10B, a second configuration 16 of a head-up display projector unit is shown (also referred to as simply the “second configuration”). The second configuration 16 differs from the first configuration 15 in that instead of expanding the field-of-view, the projectors 21, 22, …, 2K each forms a slightly offset projector head motion box 31, 32, …, 3K, and the overall head box 5 is formed as the 132652PCT1 sum (or union) of the individual motion boxes 31, 32, …, 3K. Only the first 31 and second 32 projector head motion boxes are shown in Figures 10A and 10B to avoid overcluttering the illustrations. The combiner 4 is illustrated schematically as a dashed line in Figures 10A and 10B. The chained (dash-dot) line shown in Figures 10A and 10B schematically represents the relative position for placement of a common optical element 53 (described hereinafter), when used the common optical element 53 is used. In the second configuration 16, differences between output optics 91, 92, …, 9K are limited to compensations for any curvature of the combiner 4, and each of the projectors 21, 22, …, 2Kproject the same output image Im(i,j) to the respective projector head motion box 31, 32, …, 3K. In this way image Im(i,j) is observable from any point within the overall head 5. Referring in particular to Figure 10A, after leaving the combiner 4, rays r1(n,m), r2(n,m), …, rK(n,m) corresponding to the same internal pixel are directed in parallel towards the overall head motion box 5. Referring to Figure 10B, rays r1(p≠n,m), r2(p≠n,m), …, rK(p≠n,m) corresponding to a different internal pixel coordinate (p≠n,m) are still directed in parallel towards the overall head motion box 5, but make a different angle θ1(p≠n,m) ≠ θ1(n,m). Referring also Figure 11, rays r1(n,m), r2(n,m) originating from the same pixel coordinate (n,m) of the first 21 and second 22 projectors are shown in further detail. In the second configuration 16, the output image Im(i,j) has equal dimensions to each of the projectors 21, 22, …, 2K, i.e. I=N and J=M. As shown schematically in Figure 11, rays r1(n,m), r2(n,m) originating from projector pixels P1(n,m), P2(n,m) having the coordinates (n,m) correspond to the image pixel Im(i0,j), and in parallel. This presumes a flat the combiner 4 is curved the rays r1(n,m), r2(n,m) will be compensated for the combiner 4 curvature so that the is met on exit from the combiner 4 instead. Each projector 21, 22, …, 2K is configured to form a virtual image corresponding to the output image Im(i,j) on a common virtual image plane, and projector 21, 22, …, 2K projects the same field-of-view 10, which corresponds to the total field-of-view of the overall head The projectors 21, 22, …, 2K may be configured to form the 132652PCT1 common virtual image plane in a far- for example at infinity (corresponding to the case of parallel rays illustrated), beyond 100 m and so forth. Alternatively, the projectors 21, 22, …, 2K may be configured to form the common virtual image plane at a distance less than 100 m, for example a typical focus distance appropriate for a given application. The projector head motion boxes 31, 32, …, 3Kof adjacent projectors 21, 22, …, 2Kmay overlap one another, as shown in Figures 10A and 10B. In other examples, the projector head motion boxes 31, 32, …, 3K of adjacent projectors 21, 22, …, 2K may not overlap one another. The output optics 91, 92of adjacent projectors 21, 22are separated by spacing s. Spacing s should be as small as possible, preferably less than or equal to 5 mm, and more preferably less than or equal to 3 mm. In this way, two or more projector head motion boxes 31, 32, …, 3K forming the overall head motion box 5 (and the corresponding projectors 21, 22, …, 2K) may be arranged to form an array spaced along a first direction (for example perpendicular to a central direction of the field-of-view of the overall head motion box 5). The first direction may correspond to a horizontal direction (relative to gravity in use). Expanding the size of the overall head motion box 5 may be useful in vehicles, because an operator / driver 6 can have greater flexibility to move their head whilst still being able to view the head-up display. This may prevent stiffness due to having to maintain their head within the smaller volume of a prior art head-up display. In some examples, when there are at least three or more projectors 21, 22, …, 2K, these may be arranged in a 2D array such that the corresponding three or more projector head motion boxes 31, 32, …, 3K forming the overall head motion box 5 are arranged to form an array spaced along first and second directions. The first and second directions are perpendicular to a central direction of the field-of-view of the overall head motion box 5. For example the second direction may correspond to a vertical direction (relative to gravity in use). An array of projectors 21, 22, …, 2K for forming an expanded overall head motion box 5 may be arranged in a square, rectangular, hexagonal, or any other 2D Bravais lattice type. Preferably, the output optics 91, 92, …, 9K of the projectors 21, 22, …, 2K are square, hexagonal, or any other shape which allows tessellation of the 21, 22, …, 2K. 132652PCT1 Figures 10A to 11 presume a flat combiner 4. When the combiner 4 is curved the rays, e.g. r1(n,m), r2(n,m), will be compensated for the combiner 4 curvature so that the is met on exit from the combiner 4 instead. Non-parallel projectors The two or more projectors 21, 22, …, 2Kforming an example of the first 15 or second 16 configuration are not required to have parallel optical axes. Instead, one or more pairs of projectors 21, 22, …, 2K may have non-parallel output optic axes 14. The output optics 91, 92, …, 9K will then be configured such that the hereinbefore described conditions for outgoing angles θk(n,m) are met on exit from the combiner 4 (whether flat or curved, such as a windscreen). Referring also to Figure 12, a curved head-up display projector unit 17 is illustrated. The curved head-up display projector unit 17 may be of the first 15 or second 16 configuration as required, and includes K=4 projectors 21, 22, 23, 24 which are integrated under a curved dash board 18 of a vehicle. The field-of-view 101, 102, 103, 104 of each projector 21, 22, 23, 24is compensated as described hereinbefore, so that the hereinbefore described conditions for outgoing angles θk(n,m) are met on exit from the combiner 4 in the form of a windscreen. In this way, two or more projectors 21, 22, 23, 24 arranged to form an array (in one or two directions, do not all need to be parallel to one another. This may allow embedding the curved head-up display projector unit 17 within a curved surface (such as dashboard, without enforcing a flat region. For example, the curved head-up display projector unit 17 may be seamlessly embedded in the curved vehicle dashboard 18 without disrupting the styling. Examples of projectors Referring also to Figure 13, a first example 19 of a projector 2 is shown (hereinafter “first projector”). The first projector 19 may be used to provide any or all of the projectors 2, 21, 22, …, 2K described herein. The first projector 19 includes a display 20 configured to output an image, for example the output image Im(i,j) or a sub-image SUB thereof, depending on the configuration 15, 16. 132652PCT1 The display 20 may be of any suitable type, including but not limited to a liquid crystal display (LCD), liquid crystal on silicon (LCoS), a dot matrix (DM display), an organic light emitting diode (OLED), or a micro-light emitting diode display (μLED), and so forth. A display 20 is not essential, for example referring also to Figure 14, a second example 21 of a projector 2 is shown (hereinafter “second projector”). The second projector 21 may be used to provide any or all of the projectors 2, 21, 22, …, 2K described herein. The second projector 21 uses a laser beam scanner assembly 22 configured to output an image by scanning a laser beam spot. The image (for example the output image Im(i,j) or a sub-image SUB thereof) is formed by the laser beam scanner 22. The laser beam scanner assembly 22 includes a laser module 23 creating a collimated laser beam 24 that illuminates a scanning mirror 25. As the scanning mirror changes 25 angle, the laser beam scanner assembly 22 reflected beam 26 illuminates the diffuser 27. The diffused beam 28 from the diffuser 27 illuminates the output optics 9 and creates the collimated beam required. The diffuser 28 can utilize a plurality of different technologies to form the optimal angular distribution of light. These technologies include (without limitation) embossed diffusers, holographic optical elements (HOEs), diffractive optical elements (DOEs) and so forth. Such diffusers 28 may have different angular characteristics across their surface to best fit the requirements of the projector 2kto maximise efficiency and achieve good uniformity. The laser beam scanner assembly 22 may consist of a single biaxial mirror 25 or two monoaxial mirrors 25. Each mirror 25 may operate in resonance or direct drive. The image may be formed using raster scanning (as in an old cathode ray monitor / TV) or formed using Lissajous scanning. The scanning of the mirror(s) 25 can dynamically change to maximise the brightness of the head-up display. Referring also to Figure 15, a third example 29 of a projector 2 is shown (hereinafter “third projector”). 132652PCT1 The third projector 29 may be used to any or all of the projectors 2, 21, 22, …, 2K described herein. The third projector 29 is the same as the first projector 19, except that it additionally includes one or more first actuators 30 coupled to, and configured to translate and / or rotate the display 20 relative to one or more optical elements of that projector 2k, such as the output optics 9. There may be any number of first actuators 30, each or which may be independently actuatable. In the example shown in Figure 15, two first actuators are shown, a right first actuator 30a and a left first actuator 30b. The first actuators 30 can move the display 20 up or down to allow for a variable focus for the user 6. By allowing two (or three) independent first actuators 30 to tilt and tip the display 20, the focal plane can also be shifted and aberrations introduced by the combiner 4 (for example a windscreen) may be reduced. Referring also to Figure 16, a fourth example 31 of a projector 2 is shown (hereinafter “fourth projector”). The fourth projector 31 may be used to provide any or all of the projectors 2, 21, 22, …, 2Kdescribed herein. The fourth projector 31 is the same as the first projector 19, except that it additionally includes one or more second actuators 32 coupled to, and configured to translate and / or rotate at least one optical element 9a of the output optics 9 relative to one or more other optical elements 9b of that projector 2k. Either of the optical elements 9a, 9b shown may take any form described herein in relation to the output optics 9, for example, each of the optical elements 9a, 9b may take the form of one or more lenses, one or more mirrors, one or more catadioptric arrangements / components / elements, one or more Fresnel lenses, one or more holographic optical elements (HOEs), one or more diffractive optical elements (DOEs), one or more polarization optics, and so forth. In this way, the actuatable optical element 9a may be shifted to provide variable focus. The second actuators 32 may be used to move the actuatable optical element 9a in order to shift (translate) or tilt to create variable depth cues or correct for aberrations introduced by the combiner 4 (for example a windscreen). 132652PCT1 The first 30 and second 32 actuators combined in a single projector 2k, allowing both the display 20 and actuatable optical element 9a to be moved, tilted and so forth. In a further example (not illustrated), any or all of the projectors 21, 22, …, 2Kmay have output optics 9 which include one or more adaptive optical elements (not shown). The adaptive optical elements (not shown) may be used to to introduce variable depth cues. For example, adaptive optical elements (not shown) may include (without limitation) one or more of a deformable mirror, a liquid crystal lens, a liquid lens, a phase device, or any other dynamically changing optic which can change the beam's focus and bring the virtual image to the desired depth plane. The dynamic element may operate in either a reflection mode or a transmission mode. In a still further example (not illustrated), any or all of the projectors 21, 22, …, 2Kmay have output optics 9 which include one or more polarisation optics (not shown). In such an implementation, variable depth may be achieved by using a stack of polarisation optics (such as geometric phase lenses), which perform a different optical function of the beam depending on its polarization. In such an example, a number of polarisation lenses(not shown) are stacked together along the optical path, each having half the power of the previous one. Using a half-wave plate (not shown) between each polarisation optics element (not shown) makes it possible to achieve a number 2Npo depth planes where Npo is the number of polarisation optics (not shown). While this approach results in a discretised z-positions for the image plane, the number of focal planes required to avoid fatigue may only be a handful. In another potential example (see Figures 6A, 10A and 10B), an optical path from each projector 21, 22, …, 2Kpasses through a common optical element 53 before reaching the combiner 4. For example each individual projector 2k may create a beam that is not perfectly collimated but slightly diverging (diverging rays rk(n,m) across the surface of the output optics 9). At a finite distance after the output optics 9 of each projector, a large common optical element 53 (for example a lens) may be used to perfect the collimation of the rays rk(n,m) (or if required, bring the rays rk(n,m) at a finite focal plane in front of the user). Allowing the rays rk(n,m) to diverge over a short distance in this way may enables the projector's 2k output optics to slightly increase, which may allow further reducing, or even removing any gap in the beams output from two 132652PCT1 adjacent projectors 21, 22, …, 2K. The optical element 53 may include (without being limited to) one or more of lenses, diffractive lenses, Fresnel lenses, holographic lenses, mirrors, catadioptric elements and so forth. In general the virtual image plane (where the image appears to be positioned to an observer) of each of the projectors 2 will correspond to a finite distance, so that light will not be perfectly collimated and will posses a small divergence angle. In this situation, a common optical element 53 in the form of a negative lens may be used to reduce (or even eliminate) mismatches between the virtual image planes of the projectors 2. Such mismatches may cause image degradation, fatigue / eye-strain of a user, and so forth. The term “common optical element” refers to a single component which is continuous across all the projectors 2, and which is distinct from the combiner 4. Arrays / assemblages of optical elements such as lenses which are physically joined, but which behave essentially independently in relation to each projector 2 (for example an array of lenses or micro-lenses), would not correspond to a common optical element 53. Whilst a common optical element 53 can be used, it is also possible for the optical path of each projector 21, 22, …, 2Kto be entirely independent, having only the combiner 4 in common. Referring also to Figure 17, a fifth example 33 of a projector 2 is shown (hereinafter “fifth projector”). The fifth projector 33 may be used to provide any or all of the projectors 2, 21, 22, …, 2Kdescribed herein. The fifth projector 33 includes catadioptric optics (mixture of reflective and refractive elements. In the example of the fifth projector 33, the catadioptric optics take the form of a single piece of plastic 34 shaped to form a number of lenses 35a, 35b and one or more mirrors 36. Mirrors 36 may be formed by total internal reflection and / or by silvering with a metallic layer / film. The fifth projector 33 includes a display 20 mounted on a support / substrate 37, and a catadioptric element in the form of a single piece of plastic 34 formed to have three optical surfaces 35a, 35b, 36. The first optical surface 35a operates as a (first) refractive lens, the second optical surface 36 as a curved mirror, and the third optical surface 35b as a (second) refractive lens. The curved mirror 36 may operate in 132652PCT1 reflection due to total-internal to the silvering of the outer surface of the single piece of plastic 34. The single piece of plastic 34 and support / substrate 37 may extend perpendicularly to the plan of illustration, so that two or more displays 20 may use the same prismatically extended piece of plastic 34 to provide corresponding projectors 21, 22, …, 2K. Referring also to Figure 18, a sixth example 38 of a projector 2 is shown (hereinafter “sixth projector”). The sixth projector 38 may be used to provide any or all of the projectors 2, 21, 22, …, 2Kdescribed herein. The sixth projector 38 is the same as the fifth projector 33, except that a second single piece of plastic 34b includes two optical surface 35a, 35b providing lenses and two optical surfaces 36a, 36b providing mirrors. In the second single piece of plastic 34b, a multi-faceted prism is constructed in which the optical beam bounces twice internally from a pair of optical surfaces 36a, 36b providing mirrors before it emerges from the projector 2 output optics 9. There are multiple forms that such a prism can take. In the sixth projector 38, the prism formed by the second single piece of plastic 34b has a first refractive lens 35a (or “entrance pupil”), a first mirror 36a which can be inclined by up to 45°, a second mirror 36b, and second refractive lens 35b. Referring also to Figure 19A, a seventh example 39 of a projector 2 is shown (hereinafter “seventh projector”). The seventh projector 39 may be used to provide any or all of the projectors 2, 21, 22, …, 2K described herein. The seventh projector 39 is the same as the sixth projector 38, except that a third single piece of plastic 34c has a wedge-like appearance with the light from the display 20 entering via the thick end of the wedge-like third single piece of plastic 34c, bouncing internally twice (either due to TIR or reflective coatings) before emerging from the long side of the wedge-like third single piece of plastic 34c. It should be understood that the surfaces (drawn as straight lines in Figure 19A) will in practice have surfaces curved and shaped to reduce aberrations and improve image quality. 132652PCT1 Referring also to Figure 19B, a first 2x 40 of projectors 21, 22, …, 2Kmay be formed by positioning a pair of the seventh projector 39 mirrored about the thin end (and optionally merged as a single unit). In this way, a single optical component that can accommodate two displays 20 is formed, and this structure may be tiled or extended along the direction perpendicular to the plane of the illustration to arrange the K projectors 21, 22, …, 2Kas 2x(K / 2) array. This may provide a particularly compact arrangement. Referring also to Figures 20A and 20B, a second 2x(K / 2) array 41 of projectors 21, 22, …, 2Kmay be formed by similarly to the first array 40 shown in Figure 19B, except using a single piece of plastic 34 which includes a single reflection per display 20. Figure 20A shows a side view, and Figure 20B shows a partial plan view. The second 2x(K / 2) array 41 of projectors 21, 22, …, 2K have their electronics arranged along the long sides of the second array 41. Referring also to Figure 21, an eighth example 42 of a projector 2 is shown (hereinafter “eighth projector”). In the eighth example 42, catadioptric optics are combined with polarising optics to minimise the total track length of the projector 2. (sometime termed "pancake optics"). In such an arrangement, the light propagates twice through the optical system reducing the total track length. The eighth projector 42 includes a display 20, a circular polarizer 43, a half-mirror 44, a quarter-wave plate 45 and a reflective polarizer 46. There will be a quantity of “lost light” 47 in the eighth example 42, which will not exit the projector 2 to join the output beam 8. Additionally or alternatively, in the eighth example 42 the polarisation emerging from the projector 2 may be optimised for maximum reflection from the combiner 4 (for example a windscreen). This concept may be applied to any of the examples of projectors 2 described herein, by adding appropriate polarising optics. 132652PCT1 Referring also to Figure 22, a ninth 48 of a projector 2 is shown (hereinafter “ninth projector”). The ninth projector 42 has a polarizing birdbath arrangement. In this arrangement, light leaves the display 20, with circular polarization. Light then passes through a quarter-wave plate 49 that changes the polarization to (for the sake of illustration) horizontal polarization. Light then reflects from a polarizing beam splitter 50 and passes again through the quarter-wave plate 49 and its polarization becomes anticlockwise circular. After reflection from a curved mirror 51, it again passes through the quarter-wave plate 49 and now is polarized vertically in order to pass through a reflective polarizer 52. Additional optics (not shown) on top of the polarizing beam splitter 50 may correct for aberrations, curvature of the combiner 4 and so forth. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of head-up display projectors, units and / or systems, and component parts thereof, and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. Using a display 20 with sufficiently high resolution, it would be possible to use an array of lenses or curved mirrors on top of the display 20. For example, similarly to the arrays 40, 41 shown in Figure 19B and Figures 20A and 20B, except using a single, high resolution display 20. Any of the head-up display projector units described herein (using any of the examples of projectors 2) may be arranged to use a window of a vehicle as the combiner 4. For example, the windscreen. The vehicle may be an automobile such as a car, truck of the like. In other examples, the vehicle may be an aeroplane, a ship or boat, or any other type of vehicle. Alternatively, any of the head-up display projector units described herein (using any of the examples of projectors 2) may be packaged with a suitable combiner 4 to form a stand-alone head-up display unit. 132652PCT1 The first 15 and second 16 herein represent extremal points of expanding the overall field-of-view without expanding the size of the overall head motion box 5 (the first configuration 15), and of expanding the size of the overall head motion box 5 without expanding the overall field-of-view (the second configuration 16). However, intermediate configurations are also possible, combining expansion of both the overall field-of-view and the overall head motion box 5. Any of the configurations 15, 16 (and modifications thereof) described herein may include a common optical element 53, arranged such that an optical path from each projector 2 passes through that common optical element 53 before reaching the combiner 4. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 132652PCT1

Claims

Claims 1. A head-up display projector unit comprising two or more projectors, each projector configured to project an image to a corresponding projector head motion box via a combiner, such that an overall head motion box and a total field-of-view observable from the overall head motion box are formed by the combination of the fields-of-view and projector head motion boxes of the two or more projectors; wherein an optical path from each projector passes through a common optical element before reaching the combiner.

2. A head-up display projector unit according to claim 1, wherein for each of the two or more projectors, the projector head motion box is the same as the overall head motion box, such that each projector forms a portion of a total field-of-view observable from the overall head motion box.

3. The head-up display projector unit of claims 1 or 2, configured to project an output image by: generating a number of sub-images equal to the number of projectors, each sub-image overlapping with one or more other sub-images and comprising a portion of the output image, wherein every part of the output image is comprised in at least one sub-image; projecting a respective sub-image via each of the projectors, such that observed from the overall head motion box the output image spans the total field-of- view.

4. The head-up display projector unit of claim 3, configured to adjust the relative intensity with which each projector outputs portions of the respective sub-image which overlap with one or more other sub-images, such that the output image observed from the overall head motion box has substantially uniform brightness.

5. A head-up display projector unit of claim 1, wherein each projector is configured to project an output image to the respective projector head motion box, such that the output image is observable from any point within the overall head motion box.

6. The head-up display projector unit of claim 5, wherein a maximum spacing between adjacent projectors is less than or equal to 5 mm. 132652PCT17. The head-up display projector unit of claims 5 or 6, wherein the two or more projector head motion boxes forming the overall head motion box are arranged to form an array spaced along a first direction.

8. The head-up display projector unit of claims 5 or 6, comprising three or more projectors, wherein the corresponding three or more projectors head motion boxes forming the overall head motion boxes are arranged to form an array spaced along first and second directions.

9. The head-up display projector unit of any one of claims 1 to 8, wherein the two or more projectors include first and second projectors, and wherein an output optical path of the first projector is different to an output optical path of the second projector.

10. The head-up display projector unit of any one of claims 1 to 9, wherein the two or more projectors include first and second projectors, and wherein output optic axes of the first and second projectors are not parallel.

11. The head-up display projector unit of any one of claims 1 to 10, wherein a projector of the two or more projectors comprises a display configured to output an image.

12. The head-up display projector unit of claim 11, wherein each projector that includes a display further comprises one or more first actuators configured to translate and / or rotate the display relative to one or more optical elements of that projector.

13. The head-up display projector unit of any one of claims 1 to 12, wherein a projector of the two or more projectors comprises a laser beam scanner assembly configured to output an image by scanning a laser beam spot.

14. The head-up display projector unit of claim 13, wherein each laser beam scanner assembly comprises a diffuser.

15. The head-up display projector unit of any one of claims 1 to 14, wherein a projector of the two or more projectors comprises one or more second actuators configured to translate and / or rotate at least one optical element of that projector relative to the corresponding optic axis. 132652PCT116. The head-up display projector unit of any one of claims 1 to 15, wherein a projector of the two or more projectors is configured for variable focus.

17. The head-up display projector unit of any one of claims 1 to 16, wherein a projector of the two or more projectors comprises adaptive optics.

18. The head-up display projector unit of any one of claims 1 to 17, wherein a projector of the two or more projectors comprises polarisation optics.

19. The head-up display projector unit of any one of claims 1 to 18, wherein an output beam of each projector has a shape which may be tessellated in two- dimensions.

20. The head-up display projector unit of any one of claims 1 to 19, wherein a projector of the two or more projectors comprises catadioptric optics.

21. The head-up display projector unit of claim 20, wherein the catadioptric optics comprise a single piece of plastic shaped to form one or more lenses and one or more mirrors.

22. The head-up display projected unit of any one of claims 1 to 21, wherein the common optical element comprises a negative lens.

23. A vehicle comprising the head-up display projector unit of any one of claims 1 to 22, arranged to use a window of the vehicle as the combiner.

24. A head-up display unit comprising: the head-up display projector unit of any one of claims 1 to 22; and the combiner.

25. Use of the head-up display projector unit of any one of claims 1 to 22 to project an output image to the head motion box. 132652PCT1

Images