Image projector and optical assembly

An image projector is disclosed that can include a light source and a MEMS mirror to receive a light beam emitted by the light source and oscillate to scan the light beam across multiple-beam-generators that each include a planar beam splitter arranged to receive the light beam and generate multiple beams to project an image.

FIELD OF THE INVENTION

The present invention concerns an image projector and optical assembly which comprises the image projector, which is suitable for use in a head-up-display, and which is configured to achieve a reduction in both speckle and morié.

DESCRIPTION OF RELATED ART

In projection assemblies light projected onto a projection screen can cause speckle which reduces the resolution of the projected image. For example, in projection assemblies which comprise a head-up-display, coherent light is projected onto the head-up-display so that a projected image appears on the head-up-display.

In projection assemblies which comprise a head-up-display the head-up-display is typically provided in the form of a transparent screen (e.g. a windscreen of a vehicle); the coherent light is projected onto the transparent screen so that a virtual image is visible at some point beyond the screen. The surface of the transparent screen is never perfectly smooth; accordingly when the coherent light is projected onto the transparent screen the transparent screen will randomly diffuse the coherent light thereby creating random inference which causes speckle which reduces the resolution of the virtual image.

In order to resolve this problem of speckle which occurs in projection assemblies which comprise a head-up-displays, it is known to provide the head-up-display in the form of a microlens array (or in the form of a micromirror array). The surface of each lens in the microlens array is perfectly smooth, accordingly it will not randomly diffuse the coherent light it receives; therefore no random inference or speckle is created. However, when the coherent light is projected onto the microlens array the microlens array will cause regular diffraction and regular interference known as moiré. The moiré appears as variations in intensity across the virtual image; accordingly the moiré compromises the quality of the projected image.

Many solutions in the prior art address exclusively the problem of speckle or moiré however none of the solutions provide an single adequate solutions with addresses both problems. US20040257664 discloses a solution to speckle but this solution is not effective to reduce moiré because the system is not configured to create an angle between the multiple beams which would allow the interference of the first beam to be averaged with the interference of the second beam by having the interference maxima of the second beam fit between the interference maxima of the first beam.

Other solutions of the prior art use anti-moiré filters or anti-aliasing filters but these solutions provide no means for adequate speckle reduction; moreover these solutions cannot incorporate a speckle reduction means such as a microlens array because they project polarised light and a microlens array cannot reduce the speckle in polarized light because the speckle or moiré pattern for microlens are the same for both orthogonal orientation of the polarization.

It is an aim of the present invention to mitigate or obviate at least some of the above-mentioned problems.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided an image projector comprising, a light source for providing a light beam; a MEMS mirror which is arranged such that it can receive the light beam, and which can oscillate about at least one oscillation axis to scan said light beam; one or more multiple-beam-generators each of the one or more multiple-beam-generators comprising, a planar beam splitter which is arranged to receive the light beam reflected by the MEMS mirror, and a planar reflector which can receive the part of the light beam which is transmitted through the planar beam splitter, so that each of the one or more multiple-beam-generators can generated multiple beams, and wherein the planar beam splitter and planar reflector are arranged to be in parallel; and a focusing lens which is arranged to receive multiple beams which are generated by the one or more multiple-beam-generators; wherein, in each of the one or more multiple-beam-generators the distance (h) between the planar beam splitter and planar reflector is such that the optical distance (OD) between the planar beam splitter and planar reflector is greater than, or equal to, half the coherent length of the light beam.

Preferably the light beam is a collimated light beam. Preferably the light source is configured to provide a collimated light beam.

The one or more multiple beam generators may comprise a planar beam splitter element, which defines the planar beam splitter, and a planar reflector element, which defines the planar reflector, arranged in parallel and spaced apart such that there is an air-gap between the planar beam splitter element and a planar reflector element and wherein the optical distance (OD) is defined as:

OD=tcos⁡(θ2)+hcos⁡[sin-1⁡(n⁢⁢sin⁢⁢θ2n3)]θ2=sin-1⁡(n1⁢sin⁢⁢θin)
wherein ‘t’ is the thickness of the planar beam splitter element, ‘Θi’ is the incidence angle of the light beam on the planar beam splitter element, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter element, ‘n’ is the refractive index of planar beam splitter element, ‘h’ is the distance between the planar beam splitter element and planar reflector element, ‘n3’ is the refractive index of the air in the air-gap.

The one or more multiple beam generators may comprise a block of transparent material, and a planar coating of semi-reflective material on a first surface of the block, which defines the planar beam splitter, and a planar coating of reflective material provided on a second, opposite, surface of the block which defines the planar reflector, and wherein the optical distance (OD) is defined as:

OD=hcos⁡(θ2)
wherein h is the distance between the coating of semi-reflective material and the coating of reflective material, and Θ2is

wherein ‘Θi’ is the incidence (AOI) of the light beam on the planar coating of semi-reflective material, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on planar coating of semi-reflective material, ‘n’ is the refractive index planar coating of semi-reflective material.

The image projector may comprise a plurality of multiple-beam-generators, which are arranged to be in optical communication, and wherein the planar beam splitter and planar bean reflector of each of the plurality of multiple-beam-generators lie on, or are arranged parallel to, differently orientated planes.

The planar beam splitter may be configured to have a beam splitting ratio of 40% reflection to 60% transmission. Preferably the planar beam splitter is configured to have a beam splitting ratio of 45% reflection and 55% transmission.

The planar beam splitter may be arranged such that the part of the light beam which has transmitted through the planar beam splitter and is reflected by the planar reflector, can pass directly to the focusing lens without passing through the planar beam splitter. For example the position of the planar beam splitter may be offset from the position of planar reflector so that the planar beam splitter does not completely overlay the planar reflector. Alternatively the planar beam splitter may have a smaller dimension than the planar reflector so that the planar beam splitter does not completely overlay the planar reflector. A combination of both offset positioning and smaller dimensions is also possible.

According to an aspect of the present invention there is provided an optical assembly comprising, an image projector according to any one of the preceding claims, and a screen arranged to receive light which has passed through the focusing lens, wherein the screen is configured to diffuse light it receives, and a head-up-display which is arranged to receive light which is diffused by the screen.

The screen is preferably a diffuser; the screen may be a microlens array, a micromirror array, or a white-board.

Preferably the image projector is positioned such that the focusing lens of the image projector is located at a distance from the screen which is equal to the focal length of the focusing lens.

The optical assembly may be configured to have a numerical aperture which is greater than the numerical aperture of the viewer. Preferably the optical assembly may be configured to have a numerical aperture which is greater than a predefined numerical aperture value which represents the numerical aperture of a viewer. Most preferably the optical assembly may be configured to have a numerical aperture which is greater than 0.016. The numerical aperture of the the optical assembly is defined as (d/2)/(F·M), wherein M is the magnification of the head-up-display, and F is the focal length of the of the focusing lens, and d is the distance between two consecutive multiple light beams which are output from the multiple beam generator and prior to said multiple light beams being incident on the focusing lens.

The optical assembly may be configured such that ‘d’ satisfies the condition:

d=2⁢(t⁢⁢tan⁢⁢θ2+h⁢⁢tan⁢⁢θ3)⁢cos⁢⁢θiθ2=sin-1⁡(n1⁢sin⁢⁢θin)θ3=sin-1⁡(n1⁢sin⁢⁢θ2n)
wherein ‘t’ is the thickness of the planar beam splitter, ‘Θi’ is the angle of incidence (AOI) of the light beam on the planar beam splitter, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter, ‘n’ is the refractive index of planar beam splitter, ‘h’ is the distance between the planar beam splitter and planar reflector, ‘n3’ is the refractive index of the material which occupies the space between planar beam splitter and planar reflector. For example: ‘t’ may be the thickness of a planar beam splitter element, ‘Θi’ may be the angle of incidence (AOI) of the light beam on the planar beam splitter element, ‘n1’ is the refractive index of the material in which the light beam passes before it is incident on the planar beam splitter element (e.g. ‘n1’ may be the refractive index of air), ‘n’ may be the refractive index of planar beam splitter element, ‘h’ is the distance between the planar beam splitter element and a planar reflector element, ‘n3’ is the refractive index of the material which occupies the space between planar beam splitter element and planar reflector element (e.g. ‘n3’ may be the refractive index of air).

The optical assembly may be configured such that ‘d’ satisfies the condition:

d=2⁢⁢h⁢⁢tan⁢[sin-1⁡(sin⁢⁢θin)]⁢cos⁢⁢θi
wherein ‘h’ is the distance between the planar beam splitter and the planar beam reflector, n is the refractive index of the material between the planar beam splitter and planar beam reflector, Θiis the angle of incidence of the light beam on the of semi-reflective material. For example ‘h’ may be the distance between a coating of semi-reflective material provided on a surface of a transparent block and a. coating of reflective material provided on an opposite surface of the transparent block, n may be the refractive index of a transparent block, Θimay be the angle of incidence of the light beam on the of coating of semi-reflective material.

The optical assembly may be configured such that ‘d’ satisfies the condition:

d=2⁢⁢F⁢⁢tan⁢θP4⁢(2⁢⁢k+1)
wherein F is the focal length of the of the focusing lens, k is an integer which is the order of the multiple-beam-generator, and Θpequal to:

θP=λP
wherein λ is the wavelength of the light beam and ‘P’ is the pitch of the microlens array.

k can be any integer: 0, 1, 2, 3, 4, . . . it should preferably be chosen such that it will make the OD greater than or equal to half the coherence length of the coherent light beam. It can also be chosen so as to ensure that the OD is greater than or equal to half the coherence length of the coherent light beam for all the wavelength λ used in the light beam (for example where h=5.12 choosing k=4, 5, 6 for the light bean red, green, blue respectively).

The optical assembly may be configured such that the distance ‘h’ between the planar beam splitter and the planar reflector is equal to:

h=d2⁢⁢tan⁡[sin-1⁡(sin⁢⁢θin)]⁢cos⁢⁢θi
wherein ‘n’ is the refractive index of material between the planar beam splitter and the planar reflector, Θiis the angle of incidence of the light beam on the beam splitter, and ‘d’ is the distance between two consecutive multiple light beams which are output from the multiple beam generator and prior to said multiple light beams being incident on the focusing lens.

Preferably the distance ‘h’ between the planar beam splitter and the planar reflector is between 0.5 mm and 10 mm. More preferably the distance ‘h’ between the planar beam splitter and the planar reflector is between 3 mm-4 mm.

The screen may comprise a microlens array. The microlens array may comprise different sized microlens so that the microlens array has a plurality of different pitch lengths between microlens' in the microlens array.

The screen may comprise a micromirror array. The micromirror array may comprise different sized micromirrors so that the micromirror array has a plurality of different pitch lengths between micromirrors. The pitch (P) between consecutive microlens, or micromirrors, may be equal to:
P=(Peff/cos Θscan)
wherein Peffis a predefined effective pitch value and Θscanis the angle of incidence of the light from the projector device on the microlens or micromirrors. Typically the predefined effective pitch value will be a pitch value which the users desires the scanned light to experience as it is incident on the microlens/micromirror array. Because P varies along the scanning angle (i.e. the size of the microlens or micromirrors varies (increases or decreases depending on the direction of scanning)) the Peffremains constant, at a predefined value, throughout the whole of the scanning angle.

The pitch (P) between consecutive microlens, or micromirrors, may be equal to:

P=(2⁢⁢k+1)⁢λ2⁢⁢tan-1⁢{2⁢⁢hF⁢tan⁡[sin-1⁡(sin⁢⁢θin)]⁢cos⁢⁢θi}
wherein k is an integer which is the order of the multiple beam generator, Θi is the angle of incidence of the light beam of the planar beam splitter, F is the focal length of the collimating lens, λ is the wavelength of the light beam, ‘h’ is the distance between the planar beam splitter and the planar reflector, n is the refractive index of material between the planar beam splitter and the planar reflector.

In a further embodiment the consecutive emission of independent red, green and blue light beams from the light source is synchronization with the oscillation of the MEMS mirror, so that the red, green and blue light beams are incident on predefined respective points on the focusing lens, so as to compensate for chromatic aberration ensuring that the red, green and blue light beams are focused by the focusing lens onto the same point on the screen. Preferably the red, green and blue light beams define the red, green and blue colours of a pixel; and therefore the amount of red, green and blue light in each beam is proportional to the amount of red, green and blue light in the pixel to be projected.

FIG. 1shows a plan view of an optical assembly1according to an embodiment of the present invention. The optical assembly1comprises an image projector2according to an embodiment of the present invention, and a screen3in the form of a microlens array3. The microlens array3comprises an array of microlens3′.

It should be understood that the screen3is not limited to being a microlens array; the screen3may take any suitable form, for example the screen3may be a micromirror array or a diffuser such as a wall surface, white board or standard projector-screen. Furthermore, in this exemplary embodiment each of the microlens'3′ in the microlens array3is the same size; accordingly the pitch ‘P’ between the microlens'3′ in the microlens array3is constant across the microlens array3. However, as will be described with respect to another embodiment of the invention, the microlens array may comprise microlens' which have different sizes such that the pitch between the microlens' in the microlens array differs across the microlens array. The pitch ‘P’ is the distance between the centre of a microlens to the centre of its neighbouring microlens.

The image projector2comprises, a light source4which provides a collimated light beam5; and a MEMS mirror6which is arranged such that it can receive the collimated light beam5. The MEMS mirror6is configured such that it can oscillate about two orthogonal oscillation axes7a,bso that it can scan the collimated light beam5in two-dimensions. It will be understood that the MEMS mirror6could be configured to oscillate about any number of oscillation axes. For example the MEMS mirror6could be configured to oscillate about a single oscillation axes so that it can scan the collimated light beam5in one dimension. In a further variation the image projector2may comprise two MEMS mirrors, arranged to be in optical communication with each other, one of the MEMS mirrors being configured to oscillate about one oscillation axis and the second MEMS mirror being configured to oscillate about a second oscillation axis which is orthogonal to the first oscillation axis, so that collectively the two MEMS mirrors can scan the collimated light beam5in two-dimensions.

The image projector2further comprises a multiple-beam-generator8. The multiple-beam-generator8comprises a planar beam splitter9in the form of a planar beam splitter element90which is arranged to receive the collimated light beam5reflected by the MEMS mirror6. The planar beam splitter element90reflects part5bof the collimated light beam5and transmits another part5aof the collimated light beam5. The planar beam splitter element90is configured to have a beam splitting ratio of 40% reflection to 60% transmission.

The multiple-beam-generator8further comprises a planar reflector10in the form of a planar reflector element100(e.g. a mirror) which can receive, and subsequently reflect, the part5aof the collimated light beam which is transmitted through the planar beam splitter element90. The multiple-beam-generator8thus generates multiple light beams15a-cfrom the single collimated light beam5. Only three multiple light beams15a-care illustrated in the figure for clarity, but it should be understood that the multiple-beam-generator8could generate any number of multiple light beams.

The planar beam splitter element90and planar reflector element100are arranged to be in parallel. The planar beam splitter element90and planar reflector element100are further arranged such that the optical distance between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherent length of the collimated light beam5(Typically the coherent length of the collimated light beam5will be a fixed value; the coherent length of the collimated light beam5is a property of the light source4which generates the collimated light beam5and is given as part of the specification for the light source4). The optical distance is defined as:

OD=tcos⁡(θ2)+hcos⁡[sin-1⁡(n⁢⁢sin⁢⁢θ2n3)]θ2=sin-1⁡(n1⁢sin⁢⁢θin)
wherein, ‘OD’ is the optical distance, ‘t’ is the thickness of the planar beam splitter element90, ‘Θi’ is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter element90, ‘n1’ is the refractive index of the material in which the collimated beam5passes before it is incident on the planar beam splitter element90(e.g. in this example the material in which the collimated beam passes before it is incident on the planar beam splitter element90is air so ‘n1’ is equal to the refractive index ‘air’), ‘n’ is the refractive index of planar beam splitter element90, ‘h’ is the distance between the planar beam splitter element90and planar reflector element100, ‘n3’ is the refractive index of the material which occupies the space13between planar beam splitter element90and planar reflector element100. In this embodiment the space13between the planar beam splitter element90and planar reflector element100is occupied by ‘air’ (i.e. there is an air gap13between the planar beam splitter element90and planar reflector element100) and the collimated beam5passes through ‘air’ before it is incident on the planar beam splitter element90such that n1=n3=1, so that in this example the optical distance is defined as:

While this example shown inFIG. 1illustrates the multiple-beam-generator8as comprising a planar beam splitter element90and planar reflector element100defining the planar beam splitter9and planar reflector respectively, and being separated by an air gap; it should be understood that the multiple-beam-generator8may have any other suitable configurations. For example in another embodiment a block of material, preferably transparent material (e.g glass), having a thickness ‘h’, occupies the space13. The planar beam splitter9and planar reflector10are attached, abut to, or are formed on, opposite sides of the block of material thus the planar beam splitter9and planar reflector10are fixed distance ‘h’ apart (‘h’ in this case is equal to the thickness of the block of material). In this case the optical distance is defined as:

OD=hcos⁡(θ2)
wherein ‘h’ is the distance between the planar beam splitter9and planar reflector10(i.e. the thickness of the block of material) and Θ2is

θ2=sin-1⁡(n1⁢sin⁢⁢θin)
wherein ‘Θi’ is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter element90, ‘n1’ is the refractive index of the material in which the collimated beam5passes before it is incident on the planar beam splitter element90(e.g. in this example the material in which the collimated beam passes before it is incident on the planar beam splitter90is air so ‘n1’ is equal to the refractive index ‘air’), ‘n’ is the refractive index of planar beam splitter element90.

For example, in a variation of the embodiment shown inFIG. 1, the multiple-beam-generator may comprise a glass block with a beam splitter coating on one surface of the glass block and a reflective coating on a second, opposite surface of the glass block. In this variation the beam splitter coating defines the planar beam splitter9and the reflective coating defines the planar reflector10, and the optical distance is defined as:

OD-hcos⁡(θ2)
wherein ‘h’ is the distance between the beam splitter coating (i.e. the planar beam splitter9) and the reflective coating (i.e. the planar reflector10); in other words ‘h’ is equal to the thickness of the glass block, and Θ2is

θ2=sin-1(n1⁢sin⁢⁢θin)
wherein ‘Θi’ is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter9, ‘n1’ is the refractive index of the material in which the collimated beam5passes before it is incident on the planar beam splitter9(e.g. the refractive index ‘air’), ‘n’ is the refractive index of planar beam splitter9. The image projector2further comprises a focusing lens11which is arranged to receive the multiple light beams15a-cwhich are output from the multiple beam generator8.

The image projector2is arranged to be a distance ‘F’ equal to the focal length of the focusing lens11, from the microlens array3.

Advantageously, in the embodiment shown inFIG. 1, the planar beam splitter9(planar beam splitter element90) and planar reflector10(planar reflector element100) causes the collimated light beam5to be split into multiple light beams15a-c; because the optical distance is greater than, or equal to, half the coherent length of the collimated light beam the multiple beams are ‘independent’; two beam are ‘independent’ if the difference between their optical paths is greater than the coherence length of the light source which generated the two beams. Referring to the optical assembly1illustrated inFIG. 1the light beam15ais reflected only by the planar beam splitter element90whereas the consecutive (or neighbouring) light beam15bis transmitted by the planar beam splitter element90and reflected by the planar reflector element100and transmitted again by the planar beam splitter element90. Since the optical distance between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherent length of the collimated light beam5, the beam15btravels an optical distance which is greater than the optical distance traveled by the beam15a, by at least an amount equal to the coherence length of the light source4. Thus consecutive (i.e. neighbouring) multiple light beams15aand15bare independent from each other. The other successive (i.e. neighbouring) multiple light beams (15cetc.) will also have optical paths which differ at least by a distance greater than or equal to the coherence length of the light source4.

Since the planar beam splitter9(planar beam splitter element90) and planar reflector10(the planar reflector element100) are arranged in parallel the multiple light beams15a-care then focused onto the same point16on the microlens array3using the focusing lens11, so as to define a single pixel17of a projected image at point16. Since the collimated light beam5is split into multiple independent light beams15a-cby the multiple beam generator8, the multiple independent light beams15a-ceach produce their own speckle and moiré pattern; accordingly when the multiple light beams15a-care focused onto the same point16the speckle and moiré patterns of each of the multiple independent light beams15a-cwill overlap at point16and thus will average-out; accordingly the speckle and moiré which is visible to a viewer viewing the pixel17will be reduced. In practice it is the viewer's eyes that will average out the speckle and moiré patterns upon viewing the pixel17. Each pixel of the image is projected onto the microlens array3in this manner so that there is a reduction in speckle and moiré pattern over the whole of the image projected on the microlens array3.

To ensure that the optical distance between the planar beam splitter9(the planar beam splitter element90) and the planar reflector10(the planar reflector element100) is greater than, or equal to, half the coherence length of the collimated light beam, the manufacturer will typically calculate the smallest angle of incidence ‘Θi’ of the collimated light beam5(preferably the central beam) on the beam splitter9. The smallest angle of incidence can be found for example, in the following manner: when the MEMS mirror6is at rest (i.e. the MEMS mirror6is not oscillating) the collimated light beam5reflected by the MEMS mirror6towards the beam splitter9defines what is referred to as the ‘central beam. It should be noted that in this embodiment, and for each of the other embodiments described herein, when the MEMS mirror6is at rest it is not oscillating about oscillation axes7a,b, and it is assumed that when the MEMS mirror6is at rest it will located at the centre of its amplitude of oscillation; for example if the MEMS mirror6can oscillation between ±45° then when the MEMS mirror6is at rest it will be positioned at 0°.

The manufacturer can adjust and measure the angle of incidence ‘Θi’ of the central beam on the beam splitter9. Then, the smallest angle of incidence is equal to the angle of incidence ‘Θi’ of the central beam minus half the maximum angle over which the MEMS mirror can scan the collimated light beam5. If the value for the smallest angle of incidence is less than 0° then 0° is taken to be smallest angle of incidence. Then, using the optical distance ‘OD’ formula described above and using the known refractive index of the materials composing the multiple beam generator, the manufacturer can adjust the dimension ‘t’ and ‘h’ such that the optical distance is greater than, or equal to, half the coherence length of the collimated light beam which is known.

In another type of optical assembly there may be provided a plurality of different light sources use to generate the multiple independent light beams15a-c, each light source generating one of the multiple independent light beams15a-c. Since each of the light beams will come from different light sources the multiple light beams15a-cwill be “independent”. For example a first light source could be used to generate the light beam15a, a second light source could be used to generate light beam15band a third light source could be used to generate light beam15c. In this variation no planar beam splitter9and planar reflector10is required; and the optical assembly may configured such that the first, second and third light sources generate respective light beams15a-cwhich are passed directly to the focusing lens11.

FIG. 2shows a perspective view of an optical assembly20according to a further embodiment of the present invention. The optical assembly comprises many of the same features of the optical assembly1shown inFIG. 1and like features are awarded the same reference numbers.

The optical assembly20further comprises a head-up-display21which is arranged to receive light from the microlens array3. The head-up-display21is arranged a distance ‘L’ from the microlens array3wherein ‘L’ is equal to:
(Ze−z)/M
wherein ‘Ze’ is a distance from a predefined position where the optical assembly20projects a virtual image22to a predefined position where a viewer should view the virtual image22, ‘z’ is a distance between the head-up-display21and the predefined position where a viewer should view the virtual image22, and ‘M’ is the magnification of the head-up-display21. The multiple light beams15a-cwhich are focused onto the microlens array3by the focusing lens11are diffused by the microlens array3. The diffused light23is passed through the head-up-display21to project a virtual image22at a location beyond the head-up-display21. The virtual image22will be the same image as the image which is projected onto the microlens array3, however the virtual image22may be larger (or smaller) as it may be magnified (or reduced) by the head-up-display21.

The optical-assembly20is configured such that it has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22. Preferably the optical assembly20is configured such that it has a numerical aperture which is greater than a predefined numerical aperture value which represents the numerical aperture of a viewer; the predefined numerical aperture value is usually 0.016. 0.016 is the largest numerical aperture of a viewer having a pupil size between 1 to 10 mm for a virtual image position which is greater than, or equal to, 300 mm from the viewer.

The numerical aperture of the optical assembly20is defined as:
(d/2)/(F·M)
wherein M is the magnification of the head-up-display21and F is the focal length of the of the focusing lens11, and ‘d’ is the distance between two consecutive (i.e. two neighbouring) multiple beams15a-coutput from the multiple beam generator8measured prior to being incident on the focusing lens11and is given by the equation:

d=2⁢(t⁢⁢tan⁢⁢θ2+h⁢⁢tan⁢⁢θ3)⁢⁢cos⁢⁢θiθ2=sin-1(n1⁢⁢sin⁢⁢θin)θ3=sin-1(n⁢⁢sin⁢⁢θ2n3)
wherein ‘t’ is the thickness of the planar beam splitter element90, is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter element90, ‘n1’ is the refractive index of the material in which the collimated beam5passes before it is incident on the planar beam splitter element90(which in this example is the refractive index ‘air’), ‘n’ is the refractive index of planar beam splitter element90, ‘h’ is the distance between the planar beam splitter element90and planar reflector element100, ‘n3’ is the refractive index of the material which occupies the space13between planar beam splitter element90and planar reflector element100(which in this example is the refractive index ‘air’).

In a preferred variation of the embodiment shown inFIG. 2, ‘d’ preferably satisfies the following equation:

θp=λP
wherein λ is the wavelength of the collimated light beam5and ‘P’ is the pitch of the microlens array3which is the distance from the center of a microlens in the microlens array3to the center of its neighbouring microlens, k is an integer defining the order of the multiple beam generator8, and Θpis the angular separation between the interference maxima (or minima) of one of the multiple light beams15a-c; this means that successive interference maxima of the first light beam15awill occur at 0Θp, 1Θp, 2Θp, 3Θp. . . etc. In order for the maxima of the interference pattern another one of the multiple light beams15a-c(e.g. second light beam15b) to overlay the minima of the interference pattern of the first light beam15a, its maxima of the interference pattern must fit between two successive maxima of the interference pattern of the first light beam15a; accordingly the maxima of the interference pattern of the second light beam15bmust occur at Θ=Θp/2, 1.5Θp, 2.5Θp, 3.5Θp. . . etc.

Thus calculation of ‘d’ using the condition Θ=Θp/2, 1.5Θp, 2.5Θp, 3.5Θp. . . etc gives:
d=Ftan(Θ)=Ftan(Θp/2).
Because the interference are periodic, this equation is also true for every odd integer (2k+1) multiple such that:
d=Ftan [(2k+1)Θp/2]
Advantageously when this condition for ‘d’ is satisfied, the moiré reduction is further increased.

The angle of incidence (AOI) ‘Θi’, of the collimated light beam5(preferably the central beam) on the beam splitter element90is the angle between the collimated light beam5and the normal vector to the planar surface9aof the planar beam splitter element90. Preferably the AOI is the angle between a central beam and the normal vector to the planar surface9aof the planar beam splitter element90, wherein the central beam is the beam reflected towards the planar surface9awhen the MEMS mirror6is at rest (i.e. is not oscillating about oscillation axes7a,b, in other words when the MEMS mirror6is not actuated). For example, one could use the central beam to make the calculation to find suitable dimensions for the thickness ‘t’ of the planar beam splitter element90and for the distance ‘h’ between the planar beam splitter element90and the planar reflector element100to ensure that the ‘OD’ of the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherence length of the calumniated light beam5and that the numerical aperture of the optical assembly20is larger than the numerical aperture of the viewer27:

The central beam is found by measuring the angle between the collimated beam5projected on the beam splitter9when the MEMS mirror is at rest and the normal to the surface9aof the beam splitter element90. The refractive index of the planar beam splitter (‘n’) is a known value typically given from material datasheet. Suitable dimensions for ‘t’ and ‘h’ are then chosen such that the optical distance ‘OD’ between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherence length of the collimated light beam5and such that the numerical aperture of the optical assembly20(i.e. (d/2)/(FM)) is larger than the numerical aperture of the viewer27.

The numerical aperture of the viewer27is defined by:
(y/2)/(Ze)
wherein y is the pupil size of the viewer27, and Ze is a distance from a predefined position where the optical assembly20projects a virtual image22to a predefined position where a viewer will view the virtual image22. The pupil size depends of each viewer physiognomy and ambient light conditions and may vary between 1 to 10 mm in size. The position/distance of the virtual image22with respect to the viewer27is preferably a predefined value and it is equal to ‘Ze=(L·M)+z’ wherein, ‘L’ is the distance between the microlens array3and the head-up-display21and ‘z’ is the distance between a predefined position for the eye of a viewer27and the head-up-display21. ‘L’ is determined using the equation L=(Ze−z)/M and is typically between 1 to 500 mm. ‘z’ is preferably a predefined value and is typically between 500 to 1200 mm. A distance between 500 to 1200 mm is the typical distance between the eye of a driver of a car and the position of a head-up-display21in the car. ‘M’ is preferably a predefined value which is representative of the position of the virtual image22; the position of the virtual image22can be changed by changing ‘M’. Typically a predefined value of for ‘M’ is given by the manufacturer.

During manufacturing fixed predefined values for the Ze, z, M and L are given. The magnification of the head-up-display M is preferably in the range of 1 to 20. The focal length F of the of the focusing lens is preferably in the range 5 to 1000 mm, and for d: t and h are preferably in the range 0.1 to 100 mm, and n is preferably in the range 1 to 4, Θiis preferably in the range 0 to 89°, so that d is preferably in the range 9.5×10−4mm to 55 mm.

The manufacturer will position the planar reflector element100and the planar beam splitter element90to achieve suitable values for ‘h’, ‘d’ and will adjust the position of the microlens array3and/or the position of the image projector2and replace the focusing lens11with a focusing lens having a suitable focal length, so as to achieve a suitable value for so as to ensure that the optical assembly20has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22, and such that the optical distance (OD) between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherence length of the calumniated light beam5. It is noted that if the position of the microlens array3and/or the position of the image projector2is moved then the focusing lens having a suitable focal length is a focusing lens which can be positioned between the image projector2and microlens array3at a distance from the microlens array which is equal to the focal length ‘F’ of the focusing lens.

For example, under normal illumination conditions the eye of a viewer27can measure 3 mm such that ‘y=3 mm’. The virtual image22can be desired to be at a distance of ‘Ze=2250 mm’ from the viewer27because typically for a head-up-display in automotive application it is desired to have the virtual image project to just in front of the automobile, thus the numerical aperture of the viewer27is 0.00067. Typically ‘M’ is equal 6 for the system compactness. To obtain a good virtual image quality, the pixel density must be at least 60 pixel/degree. This implies a condition on the virtual image size and resolution. For example, the image can have a 720p resolution which means that the image has 1280×720 pixels. If the image is viewed from a distance of 2250 mm it must have a maximal dimension of 846×473 mm to have a minimum of 60 pixels/degree. With a magnification of 6 the image on the microlens array3measures 141×79 mm. Thus the position of the microlens array3and/or the focusing lens11can be adjusted, and/or the focusing lens11can be replaced with a focusing lens having a suitable focal length, to achieve a suitable dimension for F which ensures that an image on the microlens array3is generated which is smaller or equal to the dimension 141×79 mm, otherwise the quality of the virtual image22will be lowered. In practice it will depend on the full optical scanning angle of the MEMS mirror6. It is always possible to obtain the right image size with a MEMS mirror projection system. If the image on the microlens array3is too small, one can replace the focusing lens11with a focusing lens with a suitable focal length, or move the image projector2further away from the microlens array3until the image dimension on the microlens array is small than or equal to the dimension 141×79 mm. Similarly if the if the image on the microlens array3is too large the focusing one can move the image projector2further away from the microlens array3and replace the focusing lens11with a focusing lens with a suitable focal length, until the image dimension on the microlens array is small than or equal to the dimension 141×79 mm. Once a value for ‘F’ is found which provides an image dimension on the microlens array3which is small than or equal to the dimension 141×79 mm, a suitable value for the thickness ‘t’ of the planar beam splitter element90and a suitable value for the distance ‘h’ between the planar beam splitter element90and the reflector element100are then determined so that the ‘OD’ between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherence length of the collimated light beam5and the numerical aperture of the optical assembly20is larger than the numerical aperture of the viewer27. Typically, ‘F=200 mm’ such that ‘d’ must be larger or equal to 2×F×M×0.00067=1.61 mm.

In this embodiment the space13between the planar beam splitter element90and planar reflector element100is occupied by an air and the collimated light beam5passes through air before it is incident on the planar beam splitter element90such that n1=n3=1, and the angle of incidence ‘Θi’ is 45°, and the beam splitter material is glass such that ‘n=1.52’, so that in this example the optical distance is defined as: ‘OD=t/0.885+h/0.707’; suitable values for ‘t’ and ‘h’ are chosen such that the optical distance (OD) between the planar beam splitter element90and the planar reflector element100is greater than, or equal to, half the coherence length of the collimated light beam5which for example in the case of a red diode laser can be 1 mm. From these numbers, one can adjust the dimension for ‘t’ and ‘h’ in the optical assembly20such that ‘OD’ of the optical assembly20is at least 1 mm and ‘d’ is at least 1.61 mm.

Advantageously, in this optical assembly20embodiment illustrated inFIG. 2, because the planar beam splitter element90and planar reflector element100are arranged in parallel and have an optical distance greater than half the coherent length of the collimated light beam5, its image projector2offers the same advantages of reduced speckle and reduced moiré patterns, as the image projector2of the optical assembly1shown inFIG. 1. Furthermore, since the optical assembly20is configured such that it has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22, the speckle patterns created by each of the multiple independent light beams15a-care de-correlated from each other; this results in a further reduction in speckle.

Furthermore since, in this optical assembly20the distance ‘d’ between two consecutive (i.e. two neighbouring) multiple beams15a-coutput from the multiple beam generator8measured prior to being incident on the focusing lens11and is equal to:
d=2(ttan θ2+htan θ3)cos θi
a better reduction in moiré is achieved because this condition for ‘d’ ensures that the maxima of the interference pattern of at least one of the multiple light beams15a-cmore precisely overlays the minima of the interference pattern of at least one other of the multiple light beams15a-c, and vice versa; in practice there will be many multiple light beams15a-cand many more precise overlays of maxima and minima of the interference patterns for different light beams.

In the embodiment in which ‘d’ satisfies the following equation:

θp=λP
wherein λ is the wavelength of the collimated light beam5and ‘P’ is the pitch of the microlens array3which is the distance from the center of a microlens in the microlens array3to the center of its neighbouring microlens, k is an integer defining the order of the multiple beam generator8, and θpis the angular separation between the interference maxima of one of the multiple light beams15a-c, then advantageously a further reduction in moiré is achieved.

FIG. 3shows a perspective view of an optical assembly30according to a further embodiment of the present invention. The optical assembly30shown inFIG. 3has many of the same features of the optical assembly20shown inFIG. 2and like features are awarded the same reference numbers.

However in contrast the embodiment illustrated inFIG. 2, the optical assembly30comprises an image projector2baccording to a further embodiment of the present invention, which comprises a multiple beam generator80having a glass block81between the planar beam splitter9and the planar reflector10(It will be understood that the invention is not limited to requiring a glass block; it will be understood that any suitable optically transparent material may be used in the multiple beam generator80). A planar beam splitter coating82is provided on one surface83of the glass block81and a planar reflective coating84is provided on a second, opposite surface85of the glass block. The planar beam splitter coating82may comprise a semi-reflective material and the planar reflective coating84may comprise reflective material. The opposing surfaces83,85of the glass block are flat and are parallel to one another so that the planar beam splitter coating82and planar reflective coating84are parallel and planar. In this embodiment the planar beam splitter coating82defines the planar beam splitter9and the planar reflective coating84defines the planar reflector10, and the optical distance between the planar beam splitter9(planar beam splitter coating82) and the planar reflector10(planar reflective coating84) is defined by:

OD=hcos⁡(θ2)
wherein ‘h’ is the distance between the planar beam splitter coating82and planar reflective coating84(i.e. the thickness ‘T’ of the block of the glass block81) and Θ2is

θ2=sin-1(n1⁢sin⁢⁢θin)
wherein ‘Θi’ is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter coating82, ‘n1’ is the refractive index of the material in which the collimated beam5passes before it is incident on the planar beam splitter element9(e.g. in this example the material in which the collimated beam passes before it is incident on the planar beam splitter9is air so ‘n1’ is equal to the refractive index ‘air’), ‘n’ is the refractive index of the planar beam splitter coating82.

In this example the distance ‘h’ between planar beam splitter coating82and planar reflective coating84is equal to:

h=d2⁢tan[sin-1(sin⁢⁢θin)]⁢cos⁢⁢θi
wherein ‘d’ is the distance ‘d’ between two consecutive (i.e. two neighbouring) multiple beams (e.g.15aand15b, or,15band15c) output from the multiple beam generator80prior to two consecutive multiple beams being incident on the focusing lens11, ‘Θi’ is the incidence (AOI) of the collimated beam5(preferably the central beam) on the planar beam splitter coating82, ‘n’ is the refractive index of the material the glass block81.

For example a manufacturer of the optical assembly30may choose a glass block having a suitable thickness ‘T’ such that the condition for ‘h’ mentioned above is met.

When this condition for ‘h’ is met a better reduction in moiré is achieved because the maxima of the interference pattern of at least one of the multiple light beams15a-cmore precisely overlays the minima of the interference pattern of at least one other of the multiple light beams15a-c, and vice versa; in practice there will be many multiple light beams15a-cand many more precise overlays of maxima and minima of the interference patterns for different light beams.

The optical-assembly30is configured such that it has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22. Preferably the optical assembly30is configured such that it has a numerical aperture which is greater than a predefined numerical aperture value which represents the numerical aperture of a viewer; the predefined numerical aperture value is usually 0.016. 0.016 is the largest numerical aperture of a viewer having a pupil size between 1 to 10 mm for a virtual image position which is greater than, or equal to, 300 mm from the viewer.

The numerical aperture of the optical assembly30is defined as:
(d/2)/(F·M)
wherein M is the magnification of the head-up-display21and F is the focal length of the of the focusing lens11, and ‘d’ is the distance between two consecutive (i.e. two neighbouring) multiple beams15a-coutput from the multiple beam generator8measured prior to being incident on the focusing lens11and is given by the equation

The optical assembly30is configured such that the distance ‘d’ between two consecutive (i.e. two neighbouring) multiple beams (e.g.15aand15b, or,15band15c) output from the multiple beam generator80measured prior to being incident on the focusing lens11is equal to:

d=2⁢h⁢⁢tan[sin-1(sin⁢⁢θin)]⁢cos⁢⁢θi
wherein ‘n’ is the refractive index of glass block81, Θiis the angle of incidence of the collimated light beam5on the planar beam splitter coating82(typically Θitaken to be the angle of incidence of central beam on the beam splitter coating82), and ‘h’ is the distance between the planar beam splitter coating82and planar reflective coating84(i.e. ‘h’ is equal to the thickness ‘T’ of the glass block81).

In a more preferred variation of the embodiment shown inFIG. 3, the optical assembly30is configured such that the distance ‘d’ between two consecutive (i.e. two neighbouring) multiple beams (e.g.15aand15b, or,15band15c) output from the multiple beam generator80measured prior to being incident on the focusing lens11is equal to:

θp=λP
wherein λ is the wavelength of the collimated light beam5and ‘P’ is the pitch of the microlens array3which is the distance from the center of a microlens in the microlens array3to the center of its neighbouring microlens, k is an integer defining the order of the multiple beam generator80, and Θpis the angular separation between the interference maxima (or minima) of one of the multiple light beams15a-c; this means that successive interference maxima of the first light beam15awill occur at 0Θp, 1Θp, 2Θp, 3Θp. . . etc. In order for the maxima of the interference pattern another one of the multiple light beams15a-c(e.g. second light beam15b) to overlay the minima of the interference pattern of the first light beam15a, its maxima of the interference pattern must fit between two successive maxima of the interference pattern of the first light beam15a; accordingly the maxima of the interference pattern of the second light beam15bmust occur at Θ=Θp/2, 1.5Θp, 2.5Θp, 3.5Θp. . . etc.

Thus, the calculation of ‘d’ using the condition Θ=Θp/2, 1.5Θp, 2.5Θp, 3.5Θp. . . etc. gives:
d=Ftan(Θ)=Ftan(Θp/2)
Because the interference are periodic, this equation is also true for every odd integer (2k+1) multiple such as:
d=Ftan [(2k+1)Θp/2]
When the optical assembly30is configured such that ‘d’ satisfies this condition a further reduction in moiré is achieved.

The angle of incidence (AOI), ‘Θi’, of the collimated light beam5on the beam splitter coating82is the angle between the collimated light beam5and the normal vector to the planar beam splitter coating82. Preferably the AOI is the angle between a central beam of the collimated light beam5and the normal vector to the planar beam splitter coating82wherein the central beam of the collimated light beam5is the beam reflected towards the planar beam splitter coating82when the MEMS mirror6is at rest.

For example, one could consider the AOI of the central beam to find suitable dimensions for ‘h’ (i.e. to find a suitable dimension for the thickness of the glass block81) when designing or manufacturing the optical assembly20so that such that ‘OD’ between planar beam splitter coating82and planar reflective coating84is greater than, or equal to, half the coherence length of the collimated light beam5and such that the numerical aperture of the optical assembly20(i.e. (d/2)/(FM)) is larger than the numerical aperture of the viewer27:

The central beam is found by measuring the angle between the collimated beam5projected on the beam splitter9when the MEMS mirror is at rest and the normal to the surface83of the beam splitter element9. The refractive index of the planar beam splitter is a known value typically given from material datasheet. Suitable dimensions for ‘h’ are then chosen such that ‘OD’ between the planar beam splitter coating82and the planar reflective coating84is greater than, or equal to, half the coherence length of the collimated light beam5and such that the numerical aperture of the optical assembly30(i.e. (d/2)/(FM)) is larger than the numerical aperture of the viewer27.

The numerical aperture of the viewer27is defined by (y/2)/(Ze) wherein y is the pupil size of the viewer27, and Ze is a distance from a predefined position where the optical assembly30projects a virtual image22to a predefined position where a viewer will view the virtual image22. The pupil size depends of each viewer physiognomy and ambient light conditions and may vary between 1 to 10 mm in size. The position of the virtual image22is preferably a predefined value and it is equal to ‘Ze=(L·M)+z’ wherein, ‘L’ is the distance between the microlens array3and the head-up-display21and ‘z’ is the distance between a predefined position for the eye of a viewer27and the head-up-display21. ‘L’ is determined using the equation L=(Ze−z)/M and is typically between 1 to 500 mm. ‘z’ is preferably a predefined value and is typically between 500 to 1200 mm. A distance between 500 to 1200 mm is the typical distance between the eye of a driver of a car and the position of a head-up-display21in the car. ‘M’ is preferably a predefined value which is representative of the position of the virtual image22; the position of the virtual image22can be changed by changing ‘M’. Typically a predefined value of for ‘M’ is given by the manufacturer.

During manufacturing fixed predefined values for the Ze, z, M and L are given. The magnification of the head-up-display M is preferably in the range of 1 to 20. The focal length F of the of the focusing lens is preferably in the range 5 to 1000 mm, and for d: h is preferably in the range 0.1 to 100 mm, and n is preferably in the range 1 to 4, Θiis preferably in the range 0 to 89°, so that d is preferably in the range 9.5×10−4mm to 55 mm.

The manufacturer of the optical assembly30may choose a glass block having a suitable thickness ‘T’ such that the condition for ‘h’ mentioned above is met and will adjust the position of the microlens array3and/or the projector2and replace the focusing lens11with a focusing lens having a suitable focal length, to achieve a suitable value for ‘F’, so as to ensure that the optical assembly30, is configured such that it has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22, and such that optical distance between the planar beam splitter coating82and the planar reflective coating84is greater than, or equal to, half the coherence length of the calumniated light beam5.

For example, under normal illumination conditions the eye of a viewer27can measure 3 mm such that ‘y=3 mm’. The virtual image22can be desired to be at a distance of ‘Ze=2250 mm’ from the viewer27because typically for a head-up-display in automotive application it is desired to have the virtual image project to just in front of the automobile, thus the numerical aperture of the viewer27is 0.00067. Typically ‘M’ is equal 6 for the system compactness. To obtain a good virtual image quality, the pixel density must be at least 60 pixel/degree. This implies a condition on the virtual image size and resolution. For example, the image can have a 720p resolution which means that the image has 1280×720 pixels. If the image is viewed from a distance of 2250 mm it must have a maximal dimension of 846×473 mm to have a minimum of 60 pixels/degree. With a magnification of 6 the image on the microlens array3measures 141×79 mm. Thus the position of the microlens array3and/or the focusing lens11can be adjusted, and/or the focusing lens11can be replaced with a focusing lens having a suitable focal length, to achieve a suitable dimension for F which ensures that an image on the microlens array3is generated which is smaller or equal to the dimension 141×79 mm, otherwise the quality of the virtual image22will be lowered. In practice it will depend on the full optical scanning angle of the MEMS mirror6. It is always possible to obtain the right image size with a MEMS mirror projection system. If the image on the microlens array3is too small, one can replace the focusing lens11with a focusing lens with a suitable focal length, or move the image projector2bfurther away from the microlens array3until the image dimension on the microlens array is small than or equal to the dimension 141×79 mm. Similarly if the if the image on the microlens array3is too large the focusing one can move the image projector2further away from the microlens array3and replace the focusing lens11with a focusing lens with a suitable focal length, until the image dimension on the microlens array is small than or equal to the dimension 141×79 mm. Once a value for ‘F’ is found which provides an image dimension on the microlens array3which is small than or equal to the dimension 141×79 mm, a suitable value for the distance ‘h’ between the planar beam splitter coating82and the planar reflective coating84are then determined so that the ‘OD’ between the planar beam splitter coating82and the planar reflective coating84is greater than, or equal to, half the coherence length of the collimated light beam5and the numerical aperture of the optical assembly30is larger than the numerical aperture of the viewer27. Typically, ‘F=200 mm’ such that ‘d’ must be larger or equal to 2×F×M×0.00067=1.61 mm.

In this embodiment the space81between the planar beam splitter coating82and planar reflective coating84is occupied by glass and the collimated light beam5passes through air before it is incident on the planar beam splitter coating82such that n1=1, and the angle of incidence ‘Θi’ is 45°, and the beam splitter material is glass such that ‘n=1.52’, so that in this example the optical distance is defined as: ‘OD=h/0.707’; suitable values for ‘h’ is chosen such that the optical distance (OD) between the planar beam splitter coating82and the planar reflecting coating84is greater than, or equal to, half the coherence length of the collimated light beam5which for example in the case of a red diode laser can be 1 mm. From these numbers, one can adjust the dimension for ‘h’ in the optical assembly30such that ‘OD’ of the optical assembly30is at least 1 mm and ‘d’ is at least 1.61 mm.

Advantageously, in this embodiment illustrated inFIG. 3, because the planar beam splitter coating82and planar reflective coating84are arranged in parallel and have an optical distance greater than half the coherent length of the collimated light beam5, its image projector2boffers the same advantages of reduced speckle and reduced moiré patterns, as the image projector2of the optical assembly1shown inFIG. 1. Furthermore, since the optical assembly30is configured such that it has a numerical aperture which is larger than the numerical aperture of a viewer27who is viewing the virtual image22, the speckle patterns created by each of the multiple independent light beams15a-care de-correlated from each other; this results in a further reduction in speckle.

Furthermore since, in this optical assembly30the distance ‘h’ between planar beam splitter coating82and planar reflective coating84is equal to:

h=d2⁢tan[sin-1(sin⁢⁢θin)]⁢cos⁢⁢θi
the optical assembly30can achieve an improved reduction in moiré is achieved because this condition for ‘h’ ensures that the maxima of the interference pattern of at least one of the multiple light beams15a-cmore precisely overlays the minima of the interference pattern of at least one other of the multiple light beams15a-c, and vice versa; in practice there will be many multiple light beams15a-cand many more precise overlays of maxima and minima of the interference patterns for different light beams.

Additionally an further more precise overlays of maxima and minima of the interference patterns of multiple light beams15a-cis achieved when the optical assembly30is configured such that ‘d’ satisfies the condition:

In the most preferred embodiment ‘h’ is equal to 5.12 mm. This provides the optimum moiré reduction when the multiple light beams15a-care blue, green and red light beams respectively; and which each have an angle of incidence Θiof 45°; and which has respective wavelength values

and respective k values,

And respective n values:

FIG. 7illustrates how the moiré reduction is achieved in the embodiments shown inFIGS. 2 and 3.FIG. 7illustrates the maxima71of the interference pattern of the second light beam15boverlaying the minima72of the interference pattern of the first light beam15a, and the maxima73of the interference pattern of the first light beam15boverlaying the minima (not visible in figure) of the interference pattern of the second light beam15b.

FIG. 4shows a plan view of an optical assembly40according to a further embodiment of the present invention. The optical assembly40shown inFIG. 4has many of the same features as the optical assembly1shown inFIG. 1and like features are awarded the same reference numbers. It will be understood that the optical assembly40could have any one or more of the features or conditions of the optical assemblies20,30shown inFIGS. 2 and 3.

In contrast the optical assembly shown inFIG. 1the optical assembly40according to a further embodiment of the present invention which comprises an image projector2caccording to a further embodiment of the present invention, which has a multiple beam generator41which comprises a glass block46(It will be understood that the invention is not limited to requiring a glass block46; it will be understood that any suitable optically transparent material may be used in the multiple beam generator41). A planar beam splitter9, in the form of a planar beam splitter coating42(which may comprise semi-reflective material), is provided on one surface49aof the glass block81, and a planar reflector10, in the form of a planar reflective coating43(which may comprise reflective material), is provided on a second, opposite surface49bof the glass block46. In this embodiment the planar beam splitter coating42defines the planar beam splitter9and the planar reflective coating43defines the planar beam reflector10. The first and second surfaces49a,bare flat and are parallel to one another so that the planar beam splitter coating42and planar reflective coating43are parallel and planar.

Importantly the planar beam splitter coating42is configured such that light45reflected from the planar reflector coating43passes directly from the planar reflector coating43to the focusing lens11without passing through the planar beam splitter42. This can be achieved by positioning the planar beam splitter coating42on only a portion of the first surface49aof the glass block46so that the planar beam splitter coating42does not overlay the whole of the planar reflector coating43and/or by providing a planar beam splitter coating42which has a smaller perimeter than the perimeter of the planar reflector coating43so that the planar beam splitter coating42does not overlay the whole of the planar reflector coating43. In this embodiment illustrated inFIG. 4the planar beam splitter coating42is provided on only a portion of the first surface49aof the glass block46so that the planar beam splitter coating42does not overlay the whole of the planar reflector coating43. Collimated light beam45which passes through the planar beam splitter coating42is thus reflected by the planar reflector coating43directly to the focusing lens11without passing through the planar beam splitter coating42. The multiple beam splitter41comprises a glass block46and a planar beam splitter coating42on one surface of the glass block46defines the planar beam splitter coating42, while a planar reflective coating43provides on an opposite surface of the glass block46defines the planar reflector43.

Advantageously, because the light beams reflected45by the planar reflector43are passed directly from the planar reflector43to the focusing lens11without passing through the planar beam splitter42only two beams48a,48bare generated by the multiple beam splitter41from a single collimated light beam45and these two beams48a,bhave similar optical power. When these two beams48a,bare focused to the same point16on the microlens array3by the focusing lens11each of the two beams48a,bwill produce an interference pattern of the same intensity and which are angularly shifted with respect to one another such as the maxima of the interference pattern of one of the light beams48aare located at the minima of the interference pattern of the other light beam48b; because the two beams48a,bhave the same optical power the interference patterns will average out perfectly to obtain a constant optical power image with reduced moiré.

It will be understood that in a variation of the embodiment shown inFIG. 4, no glass block is provided in the multiple beam generator; rather the multiple beam generator may simply take the form of the multiple beam generator8of the image projector2of the optical assembly1shown inFIG. 1, which comprises a planar beam splitter element90and planar reflector element100as mechanically independent structures separated by an air gap13. In this variation the planar beam splitter element90and planar reflector element100may be simply position so that the planar beam splitter element90does not overlay the planar reflector element100completely; or the planar beam splitter element90could be dimensioned to have a perimeter which is smaller than the perimeter of the planar reflector element100so that the planar beam splitter element90does not overlay the planar reflector element100completely; thereby allowing light45reflected from the planar reflector element100to pass directly from the planar reflector element100to the focusing lens11without passing through the planar beam splitter element90.

FIG. 6illustrates an optical assembly60according to further embodiment of the present invention. The optical assembly60contains many of the same features of the optical assemblies (1,20,30,40) shown inFIGS. 1-4and like features are awarded the same reference numbers. It will be understood that the optical assembly60could contain any of the features of the embodiments illustrated inFIGS. 1-4.

Unlike the other embodiments the optical assembly60comprises a plurality of multiple-beam-generators61,62, which are arranged to be in optical communication with one-another. Each of the multiple-beam-generators61,62may comprise one or more of the features of the multiple-beam-generators8,41,80illustrated in the any of the other optical assembly embodiments already described.

Each of the plurality of multiple-beam-generators have a different orientation such that they each lie on, or are parallel to, differently orientated planes64,65. In this example the planar beam splitter9and planar reflector10of the multiple beam generator61lie on, or are parallel to, a first plane64and the planar beam splitter9and planar reflector10of the multiple beam generator62lie on, or are parallel to, a second plane65. The first plane64and second plane65are orientated such that there is an angle of 120° between the planes64,65. Preferably the first plane64of the multiple beam generator61is orientated along the (1 0 1) and the second plane65of the multiple beam generator62is orientated along the (−1 −1 0). Then the angle between the two planes is 120°. The orientation of a plane is defined by a vector (x y z) which is normal to the plane. The first plane64is orientated perpendicular to the (1 0 1) vector66while the second plane65is orientated perpendicular to the (−1 −1 0) vector67.

It will be understood that while the optical assembly60illustrates only two multiple-beam-generators61,62which lie on different planes64,65any number of multiple-beam-generators could be provided in the optical assembly60, each multiple-beam-generator being in optical communication with another multiple-beam-generator and the planar beam splitter9and planar reflector10of each of the multiple beam generators lying on, or being parallel to, differently orientated planes.

Advantageously because the optical assembly60comprises two multiple beam generators61,62which are in optical communication and which lie on, or are parallel to, differently orientated planes64,65this enables a further reduction in speckle and moiré to be achieved because a 2D array69of multiple beams is created when using two multiple beam generators61,62. Each of the multiple beams in the 2D multiple beam array69are focused by the focusing lens11to the same point16on the microlens array3to define a single pixel17. Multiple speckle patterns or moiré patterns are then created at point16such as they average out in 2D providing improved speckle and moiré reduction. Each pixel of the projected image is projected in this manner so that there is a reduction in speckle and moiré over the whole of the projected image.

As mentioned in the embodiment illustrated inFIG. 1the sizes of the microlens'3′ in the microlens array3are all equal, however in a variation of the invention the microlens array3could be configured to have different sized microlens as shown inFIG. 5a.FIG. 5ashows a perspective view of an alternative configuration for the screen3which can be used in any of the embodiments of the present invention.FIG. 5aillustrates a screen3in the form of a microlens array50which comprises differently sized microlens51. Accordingly in the microlens array50the pitch ‘P’ between the microlens'51in the microlens array50differs across the microlens array50. More precisely in this example the sizes of the microlens'51in the microlens array50increase from a centre column53of microlens towards the outermost column54of microlens of the microlens array51. The size of the microlens along each respective column53,54are equal; however in a variation of the embodiment the size of the microlens along each respective column53,54may increase or decrease.

In a further variation of the embodiment the sizes of the microlens'51in the microlens array50can increase from a centre row56of microlens towards the outermost row of microlens57of the microlens array51. The size of the microlens along each respective row56,57may be equal; however in a further variation of the invention the size of the microlens along each respective row56,57may increase or decrease.

Specifically the size of the microlens'51in the microlens array50is such that the pitch between consecutive microlens51, is equal to:
(Peff/cos Θscan)
wherein Peffis a predefined effective pitch value and Θscanis the angle of incidence of a light beam emitted from the projector device on that microlens51. The ‘effective pitch’ is the projection of the pitch of the microlens along the incoming light direction Θscan.

FIG. 5billustrates the problem which arises with microlens arrays in which all microlens of the microlens array are of equal size: As the light beam is scanned across the microlens array, the angle of incidence of the light beam on microlens array will change over the scan amplitude; as the light beam is scanned towards the outermost microlens' in the angle of incidence will be reduced so that light beam experiences an ‘effective pitch’ which is lower than the physical distance between the centres of neighbouring microlens; this can result in a changing interference pattern along the scanning amplitude (wherein the scanning amplitude is the amplitude over which the oscillating MEMS mirror6scans the collimated light beam5). Because the angle between two maxima of the interference pattern is defined by Θp=λ/P, as the scanning amplitude increases, the ‘effective pitch’ becomes smaller, thus Θpbecomes larger and the calculated thickness of the MBG is not optimal to average out perfectly the interference patterns. Advantageously the microlens array shown inFIG. 5aresolves this problem; since the microlens' are sized such that the pitch between consecutive microlens51is equal to (Peff/cos Θscan) it compensates for the scanning of the light beam by the MEMS mirror6so that ‘effective pitch’ experienced by the light beam is equal across the whole scanning amplitude.

In a further variation of the microlens array50shown inFIG. 5; the microlens51in the microlens array may be further sized such that they compensate for the change in angle of incidence of the collimated light5beam on the planar beam splitter9, which occurs when the collimated light beam5is scanned by the oscillating MEMS mirror6. In this case the optical assembly will preferably take the form of the optical assembly30shown inFIG. 3and the optical assembly30will be configured such that the distance ‘h’ between the planar beam splitter9(beam splitter coating82) and the planar reflector10(reflective coating84)) is given by:

The thickness ‘T’ of the glass block81between the beam splitter coating82and the reflective coating84defines the distance ‘h’ between the planar beam splitter9(beam splitter coating82) and the planar reflector10(reflective coating84)), thus the optical assembly30is configured such that the above mentioned condition for ‘h’ is met by choosing a glass block81with the appropriate thickness.

The optical assembly30is configured such that the distance ‘d’ between two consecutive (i.e. two neighbouring) multiple beams15a-coutput from the multiple beam generator80measured prior to being incident on the focusing lens11is:

The optical assembly30is configured such that the above conditions for ‘h’ and ‘d’ are met for the ‘central beam’. As the amplitude of the scanning angle increases, Θiwill change and the required value of ‘h’ to satisfy the condition on ‘d’ will thus change too. But it is difficult to have a changing thickness ‘h’ because the beam splitter and reflector should preferably be planar. In the present solution the variable Θp=λ/P is changed over the canning angle by having a microlens array which has an increasing pitch between the microlens, such as the condition on ‘d’ is constant with the change in input angle Θi. Preferably pitch P between the microlens in the microlens array50should preferably vary according to the following equation:

P=(2⁢⁢k+1)⁢λ2⁢⁢tan-1⁢{2⁢⁢hF⁢tan⁡[sin-1⁡(sin⁢⁢θin)]⁢cos⁢⁢θi}
wherein all the variables of this equation are fixed by the design of the optical assembly30and Θivaries with the scanning of the MEMS mirror6.

It will be understood that although the screen3shown inFIG. 5ais in the form of a microlens array50, the screen3could alternatively be in the form of a micromirror array having the same pitch conditions between consecutive micromirrors as those described above for the microlens array50. It will also be understood that any of the above described optical assemblies could have a screen3in the form shown inFIG. 5a. Also it will be understood that the screen3may alternatively comprise a micromirror array and similarly the pitch between consecutive micromirror in the micromirror array may be equal to (Peff/cos Θscan).

Furthermore, it will be understood that in each of the above-mentioned image projector2,2b,2cand/or optical assembly embodiments1,20,30,40,60the focusing lens11may take any suitable form; for example the focusing lens11may be a simple converging lens, a plano convex lens, double convex lens or a F-theta lens. Additionally, or alternatively, the focusing lens11may be further configured to correct chromatic aberration; for example the focusing lens11may be an achromatic lens such as an achromatic doublet or lens with a surface grating which is configured to correct chromatic aberration.

In each of the above mentioned image projector2,2b,2cand/or optical assembly embodiments1,20,30,40,60the a light source4may be configured to emit collimated light beams5in pulses, each pulse of light defining a single pixel of a projected image. Each pulse may comprise the amount of red, green and blue light beams necessary for defining a corresponding pixel of the projected image; thus in this case the red, green and blue light beams are emitted by the light source simultaneously in the same pulse. One problem which may arise by having the red, green and blue light beams in the same pulse is that each of the red, green and blue light beams are incident on the same position on the focusing lens11; as a result due to chromatic aberration the red, green and blue light beams will be focused to different points on the screen3thus compromising the resolution of the projected pixel.

To obviate or mitigate this problem the light source4may be configured to emit pulses of green, red and blue light beams consecutively in independent pulses, rather than emitting them simultaneously in the same pulse. Preferably the time of the emission of the green, red and blue light beams is synchronized with the orientation of the oscillating MEMS mirror6so that the red, green and blue light beams are incident at predefined respective positions on the focusing lens11. The predefined respective positions are such that the red, green and blue light beams are focused to same point on the screen3by the focusing lens11.

The chromatic aberration of the focusing lens11is a known value; typically known from the lens manufacturer or it can be calculated using well known equations in optics; it can also be measured using a detector placed after the focusing lens11. Thus, the chromatic aberration is known for each pixel of a projected image. The optical assembly can also include a position sensor which is configured to measure the position and the speed of oscillation the MEMS mirror6about its oscillation axes7a,7b, so that the position and the speed of oscillation of the MEMS mirror6is known at all-times. Knowing the speed and the position of the MEMS mirror6, and knowing the chromatic aberrations of the focusing lens11for each red, green and blue light beam, allows calculating the time of the emission of the green, red and blue light beams to compensate the chromatic aberration such that the green, red and blue light beams are focused to the same point on the screen3by the focusing lens11:

For example, due to the effects of chromatic aberration the blue light portion of a pixel is offset by a fixed distance of ‘x mm’ with respect to the green light portion of a pixel at a given scanning angle α of the MEMS mirror. At this given scanning angle α, the speed of oscillation of the MEMS mirror6is ‘ω’ in degree/second and the scanned spot16moves on the screen3at a speed ‘v’ in mm/s. To compensate for the effects of chromatic aberration, the light source4is configured to emit independent pulses of blue, red and green light, consecutively, for each pixel to be projected; the red, green and blue pulses combine on the screen3to form an pixel. The amount of blue, red and green light in the each pulse is according to the amount of blue, red and green colour in the pixel to be projected. Importantly in present invention, when the MEMS mirror6is detected to be at its given scanning angle α, and oscillating at a speed of ‘ω’ in degree/second, the blue light pulse which is to define blue colour portion of the pixel is emitted at a time delay of ‘t=x/v’ seconds after the green light pulse which is to define the green colour portion of the pixel such that the blue and green pulses are incident at the same position16on the screen3to form the pixel17. A similar operation will be done for the emission of the red light pulse. ‘t’ defines the time delay between consecutive red, green and blue pulses. The time delay of the emission of the green, red and blue light beams can vary for each position of the MEMS mirror6because the aberration and the speed of the MEMS mirror6also vary throughout the oscillation amplitude of the MEMS mirror6will vary. Thus the synchronization of the emission of the independent red, green and blue light beams from the light source4with the oscillation of the MEMS mirror, so that the red, green and blue light beams are incident on predefined respective points on the focusing lens11such that they are focused by the focusing lens11onto the same point on the screen3, can be used to correct chromatic aberration.

Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.