EP-4740055-A1 - OPTICAL DISPLAY SYSTEM

Abstract

The present disclosure relates to an off-axis optical display system for a head mounted display, the optical system comprising: an optical combiner defining a first optical axis for directing image rays to a pupil plane; and an aberration compensating element defining a second optical axis independent from the first optical axis; wherein the optical combiner is arranged to generate an intermediate focal plane on the second optical axis which is an optical conjugate of the pupil plane and wherein the aberration compensating element is located substantially at the intermediate focal plane.

Inventors

• VOLKOV, ANDRII

Assignees

• TruLife Optics Limited

Dates

Publication Date

20260513

Application Date

20240626

Claims (19)

1. 1. An off-axis optical display system for a head mounted display, the optical system comprising: an optical combiner defining a first optical axis for directing image rays to a pupil plane; and an aberration compensating element defining a second optical axis independent from the first optical axis; wherein the optical combiner is arranged to generate an intermediate focal plane on the second optical axis which is an optical conjugate of the pupil plane and wherein the aberration compensating element is located substantially at the intermediate focal plane.
2. 2. The off axis optical display system of claim 1 , wherein the optical combiner comprises a diffractive optical element having a phase function and wherein the phase function comprises a tilt component and a quadratic component.
3. 3. The off axis optical display system of claim 2, wherein the quadratic component is a parabolic phase function.
4. 4. The off axis optical display system of claims 2 to 3, wherein the tilt component and a quadratic component are configured to achieve substantially field-independent aberrations at the optical conjugate of the pupil plane.
5. 5. The off axis optical display system of claims 2 to 4, wherein the tilt component and the quadratic component are configured to achieve substantially telecentric rays along at least a portion of the second optical axis from the optical combiner to the aberration compensating element.
6. 6. The off axis optical display of claims 2 to 5, wherein the tilt component is configured and arranged to apply substantially the same tilt to wavefronts incident on the optical combiner.
7. 7. The off axis optical display system of claims 2 to 6, wherein the quadratic component is arranged to focus all wavefronts incident on the optical combiner independent of the angle of incidence of the wavefronts on the optical combiner.
8. 8. The off axis optical display system of any preceding claim, wherein the tilt component is a diffractive tilt component and the quadratic component is a focusing component.
9. 9. The off axis optical display system of any preceding claim, wherein the diffractive optical element is one of a holographic diffractive pattern, volume diffraction grating, a surface relief diffraction grating, or a reflection grating and is configured and arranged to introduce a phase retardation to wavefronts incident thereon.
10. 10. The off axis optical display system of claims 2 to 9, wherein the total phase function is given a sum of the tilt component and the quadratic (parabolic) component: <J ( , y)tot= O ( , y)tiit + O ( , y) parab oi ic
11. 11. The off axis optical display system of claim 10, wherein the quadratic phase function is given by: where X and Y are coordinates on the optical combiner, k is a wave vector, and Zo is the eye relief from the pupil plane to the optical combiner.
12. 12. The off axis optical display system of claim 10, wherein the tilt component is given by: *(X, Y ilt = aX + bY
13. 13. The off axis optical display system of any preceding claim, further comprising focusing optics arranged between an image source and the compensating element, wherein the focusing optics are configured and arranged to image source to the optical conjugate of the pupil plane.
14. 14. The off axis optical display system of any preceding claim, further comprising additional focusing optics arranged between the compensating element and the optical combiner, wherein the additional focusing optics are arranged to relay intermediate pupil to exit the pupil.
15. 15. The off axis optical display system of any preceding claim, wherein additional focusing optics may be implemented on the optical combiner arranged to relay intermediate pupil to exit the pupil.
16. 16. The off axis optical display system of claim 15, wherein chief rays are substantially parallel to the second optical axis between the additional focusing optics and the optical combiner.
17. 17. The off axis optical display system of any preceding claim, wherein the optical combiner is a free-space optical combiner.
18. 18. The off axis optical display system of claim any preceding claim wherein an eyebox of the system is asymmetric.
19. 19. A wearable head mounted display comprising the off axis optical display system of claims 1 to 18.

Description

OPTICAL DISPLAY SYSTEM FIELD OF THE DISCLOSURE The present disclosure relates to an optical display system. In particular, the disclosure relates to an off-axis optical display system and more particularly a head mounted augmented reality display system. BACKGROUND OF THE DISCLOSURE In the field of optical display system design, so-called off-axis optical systems are systems in which at least two independent optical axes do not coincide. Off-axis optical systems relay an image to an image plane. Applications of off-axis optical systems include, but are not limited to, head mounted displays (HMDs), head up displays (HUDs), camera devices and more generally optical projection and display systems. Specific examples can include AR, VR or MR display systems where a user’s eye, and more specifically the retina of a user’s eye, is located at the image plane. Figure 1 is a generalised view of an augmented reality display to superimpose an image from a projector or image source onto a real-world view at for example the eye of a user. The off- axis optical system 100 of Figure 1 generally comprises: a light source or image source 110 with associated beam shaping optics 120; imaging optics 130; and an optical combiner 140. Broadly speaking the combination of the light source 110, beam shaping optics 120 and the imaging optics 130 are termed a projector and when in use provide the image to be viewed at the eye of the user. The function of the optical combiner 140 is to direct light from the projector to a user’s eye while also allowing ambient external environmental light from a real-world view to pass through the optical combiner 140 to the user’s eye such that images from the projector are superimposed on the real-world view. An example of an optical combiner 140 is a holographic optical element (HOE). The HOE directs light from the light source 110 and to an image plane of the optical system, which would then be visible by a user’s eye. HOEs, or more generally, diffractive optical combiners are examples of free space optical combiners, so called because they do not rely on waveguides. An optical combiner 140 has at least two independent optical axes 150, 160 as shown in Figure 1 , where the first optical axis 150 is that from the projector to the optical combiner 140 and the second optical axis 160 is that from the optical combiner 140 to the user’s eye/pupil plane. Such a system is therefore known as an off-axis optical system. When used in applications such as head-mounted display systems or augmented reality display systems, optical combiner 140 of the type described above are known to advantageously provide high transparency, good efficiency, that is, bright images at low light power and be compatible with ophthalmic glasses lens prescriptions and encapsulation into such lenses. However, such advantages need to be balanced against increased aberrations inherent with off-axis systems. The main difficulty associated with using free-space combiners in an optical system is that such combiners change the optical axis of the system, as shown in Figure 1 where the independent optical axis 150 of the projector is independent to the optical axis 160 of the combiner 140, the problems with which are discussed in more detail below. It should be noted that whilst light travels from the light source or image source 110 via the optical combiner 140 to a user’s eye, optical designers also consider systems in reverse, where notional light rays travel from the user’s eye via the optical combiner 140 to the light source or image source 110. A challenge for the optical design of off-axis display devices comprising such optical combiners is that the key specifications of eye box, form factor, and field of view, are related and constrained by a quantity known as etendue. Another challenge is the issue of aberration control. Figure 2 shows the basic components of a head mounted display where the eyebox is the area on the surface at the desired eye position (or pupil plane) within which the pupil of the user can move while still seeing the full image from the display. The form factor of such a head mounted display is the three-dimensional volume that encloses the projector and rays in the head-mounted display device, and the field of view is the angular spread of rays at the centre of the eyebox carrying the image. Figure 3 depicts a simplified ray diagram showing that the eye relief is the distance from the combiner to the eye. Also shown is the field of view and eyebox for light rays from an optical combiner. The etendue of an optical system is related to the conservation of information bandwidth through the optical system which characterises how spread out the light is in area and angle and very generally is equal to the product of the eye box size and field of view. For head mounted displays, etendue restricts the optical design in two significant ways. The first restriction imposed by the etendue of an optical system is that t