GB2635764A

GB2635764A - Opthalmic light guide system

Info

Publication number: GB2635764A
Application number: GB2318049.0A
Authority: GB
Inventors: Volkov Andrii
Original assignee: Trulife Optics Ltd
Current assignee: Trulife Optics Ltd
Priority date: 2023-11-27
Filing date: 2023-11-27
Publication date: 2025-05-28
Also published as: WO2025113997A1; GB202318049D0

Abstract

An optical system 100 comprises a light guide 110 with opposing first and second non-parallel optical surfaces 112, 114, wherein at least one of the surfaces 112, 114 is curved. An in-coupling diffractive optical element 120 receives reflected light from an object and couples the light into the guide at a first location, wherein the light propagates around the guide and exits the light guide at a second location. A light detector 140 is arranged proximal to the second location to receive the reflected light from the guide. The system may further comprise a light source that illuminates the eye of a user or a viewed scene, wherein the source is arranged to emit non-visible light, and the system may be present in a head mounted eye tracking system. Light rays exiting the guide may converge to an entrance pupil of the detector 140.

Description

OPTHALMIC LIGHT GUIDE SYSTEM

FIELD OF THE DISCLOSURE

The present disclosure relates to an optical system for use in eye tracking or world facing camera systems. The optical system may also form part of an augmented reality (AR), a virtual reality (VR) or mixed reality (MR) systems.

BACKGROUND OF THE DISCLOSURE

Eye tracking has a number of applications in user interface studies or optimisation, health, psychology and can be used to understand the behaviour of a user in response to stimuli, such as advertising. In the field of AR and VR, eye tracking systems can allow for user interface control and or helping to provide high quality images to a user.

Optical eye tracking systems for AR and VR applications can form part of a head mounted display and typically utilise a non-visible source (such as an IR light emitting diode (LED)) and an IR camera (such as a charged coupled device (CCD)) to capture an image of the eye.

Typically, both the LED and the CCD are mounted on a frame of the AR or VR device close to a user's eye. Machine vision algorithms can then determine the position of the eye based on a corneal reflection (also known as a glint or Purkinje image) of the light source from the eye onto the camera and the position of the pupil. The vector between the pupil centre and the glint determines the eye rotation and hence the gaze direction.

Typically, eye tracking systems utilise multiple light sources and camera around the user's eye within the limited space available of a head mounted device. Furthermore, for AR eye-tracking systems machine vision has a high computational power requirement even at low frame or refresh rates (typically 30-200Hz, a latency of 4-33ms). The machine vision computation is made more difficult by the off-axis view of the eye from the camera because of the asymmetric bias in the image of the user's eye.

There are several known examples of eye-tracking systems which comprise non-visible light sources, cameras, such as CCDs and light guide to capture images of a user's eye. Once such example utilises a transparent light guide in the form of a planar structure that guides IR light along the light guide and outcouples the IR light onto a user's eye. The outcoupled IR light is then coupled back into the light guide and is coupled into an IR light detector.

It is known to embed planar light guides, of the type described above, within eyeglass lenses for AR applications. However, there are limitations to embedding such light guides within lenses. Typically, lenses used in glasses are curved particularly where the lenses are prescription lenses, whereas the light guides are flat. Therefore, embedding light guides in curved lenses has the disadvantage that the thickness of lenses must be larger, and therefore heavier, to accommodate the flat light guide. Additionally, none of the known solutions discuss the problems associated with achieving the required refractive index change within an eyeglass lens to ensure that light can be guided along the light guide by total internal reflection.

In addition, optical systems such as head mounted world facing camera systems, one example of which is the commercially available Ray-Ban® Stories ®, which place a forward facing camera adjacent to each lens of a pair of glasses. However, because the location of the camera is offset from the user's eye there will be a parallax error between the scene viewed by a user and the captured scene. Whilst this may not be noticeable at long distances where scenes are essentially at essentially at infinity (greater than lm from the user), this parallax error will be noticeable at short distances (less than 1m from the user). This can be particularly problematic for virtual reality, augmented reality, or mixed reality systems where users interact with real and virtual objects at close distance viewed through the camera system.

SUMMARY OF THE DISCLOSURE

There is provided an optical system comprising: a light guide having opposing first and second non-parallel optical surfaces, wherein at least one of the first or second optical surfaces is curved; an in-coupling diffractive optical element arranged to receive reflected light from an object and couple the reflected light into the lightguide at a first location, wherein the reflected light propagates around the light guide and exits the light guide at a second location; and a light detector arranged proximal to the second location to receive the reflected light from the light guide.

The optical system may include in-coupling diffractive optical element which may comprises linear diffraction grating having a constant period. A centre thickness of the light guide may be selected such that light rays propagating through the lightguide without overlapping. The width of the in-coupling diffractive optical element may be less than four times the centre thickness of the light guide. The in-coupling diffractive optical element may comprise a lens function having a negative power (or focal length).

In the optical system according to embodiments light rays exiting the light guide may converge to an entrance pupil of the light detector. The centre thickness of the lightguide is in the range of 1mm to 10mm, and preferably 3mm. The detector may be tilted with respect to the angle at which light rays exit the light guide.

In the optical system according to embodiments the second location is an output and an outcoupler is arranged at the output. The outcoupler can vary the angle of the light rays exiting the lightguide. The outcoupler is a diffractive outcoupler, reflective outcoupler or refractive outcoupler.

The light detector may be a charge coupled device (CCD). A focusing lens may be arranged to focus light from the light guide on to the light detector. The focusing lens may be a combination of a cylindrical lens and a spherical lens.

The light guide according to embodiments may be a transparent ophthalmic lens and the first and second optical surfaces are optical surfaces of the ophthalmic lens.

A light source may arranged to illuminate the eye of a user or a viewed scene, and the light source is arranged to emit non-visible light. The light source may be a narrow band infrared light emitting diode or laser diode.

There is also provided a head mounted eye tracking system or a world facing camera system, comprising the optical system.

It is therefore and object of the embodiments disclosed herein to avoid or mitigate one or more of the disadvantages discussed above.

Against this background, there is provided an optical system for use in eye-tracking or world facing camera systems in accordance with the claims. Other preferred and optional features are defined in the other claims and discussed throughout this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the features of the present disclosure can be understood in detail, a more particular description is made with reference to embodiments, some of which are illustrated in the appended figures. It is to be noted, however, that the appended figures illustrate only typical embodiments and are therefore not to be considered limiting of its scope. The figures are for facilitating an understanding of the disclosure and thus are not necessarily drawn to scale. It should be noted that the features as illustrated in the figures have been exaggerated for illustration purposes and no dimensions (unless stated in the text or drawings) should be inferred. Advantages of the embodiments will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying figures, in which like reference numerals have been used to designate like elements, and in which: Figure 1 illustrates an eye tracking optical system utilising an eyeglass lens as a lightguide according to an embodiment; Figure 2 illustrates a world facing camera utilising an eyeglass lens as a light guide according to a further embodiment; Figure 3a illustrates optical systems according to embodiments without an outcoupler; Figure 3b illustrates propagation of light rays through the light guide in accordance with embodiments; and Figure 4 illustrates head mounted device comprising the optical system according to embodiments.

DETAILED DESCRIPTION

Figure 1 illustrates schematically the optical system 100 according to embodiments. The optical system 100 comprises a light guide (or wave guide) 110 with non-parallel opposing first and second major surfaces 112, 114. An optical incoupler 120 and an optional optical outcoupler 130 are arranged on the first major surface 112 to respectively couple light into, and out of the light guide 110. Whilst Figure 1, illustrates the optical incoupler 120 and the optical outcoupler 120 on the same side of the light guide 110, in this case the first major surface 112, the optical incoupler 120 and the optical outcoupler may be on opposing sides of the light guide 110, that is one on the first major surface 112 and the other on the second major surface 114. Furthermore, the one or both of the optical incoupler 120 or the optical outcoupler 130 may be embedded within the body of the light guide 110.

The optical incoupler 120 is arranged to collect light reflected from an object or scene, such as for example a user's eye 150, when the optical system 100 is in use. The optical outcoupler is arranged to direct light out of the light guide 110 to a light detector 140.

When in use, the optical incoupler 120 is arranged to collect light reflected from an object or scene, such as a user's eye 150. The reflected light is coupled into the light guide 110 such that the light can travel along the light guide 110 by total internal reflection. The optical incoupler 120 is arranged as a diffractive optical element (DOE) with a diverging (or concave) lens function. In other words, the optical incoupler comprises negative power to image the entire surface object and couple the image object into the light guide. Typically for optical eye tracking systems to function correctly it is preferable to image at least the white (Sclera), iris and pupil of the eye so that related software can determine rotation of the eye and thus determine the position of the eye. In this regard there is a trade-off between the thickness of the light guide 110 and the area that can be imaged. Thinner light guides are preferred so that the weight of an overall eye-tracking system is kept as low as possible which is required for head mounted optical systems. However, the thinner the light guide the smaller the area of the eye that can be imaged. In other words, the field of view (FOV) is limited by the thickness of the light guide. The FOV scales linearly and proportionally with the thickness of the light guide. Typically, the FOV is approximately equal to the thickness of the light guide. By utilising an optical incoupler comprising negative power, the field of view (FOV) can be increased, without increasing the thickness of the light guide 110 to image the white, iris and pupil so that related software can determine rotation and thus position of the eye. In other words, rays from the entire extent of the user's eye can be coupled into the light guide and propagated through the light guide, exiting the light guide to converge on at the detector, or the entrance pupil of the detector.

For thickness of the light guide is a balance between weight and robustness. Too thin and the light guide will be easily damaged. Too thick and the light guide 110 may be too heavy for all day wearable use. Typically, therefore the thickness of the light guide 110 will be between 1 and 10mm, and preferably 5mm. In the context of the present disclosure, the thickness of the light guide is measured as the centre thickness along the optical axis.

For a lightguide thickness of 3mm and incoupler width of 12mm (where width of the incoupler is defined as the net direction of light propagation along the light guide), an eye relief of 25mm to image the entire extent of a typical eyeball with diameter of approximately 24mm, the power of the negative lens function of the optical incoupler 120 should be -40 diopters (m-1) (or 1/0.025m). The width of the optical incoupler is less than or equal to four times the centre thickness of the light guide because the negative power of the optical incoupler demagnifies the image. As the eye relief increases the power of the negative lens function will decrease proportionally. In this context, eye relief is the distance from the surface of the user's eye to the optical incoupler 120 and can typically be between 10mm, for glasses, and 50mm, helmet visors, dependent on the type of head mounted system the optical system 100 is used. The field of view (FOV) due to the negative power of the incoupler 120 may be approximated by the following expression: FOV = arctan (Incoupler Width x Incoupler Power) .... (1) Where: Incoupler Width is defined in meters and Incoupler Power is defined in diopters.

The optical incoupler may be volume holographic material applied to the surface of the for light guide 110. For example, the incoupler may be formed on a photopolymer (for instance, Bayfol (RTM) as marketed by Covestro AG or a silver halide film), then attached onto the surface of the light guide conforming to the surface thereof. Alternatively, the optical incoupler 120 may be a surface relief grating or a Fresnel lens formed on the surface of the light guide 110 or embedded therein. Alternatively, the incoupler 120 may be formed from a holographic material, where the holographic material is selected from one of a photopolymer, silver halide, dichromated gelatin or SHSG. Where the optical incoupler 120 is a holographic, it may be a transmission or a reflection holographic incoupler. The incoupler 120 is advantageously positioned close to the lens optical axis.

As well as the negative power, which demagnifies the image the incoupler 120 also comprises a linear grating term to deflect light into the light guide 110 at an angle greater than the angle for total internal reflection to occur in the light guide 110 so that incoupled light can propagate around the light guide 110. The negative power of the optical incoupler 120 results in an increased field of view compared to an incoupler (of the same dimensions) without negative optical power. The negative power function has benefits, compared to increasing the dimensions of the incoupler, because increasing the field of view without negative power will result in overlapping images at the light detector. In other words, footprints of light rays at points of incidence with the surfaces of the light guide 110 do not substantially overlap within the light guide, which avoids overlapped images at the light detector 140. The overall phase function of the optical incoupler can be described by the following expression: Phase = Ax + By + Cx2 + Dy2.... (2) Where: x and y define coordinates on the incoupler A and B are coefficients defining the amount of light deflection in respective x and y directions; and C and D are the negative power coefficients in respective x and y directions When in use, the optional optical outcoupler 130 is arranged to output light that has propagated along the light guide 110, by total internal reflection, to the light detector 140. In this way an image of a user's eye 150 projected onto the light detector. The optical outcoupler 130 is arranged as a diffractive optical element (DOE) with a converging (or convex) lens component. Alternatively, the optical outcoupler 130 may comprise a linear diffraction term and an optional convex lens element 150 may be placed between the optical outcoupler 130 to project an image of a user's eye 150 onto the detector 140. As with the optical incoupler 120, the optical outcoupler 130 be volume holographic material applied to the surface of the for light guide 110. For example, the incoupler may be formed on a photopolymer (for instance, Bayfol (RTM) as marketed by Covestro AG or a silver halide film), then attached onto the surface of the light guide 110 conforming to a major surface thereof. Alternatively, the optical outcoupler 130 may be a surface relief grating or a Fresnel lens formed on the surface of the light guide 110 or embedded therein. The optical outcoupler 130 may be refractive or reflective element such as an optical facet or prism formed on a surface of, or embedded in, the lightguide 110 facing the light detector 140 to direct rays from the lightguide 110 into the light detector 140. However, following the discussion below in relation to Figure 3b, the skilled person will appreciate that a specific outcoupler is not essential. The outcoupler 130 may be formed from a holographic material, where the holographic material is selected from one of a photopolymer, silver halide, dichromated gelatin or SHSG.

The light guide 110 is formed of a transparent material which has a higher refractive index than the surrounding environment, such as air. The light guide 110 therefore guides light from the optical incoupler 120 to the optical outcoupler 130 by total internal refraction. The light guide 110 is also transparent to environmental light such that a user can view the external world through the light guide 110. In this way the light guide 110 acts as an ophthalmic lens and in this regard the skilled person will appreciate the light guide 110 is a curved light guide 110. That is first and/or second major interfaces 112, 114 will be appropriately curved dependent ophthalmic nature of the light guide as discussed below. The skilled person will also appreciate that one of the major surfaces may be planar and that the first and second major surfaces are not parallel.

The first major interface 112 may be considered to be the eye facing surface and the second major interface 114 may be considered to be the world facing surface. One or both of the first and second major interfaces 112, 114 of the light guide 110 may be curved in the same way that an ophthalmic lens may have respective curved surfaces. In this regard the light guide 110 may be a corrective (that is, with optical power) ophthalmic lens or a zero prescription (that is, without optical power) lens. For example, the first major interface 112 may have a radius of curvature of 240mm and the second major interface 114 may have a second radius of curvature of 120mm giving an optical power of the lightguide 110 of 2 diopters (m-1), such that the lens would be a positive lens. Similarly, the radius of curvature of the first and second major interfaces 112, 114 may be equal resulting in a zero power lens. Furthermore, the radii of curvature of the first and second major interfaces 112, 114 may be such that the optical power of the light guide 110 may be negative. The light guide may also incorporate bi-focal or varifocal lens functions. The skilled person will appreciate that the curved nature of the light guide 110, negative power of the incoupler 120 will cause distortion of the image, that is deviation from rectilinear projection, and that the distortion will be constant for a specific geometric configuration of the light guide 110. Whilst outside the scope of the present disclosure, any distortion may be corrected for using appropriate image processing algorithms. Such algorithms may advantageously carryout remapping of the image one example of which is known as warping. Remapping may be fixed such as applying a constant look-up table to the distorted image.

The thickness of the lightguide 110 is greater than a quarter of the width, w of the optical incoupler 120, where the width of the incoupler is the transverse direction across the surface of the light guide. In this way light rays from a specific point on the human eye will propagate through the light guide 110 without overlapping and the image of the user's eye will be relayed to the output of the lightguide 110. In this way the rays will propagate through the lightguide and exit the lightguide 110 such that they converge to the entrance pupil of the light detector 140.

The light detector 140 can be any appropriate light detector such as a CCD which is used to detect light which is reflected from the eye 150 and coupled out of the light guide 110. The light detector 140 will be selected to operate at the appropriate wavelength to detect the wavelength of light reflected from the eye, for example infrared, but any non-visible wavelength of light will be appropriate. Typically, eye tracking systems use infrared light. The light detector generates data corresponding the light reflected from the user's eye 150. This data is used by a computation system (not illustrated) to determine the position of the user's eye 150. The light detector 140 will be positioned relative to the light guide 110 at a suitable point where the light exits the light guide 110.

A focusing lens 160 may be placed between the outcoupler 130 and the light detector 140 to image the output of the light guide 110 on to the light detector. In other words, the image of the user's eye, as propagated along the light channel from the optical incoupler 120 to the optical outcoupler 130 by total internal refraction is imaged onto the light detector by the focusing lens 160. The focusing lens 160 may be formed on the surface of the light guide. Alternatively, it may be integrally formed with the light detector 140. The problem of astigmatism at the light detector 140 may be corrected for by using a combination of a spherical lens and a cylindrical lens as the focusing lens 160. Similarly, the detector may be tilted with respect to the optical axis of the detector. Or in other words, angled with respect to the normal of the central ray of the detector. Astigmatic correction and detector tilt improve the focus of the image across the field of view.

Where ambient light is used, a narrow band optical filter (not illustrated) may be incorporated into or onto the light detector 140 to filter out any unwanted light. This improves the contrast between image light and stray light.

As an alternative to or in addition to ambient light as a source of illumination of the eye one or more light sources (not illustrated) may be placed in proximity to the user's eye 150. The light source may be an LED operable to emit non-visible light, for example infrared light. The wavelength of operation of the LED will be matched to the wavelength of operation of the light detector. A narrowband optical filter may be arranged at the light detector to filter out any unwanted wavelengths of light, and this is particularly beneficial where illumination of the eye is achieved by ambient light. The skilled person will appreciate however that the optical filter will not be necessary where illumination of the eye is achieved by a narrow band light source, such as a narrow LED or laser diode, which is matched to the detection wavelength of the light detector.

With regard to the above discussion, the skilled person will understand that the arrangement of Figure 1 describing optical system utilising an eyeglass lens may be used in an optical eye tracking system. Similarly, and with reference to Figure 2, the skilled person will appreciate that the optical system may be used in a world facing camera system. The optical system 200, which is a world facing camera system, of Figure 2 comprises a light guide (or waveguide) 210 with opposing first and second major interfaces 212, 114. An optical incoupler 220 and an optional optical outcoupler 230 are arranged on the first major surface 212 to respectively couple light into, and out of the light guide 210. As with the arrangement of Figure 1, the incoupler and the outcoupler may be arranged on opposing surfaces or embedded in the body of the light guide. A light detector 240 is arranged to collect light coupled out of the light guide 210 by the optical outcoupler 230. The optical incoupler 220 is arranged to collect light reflected from an object or objects 252 (the viewed scene) visible to the user's eye 250 in the distance when the optical system 200 is in use. Typically, the object or objects will be the viewed scene at which the user is looking. In the present embodiment, the optical incoupler 220 is arranged on the first major surface 212 of the light guide 210, that is on the eye-side of the light guide 210. Light rays from the objects 252 are coupled into the light guide 210 by the optical incoupler 220, propagate around light guide 210 and exit the light guide 210 at the optical outcoupler 230. Light rays from the objects 252 will also pass through the light guide to the user eye 250 so that the scene can be viewed. As discussed above the light guide 210 may be in the form of an ophthalmic lens which may correct the user's vision so that the scene can be viewed. The optical outcoupler 230 couples the light rays into the light detector 240. In this way an image of the objects or scene as view by the user may be captured without a parallax error. As discussed below with regard to Figure 3b a specific optical outcoupler 230 is not essential as light may exit the light guide 210 and enter the light detector when the critical angle for total internal reflection within the light guide 210 is broken. For the arrangement of Figure 2, which is a world facing camera system, the negative power of the incoupler 220 such be large enough the capture a wide field of view. For example, a negative focal length of -12mm would give a field of view of approximately 60 degrees. The world facing camera system 200 may also include light sources, such as infra-red LEDs adapted to illuminate the world environment being viewed and may include time-of-flight detection.

Figure 3a illustrates an alternative arrangement to the optical system 100 of Figure 1. Similar to the optical system 100 of Figure 1, the optical system 300 of Figure 3a comprises a light guide (or waveguide 110) 310 with opposing first and second major interfaces 312, 314 and an optical incoupler 320 to couple light into the light guide 310. A light detector 340 is arranged to collect light coupled out of the light guide by the optical outcoupler 330. The optical incoupler 320 is arranged to collect light reflected from a user's eye 350 when the optical system 200 is in use. Unlike the optical systems 100, 200 of Figures 1 and 2, the optical system 100 of Figure 3a does not comprise a specific optical outcoupler. In the present arrangement of Figure 3a, the light detector 340 is placed at an angle with respect to the light guide 310. More specifically, the light detector 340 follows the angle of the ray (x-x) propagating through and then exiting the light guide 340. The angle of ray x-x will depend on the refractive index n of the light guide material, but the angle B, is greater than the critical angle of the lightguide 340. Specifically, the angle B is the angle of the ray with respect to a surface normal at the point of incidence with the surface of the light guide 110. In this regard therefore the skilled person will see therefore that a specific optical outcoupler is not required, but light can exit the light guide 340 when the angle of the light ray is greater than the critical angle of the light guide 340.

Figure 3b illustrates how light propagates through the light guide 110 and exits the light guide 110 at the exit point 132, without the need for a specific optical outcoupler. For clarity only a single light ray propagating through and exiting the light guide 110 is illustrated. A light ray X from an object (in the case a reflection from a user's eye) is coupled into the light guide 110 by the optical incoupler 120 at the first major interface 112. The optical incoupler 110 in-couples the light ray X at an incoupling angle Gi which is greater than the critical angle B. of the light guide 120 so that the light ray is internally reflected from the second major interface 114 at point A and due to Snell's Law, the light ray X will be reflected at an angle 0,1which is equal to the angle of incidence of the light ray X from the optical incoupler 120. The light ray Xis then incident at an angle B2 with respect to the surface normal at point B on the first major interface 112 and thus reflected toward, and incident at point C on the second major interface 114 at an angle B. The light ray X is then incident at an angle at at the exit point 132 and because the angle 04 is less than the critical angle 0. of the light guide 110 the light will exit the light guide 110. Because the first and second major interfaces 112 are not parallel the angle of reflection is reduced after each successive reflection until the angle of incidence is less than the critical angle of the light guide 110 and the light exits the light guide. This allows light to be coupled into the light guide and subsequently out coupled without the need for an output coupler.

In the foregoing all incident or reflection angles el and so on are given with respect to surface normal at the point of incidence or reflection at that surface.

For example, where the light guide 110 material is Trivex ®, with a refractive index n1= 1.53 at an air interface with a refractive index n2= 1, the critical angle O will be 40°. The optical incoupler 120 should therefore be arranged to in-couple light rays into the light guide 110 at an angle of greater than 40°. For each successive total internal reflection from the respective major interface the angle of incidence will decrease so that eventually light will exit from the lightguide 110 when the critical angle is broken. The number of reflections as the light propagates through the light guide 110 will depend on the in-coupling angle into the light guide 110 at the optical incoupler 120, the refractive index of the material and the variation in thickness of the light guide 110 from the incoupler to the point of exit. A mirrored surface 115 may be provided on the second major surface 114 of the light guide 110 to assist with reflection of the rays, at that point on the light guide, to the output of the light guide.

Whilst Figure 3b shows propagation of the light rays through and exiting of light from the light guide 100 this is also applicable to the arrangements of Figures 1, 2 or Figure 3a. The skilled person will also understand that for light guides 110, 210 with non-uniform distances between first and second major surfaces, multiple images or ghost images will occur due to light rays overlapping as they propagate around the light guide 110, 210. For the optical systems 100, 200 discussed herein, the negative power of the incoupler 120, 220 overcomes this problem.

Also, because the negative power of the incoupler 120, 220 increases the field of view, the width of the incoupler 120, 220 can be reduced when compared to an incoupler without negative power.

Figure 4 illustrates a head mounted 400 comprising at least one optical system 100, 200 as described above. The head mounted eye tracking system may form part of a wider head mounted display such as a pair of augmented reality glasses. As with known types of glasses, head mounted eye tracking system 400 includes a frame 402. The frame 402 includes arms 404, and lens mounting portions 406 connected by a bridge portion 408. One of the arms 404 includes a mounting portion 410 in which the light detector 140, 240 (discussed above) is fixedly mounted to collect light exiting the light guide 110, 210. Positioning the light detector 140, 240 in this way places it out of the field of view of the user. The light guide 110, 210 (as discussed above) can be mounted in one of the lens mounting portions 406 of the frame 402.

The skilled person will appreciate that the light guide 110, 220 will be mounted in the lens mounting portion adjacent to the light detector 140, 240. The skilled person will also appreciate that two light guides 110, 210 (of the type discussed above) may be mounted in the frame 402, one in each of the respective lens mounting portions 306. Where two light guides 110, 210 are utilised two light detectors 140, 240 will be appropriately mounted on the arms 404 of the frame 402.

The circumferential edges of the light guide 110, 210 may include an absorbing surface, such as a blackened coating to block stray light from entering the light guide 110, 210 and thus improve image contrast.

One or both of the arms 404 may also be adapted to house a battery (not illustrated) to power the light detector(s) 140, 240, illumination sources, and processing electronics (not illustrated). In addition, one or both of the arms 404 may also be adapted to house the processing electronics, where the processing electronics compute movement of the user's eye(s) 150, 250, and execute any algorithm to correct for distortion.

Similarly, for the world facing camera system of Figure 2, the skilled person will appreciate that the light guides 210 may be mounted in the lens mounting portions and the light detector may be placed at an appropriate forward facing position on the frame, at the junction of the arms lens mounting portions, for example. The skilled person will also appreciate that a light detector may be placed at each position with a corresponding light guide according to embodiments mounted in the lens mounting portions.

Particular and preferred aspects of the disclosure are set out in the accompanying independent claims. Combinations of features from the dependent and/or independent claims may be combined as appropriate and not merely as set out in the claims.

The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed disclosure or mitigate against any or all of the problems addressed by the present disclosure. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination.

The term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality. Reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS1. An optical system comprising: a light guide having opposing first and second non-parallel optical surfaces, wherein at least one of the first or second optical surfaces is curved; an in-coupling diffractive optical element arranged to receive reflected light from an object and couple the reflected light into the lightguide at a first location, wherein the reflected light propagates around the light guide and exits the light guide at a second location; and a light detector arranged proximal to the second location to receive the reflected light from the light guide.
2. The optical system of claim 1, wherein the in-coupling diffractive optical element comprises a linear diffraction grating having a constant period.
3. The optical system of claim 1, wherein a centre thickness of the light guide is such that light rays propagating through the lightguide without overlapping.
4. The optical system of claim 3, wherein the width of the in-coupling diffractive optical element is less than four times the centre thickness of the light guide.
5. The optical system of claim 1, wherein the wherein the in-coupling diffractive optical element comprises a lens function having a negative power.
6. The optical system of claim 1 to 5, wherein light rays exiting the light guide converge to an entrance pupil of the light detector.
7. The optical system of any preceding claim wherein the centre thickness of the lightguide is in the range of 1mm to 10mm, and preferably 3mm.
8. The optical system of any preceding claim, wherein the detector is tilted with respect to the angle at which light rays exit the light guide.
9. The optical system of claim 1, wherein the second location is an output and an outcoupler is arranged at the output.
10. The optical system of claim 9, wherein the outcoupler can vary the angle of the light rays exiting the lightguide.
11. The optical system of claim 9 or 10, wherein the outcoupler is a diffractive outcoupler, reflective outcoupler or refractive outcoupler.
12. The optical system of any preceding claim, wherein the light detector is a charge coupled device (CCD).
13. The optical system of any preceding claim, further comprising a focusing lens arranged to focus light from the light guide on to the light detector.
14. The optical system of claim 13, wherein the focusing lens is a combination of a cylindrical lens and a spherical lens.
15. The optical system of any preceding claim, wherein the light guide is a transparent ophthalmic lens and wherein the first and second optical surfaces are optical surfaces of the ophthalmic lens.
16. The optical system of any preceding claim, further comprising a light source arranged to illuminate the eye of a user or a viewed scene, wherein the light source is arranged to emit non-visible light.
17. The optical system of claim 16, wherein the light source is a narrow band infrared light emitting diode or laser diode.
18. A head mounted eye tracking system, comprising the optical system of any preceding claim.
19. A world facing camera system, comprising the optical system of any preceding claim.