Opthalmic lightguide system

Abstract

The present disclosure relates to an optical lightguide system comprising: a first lens part comprising an eye facing surface, a second lens part comprising a world facing surface, the first lens part affixed to a second lens part, with a cylindrically shaped interface therebetween; the first and second lens parts defining a lightguide wherein the eye facing surface and the world facing surface are non-parallel; an in-coupling diffractive optical element arranged between first and second lens parts at the cylindrically shaped interface to receive reflected light from the and object and couple the reflected light into to the lightguide such that light propagates around the lightguide to a second location and wherein the in-coupling diffractive optical element comprises a lens function having negative power.

Description

[0001] OPTHALMIC LIGHTGUIDE SYSTEM

[0002] FIELD OF THE DISCLOSURE

[0003] The present disclosure relates to an optical lightguide system suitable 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) eye tracking or camera systems.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] 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.

[0006] Optical eye tracking systems for AR or 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.

[0007] 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.

[0008] Typically, eye tracking systems utilise multiple light sources and cameras 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.

[0009] There are several known examples of eye-tracking systems which comprise non-visible light sources, cameras, such as CCDs and lightguide to capture images of a user’s eye. Once such example utilises a transparent lightguide in the form of a planar structure that guides IR light along the lightguide and outcouples the IR light onto a user’s eye. The outcoupled IR light is then coupled back into the lightguide and is coupled into an IR light detector.

[0010] It is known to embed planar lightguides, of the type described above, within eyeglass lenses for AR applications. However, there are limitations to embedding these lightguides within lenses. Typically, lenses used in glasses are curved, particularly where the lenses are prescription lenses, whereas the lightguides are flat. Therefore, embedding lightguides in curved lenses has the disadvantage that the thickness of lenses must be larger, and therefore heavier, to accommodate the flat lightguide.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 lightguide by total internal reflection.

[0011] 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 on the frame of the glasses 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 infinity (greater than 1 m from the user), this parallax error will be noticeable at short distances (less than 1 m 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.

[0012] SUMMARY OF THE DISCLOSURE

[0013] There is provided an optical lightguide system comprising: a first lens part comprising an eye facing surface, a second lens part comprising a world facing surface, the first lens part affixed to a second lens part, with a cylindrically shaped interface therebetween and wherein the first and second lens parts define a lightguide; an in-coupling diffractive optical element, comprising a lens function having negative power, arranged at a first location between first and second lens parts at the cylindrically shaped interface to receive reflected light from the object and couple the reflected light into to the lightguide at the first location such that light propagates around the lightguide to a second location.

[0014] Further optional aspects are set out in the dependent claims.

[0015] There is also provided a head mounted eye tracking system, comprising the optical lightguide system. There is also provided a world facing camera system, comprising the optical lightguide system.

[0016] It is therefore an 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 or optional features are defined in the other claims and discussed throughout this disclosure.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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 ordrawings) 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:

[0019] Figure 1 illustrates an eye tracking optical system utilising an ophthalmic lens formed of first and second parts as a lightguide according to an embodiment;

[0020] Figure 2 illustrates a world facing camera utilising an ophthalmic lens formed of first and second parts as a lightguide according to a further embodiment;

[0021] Figure 3a illustrates an optical system according to embodiments without an outcoupler; Figure 3b illustrates propagation of light rays through the lightguide in accordance with embodiments; and

[0022] Figure 4 illustrates head mounted device comprising the optical system according to embodiments.

[0023] DETAILED DESCRIPTION

[0024] Figure 1 illustrates schematically the optical system 100 according to embodiments. The optical system 100 comprises a lightguide (or waveguide) 110 which has opposing first 112 and second 114 major surfaces where the first 112 and second 114 major surfaces are respectively eye-side and world-side surfaces of the lightguide 110. The lightguide 110 is formed of a first lens part 111 and a second lens part 113. The first 111 and second 113 lens parts each comprise a cylindrical major surface opposite the respective first 112 and second 114 major surface. The first lens part 111 is affixed to the second lens part 113 so as to form a cylindrical interface 115 between the first 111 and second 113 lens parts. An optical incoupler 120 formed of a diffractive optical element is arranged between the first 111 and second 113 lens parts at the cylindrical interface 115, as described in GB Patent No. 2604894, in the name of TruLife Optics Limited, the contents of which are incorporated herein by reference. In this regard the in-coupling diffractive optical element may be considered to be a thin film based holographic optical element, where the thin film is for example a holographic photopolymer.

[0025] The optical incoupler 120, and an optional optical outcoupler 130, are arranged on the cylindrical interface 115 between the first and second 111 , 113 lens parts to respectively couple light into, and out of the lightguide 110. In this way the optical incoupler 120 and the optical outcoupler 130 may be embedded within the body of the lightguide 110. The optional optical outcoupler 130 may also be a film based holographic optical element. The first lens part 111 and a second lens part 113 may be bonded together with an adhesive material which as the same refractive index as the first and second lens parts 111 , 113 to ensure that total internal reflection does not occur at the cylindrical interface 115.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 130 is arranged to direct light out of the lightguide 110 to a light detector 140.

[0026] The reflected light from the object or scene is coupled into the lightguide 110 such that the light can travel along the lightguide 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 120 comprises negative power to image the entire surface of the object, such as a human eye, and couple the image of the object into the lightguide 110. 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 lightguide 110 and the area that can be imaged. Thinner lightguides 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 lightguide 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 lightguide. The FOV scales linearly and proportionally with the thickness of the lightguide. Typically, the FOV is approximately equal to the thickness of the lightguide. By utilising an optical incoupler 120 comprising negative power, the field of view (FOV) can be increased, without increasing the thickness of the lightguide 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 lightguide and propagated through the lightguide 110, exiting the lightguide 110 to converge on a detector 140, or the entrance pupil of the detector 140.

[0027] The thickness of the lightguide 110 is a balance between weight and robustness. Too thin and the lightguide 110 will be easily damaged. Too thick and the lightguide 110 may be too heavy for all day wearable use. Typically, therefore the thickness of the lightguide 110 will be between 1 and 10mm, and preferably 5mm. In the context of the present disclosure, the thickness of the lightguide 110 is measured as the centre thickness along the optical axis.

[0028] For a lightguide thickness of 3mm and incoupler width, W of 12mm (where width of the incoupler is defined as the net direction of light propagation along the lightguide), 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 (nr1) (or 1 / 0.025m). The width of the optical incoupler is less than or equal to four times the centre thickness of the lightguide 110 because the negative power of the optical incoupler 120 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:

[0029] FOV ~ arctan (Incoupler Width x Incoupler Power) .... (1)Where:

[0030] Incoupler Width is defined in meters and Incoupler Power is defined in diopters.

[0031] The optical incoupler 120 may be volume holographic material. For example, the optical incoupler 120 may be formed on a photopolymer (for instance, Bayfol (RTM) as marketed by Covestro AG or a silver halide film), then attached onto the cylindrical surface 115 of the lightguide and conforming to the surface thereof. Alternatively, the optical incoupler 120 may be a surface relief grating or a Fresnel lens. 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 lightguide 110 optical axis.

[0032] As well as the negative power, which demagnifies the image, the incoupler 120 also comprises a linear grating term to deflect light into the lightguide 110 at an angle greater than the angle for total internal reflection to occur in the lightguide 110 so that incoupled light can propagate around the lightguide 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 140. In other words, footprints of light rays at points of incidence with the surfaces of the lightguide 110 do not substantially overlap within the lightguide 110, which avoids overlapped images at the light detector 140. The overall phase function of the optical incoupler 120 can be described by the following expression:

[0033] Phase = Ax + By + Cx2+ Dy2.... (2)

[0034] Where: x and y define coordinates on the incoupler

[0035] A and Bare coefficients defining the amount of light deflection in respective x and / directions; and

[0036] C and D are the negative power coefficients in respective x and y directions

[0037] When in use, the optional optical outcoupler 130 is arranged to output light that has propagated along the lightguide 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 140. 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 160 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. For example, the incoupler may be formed on a photopolymer (for instance, Bayfol (RTM) as marketed by Covestro AG or a silver halide film). Alternatively, the optical outcoupler 130 may be a surface reliefgrating or a Fresnel lens. Yet further, 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.

[0038] The first and second lens parts 111 , 113 of lightguide 110 are formed of a transparent material which has a higher refractive index than the surrounding environment, such as air. The lightguide 110 therefore guides light from the optical incoupler 120 to the optical outcoupler 130 by total internal refraction. The lightguide 110 is also transparent to environmental light such that a user can view the external world through the lightguide 110. In this way the lightguide 110 can act as an ophthalmic lens and in this regard the skilled person will appreciate the lightguide 110 is a curved lightguide 110. That is first and / or second major interfaces 112, 114 will be appropriately curved dependent ophthalmic nature of the lightguide as discussed below. The skilled person will also appreciate that one of the major surfaces 112, 114 may be planar and that the first and second major surfaces 112, 114 may not be parallel.

[0039] 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 surfaces 112, 114 of the lightguide 110 may be curved in the same way that an ophthalmic lens may have respective curved surfaces. In this regard the lightguide 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 surface 112 may have a radius of curvature of 240mm and the second major surface 114 may have a second radius of curvature of 120mm giving an optical power of the lightguide 110 of 2 diopters (nr1), such that the lens would be a positive lens. Similarly, the radius of curvature of the first and second major surfaces 112, 114 may be equal resulting in a zero power lens. Furthermore, the radii of curvature of the first and second major surfaces 112, 114 may be such that the optical power of the lightguide 110 may be negative. The lightguide 110 may also incorporate bi-focal or varifocal lens functions. The skilled person will appreciate that the curved nature of the lightguide 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 lightguide 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.

[0040] 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 lightguide. In this way light rays from a specific point on the human eye will propagate through the lightguide 110 without overlapping and the image of the user’s eye will be relayed to the output of the lightguide 110. In thisway the rays will propagate through the lightguide 110 and exit the lightguide 110 such that they converge to the entrance pupil of the light detector 140.

[0041] 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 lightguide 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 may 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 lightguide 110 at a suitable point where the light exits the lightguide 110.

[0042] A focusing lens system 160 may be placed between the outcoupler 130 and the light detector 140 to image the output of the lightguide 110 on to the light detector. In other words, the image of the user’s eye, as propagated along the lightguide 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 lightguide. Alternatively, the focusing lens 160 may be integrally formed with the light detector 140.

[0043] 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 system 160. Similarly, the detector may be tilted with respect to the optical axis of the detector 140. 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.

[0044] Where ambient light is used to illuminate the eye 150, 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.

[0045] 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 140 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.

[0046] 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 200 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 lightguide (or waveguide) 210 with opposing first and second majorinterfaces 212, 114. As with the embodiment of Figure 1 , the lightguide 210 is formed of first and second 211 , 213 lens parts with a cylindrical interface 215 therebetween.

[0047] An optical incoupler 220 and an optional optical outcoupler 230 are arranged on the cylindrical interface 215 to respectively couple light into, and out of the lightguide 210. As with the arrangement of Figure 1 . A light detector 240 is arranged to collect light coupled out of the lightguide 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. Light rays from the objects 252 are coupled into the lightguide 210 by the optical incoupler 220, propagate around lightguide 210 and exit the lightguide 210 at the optical outcoupler 230. Light rays from the objects 252 will also pass through the lightguide 210 to the user eye 250 so that the scene can be viewed. As discussed above the lightguide 210 may be in the form of ophthalmic lens parts 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 lightguide 210 and enter the light detector when the critical angle for total internal reflection within the lightguide 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 lightguide (or waveguide 110) 310 and an optical incoupler 320 to couple light into the lightguide 310. A light detector 340 is arranged to collect light coupled out of the lightguide 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 arrangement of Figure 3a, the light detector 340 is placed at an angle with respect to the lightguide 310. More specifically, the light detector 340 follows the angle of the ray propagating through and then exiting the lightguide 310. The angle of rays will depend on the refractive index n of the lightguide material and is greater than the critical angle of the lightguide 340. Specifically, the angle is the angle of the ray with respect to a surface normal at the point of incidence with the surface of the lightguide 110. In this regard therefore the skilled person will see therefore, that a specific optical outcoupler is not required, but light can exit the lightguide 340 when the angle of the light ray is greater than the critical angle of the lightguide 340. A focusing lens system 360 may also be provided as described above.

[0048] Figure 3b illustrates how light propagates through the lightguide 110 and exits the lightguide 110 at the exit point 132, without the need for a specific optical outcoupler. For clarity only a single light ray X propagating through and exiting the lightguide 110 is illustrated. A light ray X from an object (in the casea reflection from a user’s eye) is coupled into the lightguide 110 by the optical incoupler 120. The optical incoupler 110 in-couples the light ray X at an in-coupling angle 0 which is greater than the critical angle of the lightguide 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 01 which is equal to the angle of incidence of the light ray X from the optical incoupler 120. The light ray X is then incident at an angle 02 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 03. The light ray X is then incident at an angle 04 at the exit point 132 and because the angle 04 is less than the critical angle of the lightguide 110 the light will exit the lightguide 110. Because the first and second major interfaces 112, 114 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 lightguide 110 and the light exits the lightguide 110. Indeed, one of first and second major interfaces 112, 114 may be curved and the other may be flat. Non-parallel surfaces allows light to be coupled into the lightguide and subsequently out coupled without the need for an output coupler. In the foregoing all incident or reflection angles 01 and so on are given with respect to surface normal at the point of incidence or reflection at that surface.

[0049] For example, where the lightguide 110 material is Trivex ®, with a refractive index n1= 1.53 at an air interface with a refractive index n2= 1 , the critical angle 0c will be 40°. As mentioned above, the skilled person will also understand that the adhesive material for bonding the lens parts will also have a refractive index matching that of Trivex®. The optical incoupler 120 should therefore be arranged to incouple light rays into the lightguide 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 lightguide 110 will depend on the in-coupling angle into the lightguide 110 at the optical incoupler 120, the refractive index of the material and the variation in thickness of the lightguide 110 from the incoupler to the point of exit. Optionally, a mirrored surface 117 may be provided on the second major surface 114 of the lightguide 110 to assist with reflection of the rays, at that point on the lightguide, to the output of the lightguide.

[0050] Whilst Figure 3b shows propagation of the light rays through and exiting of light from the lightguide 100 this is also applicable to the arrangements of Figures 1 , 2 or Figure 3a. The skilled person will also understand that for lightguides 110, 210 with non-uniform distances between first and second major surfaces (that is non-parallel), multiple images or ghost images will occur due to light rays overlapping as they propagate around the lightguide 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.

[0051] 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 406connected 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 lightguide 110, 210. Positioning the light detector 140, 240 in this way places it out of the field of view of the user. The lightguide 110, 210 (as discussed above) can be mounted in one of the lens-mounting portions 406 of the frame 402.

[0052] The skilled person will appreciate that the lightguide 110, 220 will be mounted in the lens mounting portion adjacent to the light detector 140, 240. The skilled person will 5 also appreciate that two lightguides 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 lightguides 110, 210 are utilised two light detectors 140, 240 will be appropriately mounted on the arms 404 of the frame 402.

[0053] The circumferential edges of the lightguide 110, 210 may include an absorbing surface, such as a blackened coating to block stray light from entering the lightguide 110, 210 and thus improve image contrast.

[0054] 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.

[0055] Similarly, for the world facing camera system of Figure 2, the skilled person will appreciate that the lightguides 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 lightguide 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.

[0056] 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 lightguide system comprising:a first lens part comprising an eye facing surface, a second lens part comprising a world facing surface, the first lens part affixed to a second lens part, with a cylindrically shaped interface therebetween and wherein the first and second lens parts define a lightguide;an in-coupling diffractive optical element, comprising a lens function having negative power, arranged at a first location between first and second lens parts at the cylindrically shaped interface to receive reflected light from the object and couple the reflected light into to the lightguide at the first location such that light propagates around the lightguide to a second location.

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 lightguide is such that light rays propagate 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 lightguide.

5. The optical system of any preceding claim wherein the centre thickness of the lightguide is in the range of 1 mm to 10mm.

6. The optical system of any preceding claim wherein a light detector arranged proximal to the second location to receive the reflected light from the lightguide.

7. The optical system of claim 6, wherein the light rays exiting the lightguide converge to an entrance pupil of the light detector.

8. The optical system of claims 6 and 7, wherein the detector is tilted with respect to the angle at which light rays exit the lightguide.

9. The optical system of claims 6 to 8, wherein the light detector is a charge coupled device (CCD).

10. The optical system of claims 6 to 9, further comprising a focusing system arranged to focus light from the lightguide on to the light detector.11 . The optical system of claim 10, wherein the focusing system is a combination of a cylindrical lens and a spherical lens.

12. The optical system of any preceding claim, wherein the lightguide is a transparent ophthalmic lens and wherein the world facing and eye-facing surfaces are optical surfaces of the ophthalmic lens and the eye facing surface and the world facing surface are curved.

13. The optical system of any preceding claim, further comprising a light source, wherein the light source is arranged to emit non-visible light.

14. The optical system of claim 13, wherein the light source is a narrow band infrared light emitting diode or laser diode.

15. A head mounted eye tracking system, comprising the optical system of any preceding claim.

16. A world facing camera system, comprising the optical system of any preceding claim.

Images