Patent Publication Number: US-2019191992-A1

Title: Eye Monitoring Apparatus

Description:
TECHNOLOGICAL FIELD 
     Examples of the disclosure relate to an eye monitoring apparatus. In examples of the disclosure images may be projected onto the user&#39;s eye to enable the eye to be monitored. 
     BACKGROUND 
     Apparatus for monitoring a user&#39;s eye can be used to obtain information about the condition of the user&#39;s eye. These could be used to detect changes in the shape of the eye or the presence of any abrasions or other irregularities within the eye. 
     BRIEF SUMMARY 
     According to various, but not necessarily all, examples of the disclosure there is provided an, apparatus comprising: an image source; an optical arrangement comprising at least one light guide and an exit pupil where the exit pupil is arranged to be positioned proximate to an eye of a user and the at least one light guide is arranged to convey a beam of light from the image source to the exit pupil to enable an image from the image source to be projected onto the user&#39;s eye; a detector arranged to detect the image projected onto the user&#39;s eye; and means for enabling the detected image to be analysed so as to identify changes in the projected image. 
     The means for enabling the detected image to be analysed may comprise image processing circuitry. 
     The means for enabling the detected image to be analysed may comprise one or more transceivers arranged to transmit the detected image to a remote device to enable the remote device to analyse the image. 
     The apparatus may be provided in a wearable device. 
     The image source may comprise a light source. 
     The image source may comprise a plurality of collimated light sources. 
     The image source may comprise at least one display. The display may be curved. 
     The image provided by the image source may comprise an array of objects. 
     The light guide may comprise one or more diffraction gratings. The light guide and the one or more diffraction gratings may provide an exit pupil expander. 
     The optical arrangement may be arranged to duplicate the image provided by the image source so that a plurality of duplicated images are projected onto the user&#39;s eye. 
     The diffraction gratings in the light guide element may be arranged to cause duplication of the image. 
     The detector may comprise an image capturing device configured to capture an image of the user&#39;s eye and the image projected onto the user&#39;s eye. 
     The detector may comprise a light source configured to illuminate the user&#39;s eye using infrared light. 
     The apparatus may comprise means for using the image detected by the detector to determine the position of the user&#39;s pupil. 
     The optical arrangement may comprise adjustment means arranged to adjust the position of the projected image to be aligned with the user&#39;s pupil. 
     According to various, but not necessarily all, examples of the disclosure there is provided an, apparatus comprising: an image source; an optical arrangement comprising at least one light guide and an exit pupil where the exit pupil is arranged to be positioned proximate to an eye of a user and the at least one light guide is arranged to convey a beam of light from the image source to the exit pupil to enable an image from the image source to be projected onto the user&#39;s eye; a detector arranged to detect the image projected onto the user&#39;s eye and enable the detected image to be analysed so as to identify changes in the projected image. 
     The apparatus may comprise image processing circuitry for analysing the detected image. 
     The apparatus may comprise one or more transceivers arranged to transmit the detected image to a remote device to enable the remote device to analyse the image. 
     According to various, but not necessarily all, examples of the disclosure there is provided a method comprising: providing an image from an image source; conveying a beam of light through an optical arrangement to enable an image from the image source to be projected onto the user&#39;s eye wherein the optical arrangement comprises at least one light guide and an exit pupil where the exit pupil is arranged to be positioned proximate to an eye of a user and the at least one light guide is arranged to convey a beam of light from the image source to the exit pupil; detecting the image projected onto the user&#39;s eye; and enabling the detected image to be analysed so as to identify changes in the projected image. 
     According to various, but not necessarily all, embodiments of the invention there is provided examples as claimed in the appended claims. 
    
    
     
       BRIEF DESCRIPTION 
       For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which: 
         FIG. 1  illustrates an apparatus; 
         FIG. 2  illustrates an apparatus; 
         FIGS. 3A to 3D  illustrate an example apparatus and image; 
         FIGS. 4A to 4D  illustrate an example apparatus and image; 
         FIGS. 5A to 5D  illustrate an example apparatus and image; 
         FIGS. 6A to 6D  illustrate an example apparatus and image; 
         FIGS. 7A to 7D  illustrate an example apparatus and image; 
         FIGS. 8A to 8D  illustrate an example apparatus and image; 
         FIGS. 9A to 9D  illustrate example images; 
         FIGS. 10A and 10B  illustrate images projected onto a users eye; 
         FIG. 11  shows light beams required for coverage of the user&#39;s cornea 
         FIG. 12  shows the coverage of the user&#39;s cornea obtained with a narrow light beam; 
         FIGS. 13A to 13C  show apparatus arranged to obtain extended coverage of the cornea of the user&#39;s eye; 
         FIGS. 14A to 14C  show diffraction gratings arranged to obtain extended coverage of the cornea of the user&#39;s eye; and 
         FIG. 15  illustrates an example method. 
     
    
    
     DETAILED DESCRIPTION 
     Examples of the disclosure relate to an apparatus  1  for monitoring a user&#39;s eye  11 . The apparatus  1  uses an optical arrangement  5  comprising at least one light guide  13  and an exit pupil  15  to enable an image  31  to be projected onto the user&#39;s eye  11 . Any distortion or other changes in the image  31  may be indicative of an irregularity in the user&#39;s eye  11 . As the optical arrangement  5  comprises a light guide  13  and an exit pupil  15  this provides for a light weight apparatus  1  which could be integrated into a wearable device such as a near eye display. Also the use of the diffractive optical elements such as the light guide  13  means that there is no need to shine light directly into the user&#39;s eye  11 . This provides for an apparatus  1  which is more convenient for a user  33  to use and may enable the user&#39;s eye  11  to be monitored over longer periods of time. 
       FIG. 1  schematically illustrates an apparatus  1  according to examples of the disclosure. The apparatus  1  comprises an image source  3 , an optical arrangement  5 , a detector  7  and analysis means  9  for enabling the detected image to be analysed. It is to be appreciated that the apparatus  1  may comprise additional components in some examples of the disclosure. For instance, the apparatus  1  could comprise additional optical elements which are not shown in  FIG. 1 . 
     The image source  3  may comprise any means for providing an image. In some examples the image source  3  comprises one or more light sources. The one or more light sources could comprise one or more light emitting diodes or any suitable light sources. The light source could be arranged to provide light of a single wavelength or a range of different wavelengths. 
     In some examples the image source  3  comprises a plurality of collimated light sources  34 . The plurality of collimated light sources maybe arranged so as to produce a particular image. For example the plurality of collimated light sources may be arranged in a curved line or in any other suitable arrangement. An array of lenses  38  or other optical elements may be provided to focus the light from the collimated light sources. 
     In some examples the image source  3  may comprise means for providing structured light. The structured light may comprise a beam of light which comprises light and dark fringes. The light and dark fringes could comprise different intensities of light. The darker fringes may have a lower intensity compared to the light fringes so that less light is provided within the dark fringes than the light fringes. In some examples the dark fringes could be arranged so that none, or very few, of the photons from the beam of light fall within the dark fringes. In such examples the intensity of the light in the dark fringes could be zero or close to zero. The intensity of the light in the light fringes may be much higher than the intensity of light in the dark fringes. The light and dark fringes may be arranged in lines, such as parallel lines or in any other suitable pattern. 
     The means for providing the structured light could comprise a light source and a mask, or other optical element, which enables a pattern to be projected onto the beam of light provided by the light source. The pattern within the structured light will deform when it is incident on an object such as a user&#39;s eye  11 . The deformation of the pattern can be used to determine the shape of the user&#39;s eye  11  or any other irregularities within the user&#39;s eye  11 . 
     In some examples the image source  3  may comprise one or more displays  51 . The displays  51  could be LED displays or any other suitable type of displays. The display  51  may be controlled by controlling circuitry. The controlling circuitry may enable different images to be displayed on the display  51 . This may enable different images  31  to be projected onto the user&#39;s eye  11 . This may enable different conditions of the user&#39;s eye  11  to be identified. For example, different images  31  may be arranged to make different types of deformations to be easier detect. This may enable different types of deformations, and the different conditions which cause the deformation to be detected. For instance, an abrasion of the cornea might be more easily detected with a coloured image  31  while a change in shape of the user&#39;s eye  11  could be detected with a black and white image  31 . 
     The image  31  provided by the image source  3  may comprise any suitable image which can be conveyed through the optical arrangement  5  and projected onto the users eye  11 . The image  31  may be arranged so that changes in the image  31  projected onto the user&#39;s eye  11  can be detected by one or more detectors  7 . The changes in the image  31  could comprise a distortion of the image  31 . This distortion could be detected as a change in position of one or more elements within the image  31  or a change in the separation between one or more elements within the image  31 . In some examples the change in the image  31  could comprise a change in the colour of the projected image  31 . For example, if there is an abrasion on the surface of the user&#39;s eye  11  this may change the colour of projected image  31  in the region of the abrasion. Example images  31  that could be used in examples of the disclosure are shown in  FIGS. 9A to 9D . 
     The optical arrangement  5  is coupled to the image source  3  so that a beam of light  17  provided by the image source  3  is coupled into the optical arrangement  5 . The optical arrangement  5  may comprise any means for conveying a beam of light  17  from the image source  3  to the users eye  11 . The optical arrangement  5  therefore enables an image  31  from the image source  3  to be provided to the user&#39;s eye  11 . 
     In examples of the disclosure the optical arrangement  5  may comprise one or more diffractive optical elements. The diffractive optical elements may be arranged to diffract the beam of light  17  from the image source  3  towards the user&#39;s eye  11 . 
     The optical arrangement  5  may comprise at least one light guide  13  and an exit pupil  15 . The optical arrangement  5  may be arranged within the apparatus  1  so that, when the apparatus  1  is in use, the exit pupil  15  is arranged to be positioned proximate to a user&#39;s eye  11 . The light guide  13  is arranged to convey a beam of light  17  from the image source  3  to the exit pupil  15  to enable an image from the image source  3  to be projected onto the user&#39;s eye  11 . The optical arrangement  5  may be controlled so that the image  31  is projected onto the cornea of the users eye  11  or onto any other suitable part of the users eye  11 . 
     In some examples a plurality of light guides  13  may be provided within the optical arrangement  5 . In some examples two or more light guides  13  may be arranged to enable images  31  from a single image source  3  to be provided to different eyes  11 . In other examples a plurality of light guides  13  could be arranged to enable an image  31  from a single image source  3  to be provided to a single eye  11 . 
     The optical arrangement  5  may be arranged so to increase the area of the user&#39;s eye  11  which is covered by the projected image  39 . For example the optical arrangement  5  maybe arranged to expand or duplicate the image  31  to increase the area of the image before it is projected onto the user&#39;s eye  11 . 
     The optical arrangement  5  may be arranged to project the image onto a particular part of the user&#39;s eye  11 . For example, the optical arrangement maybe arranged to project the image  31  onto the cornea or other part of the user&#39;s eye  11 . In some examples the apparatus  1  may comprise means for determining the position of the pupil, or other part of the user&#39;s eye  11  and means for aligning the exit pupil  15  with the pupil or other part of the user&#39;s eye  11 . For example the apparatus  1  may comprise gaze tracking means which can detect the position of the pupil and the optical arrangement  5  may be adjustable so that the position of the exit pupil  15  can be aligned with the position of the pupil. 
     The detector  7  may comprise any means which may be arranged to detect the image  31  projected onto the user&#39;s eye  11 . The detector  7  may be positioned within the apparatus  1  so that it can detect the images  31  projected onto the user&#39;s eye  11 . In some examples the detector  7  may comprise an image  31  capturing device which is configured to capture an image of the user&#39;s eye  11  and the image projected onto the user&#39;s eye  11 . 
     In some examples the detector  7  may comprise a detecting light source which may be arranged to illuminate the user&#39;s eye  11 . The detecting light source may be provided in addition to a light source within the image source  3 . The detecting light source may have a different wavelength of light to the light source  3  which provides the image  21 . In some examples the detecting light source may use infrared light. 
     In some examples the apparatus  1  could be arranged so that the image detected by the detector  7  could be used to determine the position of the user&#39;s pupil or other part of the user&#39;s eye  11 . In such examples the apparatus  1  may comprise image processing circuitry or any other suitable means which may be arranged to analyse the image and determine the position of the user&#39;s pupil or other part of the eye  11 . Once the position of the pupil or other part of the eye  11  has been determined this information may be used to adjust the optical arrangement  5  so that the exit pupil  15  is aligned with the pupil, or other part of the user&#39;s eye  11 . 
     The detector  7  is coupled to the analysis means  9  so that the images detected by the detector  7  are provided to the analysis means  9 . 
     In some examples the analysis means  9  comprises image processing circuitry. The image processing circuitry may be provided within the apparatus  1 . In some examples the analysis means  9  may comprise one or more transceivers arranged to transmit the detected image to a remote device to enable image processing circuitry within the remote device to analyse the image. In such examples the apparatus  1  might not comprise any image processing circuitry or might only comprise limited image processing circuitry. 
     The image processing circuitry, whether it is contained within the apparatus  1  or within a remote device, may be arranged to analyse the detected image to determine if there is any distortion or other types of changes between the image  31  projected onto the user&#39;s eye  11  and the image  31  detected by the detector  7 . The distortion or other changes in the image  31  could be an indication that there is an irregularity in the user&#39;s eye  11 . For example, a distortion of the image  31  could provide an indication that the shape of the user&#39;s eye  11  is irregular. A change in the colour of the image  31  could provide an indication that there is an abrasion or other irregularity within the user&#39;s eye  11 . If the image processing circuitry determines that there is a change in the image  31  then information about the change could then be provided to the user  33  or to a medical practitioner to enable further tests on the condition of the user&#39;s eye  11  to be carried out. 
     In examples of the disclosure the apparatus  1  may enable the user&#39;s eye  11  to be monitored over periods of time. For example the apparatus  1  could be worn by the user  33  for a continuous period of time. In some examples the apparatus  1  could be used by the same user  33  at intervals over a period of time to enable changes in their eye  11  to be monitored. The intervals could be days, weeks or months so as to enable long term conditions of the user&#39;s eye  11  to be monitored. In such examples the images  31  that are detected by the detector  7  could be stored and used so that images  31  obtained at different times may be compared with each other. 
       FIG. 2  illustrates an example apparatus  1  being used by a user  33 . The example apparatus  1  comprises an image source  3 , an optical arrangement  5 , a detector  7  and analysis means  9 . 
     In the example of  FIG. 2  the apparatus  1  is provided within a wearable device  21  such as a head mounted display. The apparatus  1  is provided within the wearable device  21  so that the exit pupil  15  of the optical arrangement  5  is positioned adjacent to the user&#39;s eye  11  when the user  33  is wearing the wearable device  21 . 
     In the example apparatus  1  of  FIG. 2  the image source  3  is provided to the side of the user&#39;s eye  11 . The image source  3  is positioned in the wearable device  21  so that it as adjacent to the user&#39;s nose. The light guides  13  within the optical arrangement  5  are arranged to convey the beam of light  17  provided by the image source  3  laterally toward the user&#39;s eye  11 . This means that there is no need for a beam of light  17  to be shone directly into the user&#39;s eye  11 . 
     The detector  7  is positioned within the wearable device  21  so that the detector  7  is positioned in front of the user&#39;s eye  11 . One or more optical elements  25  may be provided within the apparatus  1  to direct images of the user&#39;s eye  11  onto to the detector  7 . The detector  7  may be positioned on the opposite side of the optical arrangement  5  so that the images captured by the detector  7  pass through one or more elements of the optical arrangement  5  before they are detected by the detector  7 . 
     In the example of  FIG. 2  the detector  7  comprises an image capturing device. The image capturing device may be an infrared camera or any other suitable type of camera. The image capturing device may comprise a CMOS (complementary metal-oxide-semiconductor) sensor, a CCD (charge coupled device) sensor, or any other suitable type of sensor. 
     In the example of  FIG. 2  the detector  7  is mounted on a circuit board  23 . Other components of the apparatus  1  may also be mounted on the same circuit board  23 . For example, the analysis means  9  may also be provided on the circuit board  23 . The analysis means  9  could comprise image processing circuitry and/or one or more transceivers as described in this description. 
     In the example of  FIG. 2  the apparatus  1  is provided within the wearable device  21 . In other examples other means could be used to attach the apparatus  1  to a wearable device  21 . For example the apparatus  1  could be a separate attachment which could be attached to a user&#39;s glasses or sunglasses. In some examples the apparatus  1  could be a separate attachment which could be attached to a virtual or augmented reality headset or other head mounted display. 
       FIGS. 3A to 3D  illustrate an example apparatus  1  and projected image  31  that may be used in some examples of the disclosure.  FIG. 3A  shows a front view of the apparatus  1 ,  FIG. 3B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 3C  shows the image source  3  in more detail and  FIG. 3D  shows the image  31  projected by the image source  3 . 
     In the examples of  FIGS. 3A to 3D  the apparatus  1  is provided within a wearable device  21 . The wearable device  21  is configured to be worn on the head of the user  33 . In the example of  FIGS. 3A to 3D  the wearable device  21  comprises a head mounted display which enables the exit pupil  15  of the optical arrangement  5  to be positioned adjacent to the user&#39;s eye  11 . Other types of wearable device  21  which enable the exit pupil  15  to be positioned adjacent to the user&#39;s eye  11  may be used in other examples of the disclosure. 
     In  FIG. 3B  the apparatus  1  is illustrated positioned adjacent to the head of the user  33  as it may be arranged during use. The cross section of the face of the user  33  is illustrated to provide an indication of how the apparatus  1  may be positioned in use. 
     In the examples of  FIGS. 3A to 3D  the light guides  13  within the optical arrangement  5  comprise an exit pupil expander  35 . The exit pupil expander  35  may comprise means for increasing the size of the exit pupil  15  of the optical arrangement  5 . The exit pupil expander  35  may be configured to increase the size of the exit pupil  15  of the optical arrangement  5  so that the exit pupil  15  of the optical arrangement  5  may be larger than the exit pupil of the image source  3 . This may increase the size of the image before it is projected onto the user&#39;s eye  11 . It is to be appreciated that other types of light guide elements  13  could be used in other examples of the disclosure. 
     The exit pupil expander  35  may comprise one or more substrates  36 . The substrates  36  of the exit pupil expander  35  comprise a first diffraction grating  37  and a second diffraction grating  39 . The diffraction gratings  37 ,  39  may comprise any suitable periodic structure. The first diffraction grating  37  may be an in-coupling grating which couples an incident beam of light  17  into the substrate  35 . The incident beam of light  17  may be provided by the image source  3 . The second diffraction grating  39  may be an out-coupling grating which couples the beam of light out of the substrate  36 . The second diffraction grating  39  is positioned so that the out-coupled beam of light is incident on the user&#39;s eye  11 . This enables the image provided by the image source  3  to be projected onto the user&#39;s eye  11 . 
     The first diffraction grating  37  and the second diffraction grating  39  may have different sizes. In some examples the second diffraction grating  39  may be larger than the first diffraction grating  37  such that it covers a larger area of the surface of the substrate  35 . This may cause expansion of the exit pupil of the beam of light  17 . The sizes and period of the diffraction gratings  37 ,  39  may be selected to enable a focused image to be projected onto the user&#39;s eye  11 . 
     The thickness of the substrate  36  which is used may be dependent upon the width of the diffraction gratings  37 ,  39  which are used, the wavelength of the light provided by the image source  3 , the proximity of the light guide  13  to the user&#39;s eye  11  and any other suitable factor. The thickness of the substrate  36  that is used in the light guide  13  may be thinner than the thickness of the substrates used for exit pupil expanders in near eye displays which may be used for virtual or augmented reality applications. This may increase the number of internal reflections of the beam of light  17  within the substrate  36  which may decrease the quality of the image that is projected onto the user&#39;s eye  11 . However the image quality required for monitoring the user&#39;s eye  11  may be lower than the image quality required for augmented or virtual reality applications. This may enable a thinner and/or more light weight apparatus  1  to be used to monitor the user&#39;s eyes  11 . 
     In the example embodiment of  FIGS. 3A to 3D  the first diffraction grating  37  is arranged so that a beam of light  17  from the image source  3  is incident on the first diffraction grating  37 . The first diffraction grating  37  couples the incident beam of light  17  into the substrate  36 . The in-coupled beam of light propagates through the substrate  36 . The exit pupil expander  35  is configured to enable total internal reflection of the in-coupled beam of light. This means that the in-coupled beam of light remains inside the substrate  36  until it is incident upon the second diffraction grating  39 . 
     The second diffraction grating  39  may be configured to diffract the in-coupled beam of light out of the substrate  36  to provide an out-coupled beam of light. As the light guide  13  comprises an exit pupil expander  35  the out-coupled beam of light may have a larger exit pupil  15  than the beam of light  17  which is provided by the image source  3 . 
     The image source  3  is positioned within the apparatus  1  so that, when the user  33  is wearing the apparatus  1  the image source  3  is positioned to the side of the user&#39;s eye  11 . This means that any light provided by the image source  3  is not shone directly into the user&#39;s eye  11 . In the examples of  FIGS. 3A to 3D  the image source  3  is positioned in the center of the apparatus  1 . In the example of  FIGS. 3A to 3D  the image source  3  is positioned within the apparatus  1  so that when the user  33  is wearing the apparatus  1  the image source  3  is positioned over the user&#39;s nose or adjacent to the user&#39;s nose. 
     In the example of  FIGS. 3A to 3D  the image source  3  comprises a plurality of collimated light sources  34 . In the example of  FIG. 3A  each of the light sources  34  is identical so that each of the light sources  34  are the same size and provide light the same wavelength and the same intensity. 
     In the example of  FIGS. 3A to 3D  the image source  3  also comprises a plurality of lenses  38  positioned between the light sources  34  and the first diffraction grating  37  of the optical arrangement  5 . The lenses  38  may be arranged to focus the light from the plurality of collimated light sources  34  onto the first diffraction grating  37  so that the light from the light sources  34  is coupled into the exit pupil expander  35 . 
     In the examples of  FIGS. 3A to 3D  the plurality of light sources  34  are provided in a curved array so as to provide a curved image. The curvature of the array may correspond to the curvature of the user&#39;s eye  11  so that when the image  31  is projected onto the user&#39;s eye  11  any irregularities in the shape of the user&#39;s eye  11  may be detected. 
     In the example of  FIGS. 3A to 3D  the curved array comprises an arc which spans an angle of 30 degrees. The curved array could span a different angle in other examples of the disclosure. 
     In the example of  FIGS. 3A to 3D  the collimated light sources  34  are provided in a line so that the image  31  provided by the image source  3  comprises a single bright line  32  corresponding to the plurality of light sources  34  as shown in  FIG. 3D . This image could be used to detect any changes in the shape of the user&#39;s eye  11 . For example, if the user&#39;s eye  11  has an irregular shape this would distort the line  32  when the image  31  is projected onto the user&#39;s eye  11 . This distortion could be detected by the detector  7  and the analysis means  9  which are not shown in  FIGS. 3A to 3D . 
     In the example apparatus of  FIGS. 3A to 3D  the apparatus  1  comprises a single image source  3 . The optical arrangement  5  is arranged so that the image  31  provided by the image source  3  is conveyed to both of the user&#39;s eyes  11 . In this example the image  31  that is provided to each of the eyes  11  may be identical. In other examples of the disclosure more than one image source  3  could be provided.  FIGS. 4A and 4D  illustrate another example embodiment in which two image sources  3  are provided.  FIG. 4A  shows a front view of the apparatus  1 ,  FIG. 4B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 4C  shows one of the image sources  3  in more detail and  FIG. 4D  shows an image  31  projected by one of the image sources  3 . 
     The apparatus  1  in the examples of  FIGS. 4A to 4D  is similar to the apparatus  1  shown in the examples of  FIGS. 3A to 3D  and corresponding reference numerals are used for corresponding components. The apparatus  1  in the examples of  FIGS. 4A to 4D  differs from that of the examples of  FIGS. 3A to 3D  because in the example of  FIGS. 4A to 4D  the apparatus  1  comprises two image sources  3  whereas in the example of  FIGS. 3A to 3D  the apparatus  1  only comprises a single image source  3 . Also in the example of  FIGS. 4A to 4D  the optical arrangement  5  comprises two first diffraction gratings  37  so that a first diffraction grating  37  is provided for each of the image sources  3 . Each of the first diffraction gratings  37  are positioned within the apparatus  1  so that a beam of light  17  from an image source  3  is incident on the first diffraction grating  37 . 
     In the example of  FIGS. 4A to 4D  both of the image sources  3  comprises a plurality of collimated light sources  34 . The plurality of collimated light sources  34  may be as shown in  FIGS. 3A to 3D  to provide an image  31  comprising a single line  32 . In the example of  FIGS. 4A to 4D  the two image sources  3  are identical so that two identical images  31  are provided. In other examples the image sources  3  could be different so that different images can be provided to each of the user&#39;s eyes  11 . 
       FIGS. 5A to 5D  illustrate another example embodiment in which a different type of image source  3  is used.  FIG. 5A  shows a front view of the apparatus  1 ,  FIG. 5B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 5C  shows the image sources  3  in more detail and  FIG. 5D  shows an example image  31  which could be projected by the image sources  3 . 
     The optical arrangement  5  in the examples of  FIGS. 5A to 5D  is similar to the optical arrangement  5  shown in the examples of  FIGS. 3A to 3D  and corresponding reference numerals are used for corresponding features. The apparatus  1  in the examples of  FIGS. 5A to 5D  differs from that of the examples of  FIGS. 3A to 3D  because in the example of  FIGS. 5A to 5D  the image source  3  comprises a display  51  rather than the plurality of collimated light sources  34 . 
     The display  51  used in the image source  3  may be any suitable type of display such as an OLED (organic light emitting diode) display. The display  51  may be a curved display so as to provide a curved image. The curvature of the display  51  may correspond to the curvature of the user&#39;s eye  11  so that when the curved image generated by the display  51  is projected onto the user&#39;s eye  11  any irregularities in the shape of the user&#39;s eye  11  may be detected. 
     In the example of  FIGS. 5A to 5D  the curved display  51  comprises an arc which spans an angle of 30 degrees. The curved display  51  could span a different angle in other examples of the disclosure. 
     The image source  3  also comprises an array of collimating lenses  53 . The collimating lenses  53  are provided overlaying the display  51  and arranged to focus the image  31  from the display  51  onto the first diffraction grating  37  of the optical arrangement  5 . 
       FIG. 5D  illustrates an example image  31  that may be provided by the display  51 . The example image comprises an array  55  of objects. The objects may comprise two dimensional shapes. Each of the objects comprises a different two dimensional shape in the example of  FIG. 5D . In other examples two or more of the same shape could be repeated within the array  55 . In examples of  FIG. 5D  each of the objects is a single colour. In other examples the objects could be multicolored. 
     The array  55  may be arranged so that when the image  31  is projected onto the user&#39;s eye  11  the spacing between the objects within the array  55  may be used to determine if there has been any distortion in the image  31  as detected by the detector  7 . In the example of  FIG. 5D  the array  55  comprises a plurality of objects arranged in a plurality of columns and rows. In the example of  FIG. 5D  the columns are perpendicular to the rows. Other arrangements for the array  55  may be used in other examples of the disclosure. The arrangement of the objects within the array  55  may depend on the optical arrangement  5  or any other suitable factors. 
     In the example array  55  in  FIG. 5D  each of the objects are different. That is, there is no repeated object within the image  31 . In other examples some of the objects may be replicated at different positions within the array  55 . 
     The image  31  may be provided in two colours so that the objects are provided in a first colour and the background may be provided in a second colour. For examples the objects may be provided as white objects on a black background. In other examples the images  31  provided by the display  51  may be multi colour. 
     The display  51  may be arranged so that the image  31  provided by the display  51  can be changed. For example, this may enable different arrays  55  to be used at different times. Alternatively the display  51  could be used to display an image  31  that is not an array. Other types of images are shown in  FIGS. 9A to 9D . 
       FIGS. 6A to 6D  illustrate another example embodiment in which two image sources  3 , each comprising a display  51 , are provided.  FIG. 6A  shows a front view of the apparatus  1 ,  FIG. 6B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 6C  shows one of the image sources  3  in more detail and  FIG. 6D  shows an image  31  projected by one of the image sources  3 . 
     The apparatus  1  in the examples of  FIGS. 6A to 6D  is similar to the apparatus  1  shown in the examples of  FIGS. 5A to 5D  and corresponding reference numerals are used for corresponding components. The apparatus  1  in the examples of  FIGS. 6A to 6D  differs from that of the examples of  FIGS. 5A to 5D  because in the example of  FIGS. 6A to 6D  the apparatus  1  comprises two image sources  3  whereas in the example of  FIGS. 5A to 5D  the apparatus  1  only comprises a single image source  1 . Also in the example of  FIGS. 6A to 6D  the optical arrangement  5  comprises two first diffraction gratings  37  so that a first diffraction grating  37  is provided for each of the image sources  3 . Each of the first diffraction gratings  37  are positioned within the apparatus  1  so that a beam of light  17  from an image source  3  is incident on the first diffraction grating  37 . 
     In the example of  FIGS. 6A to 6D  both of the image sources  3  comprises a curved display  51 . The curved displays  51  may be arranged as shown in  FIGS. 5A to 5D . Each of the curved displays  51  may provide an image  31  comprising an array  55 . 
     In the example of  FIGS. 6A to 6D  the two displays  51  are controlled so that two identical images  31  are provided. In other examples the displays  51  could be controlled so that different images  31  are provided by each of the displays  51 . This enables different images  31  to be provided to each of the user&#39;s eyes  11 . 
     In each of the examples in  FIGS. 3A to 6D  the image sources  3  are provided in the centre of the apparatus  1  so that when the apparatus  1  is worn by the user  33  the image sources  3  are positioned over the nose of the user  33 .  FIGS. 7A to 8D  illustrate examples in which the image sources  3  are provided in a different position within the apparatus  1 . 
       FIGS. 7A to 7D  illustrate an example embodiment in which two image sources  3  comprising a display  51  are provided.  FIG. 7A  shows a front view of the apparatus  1 ,  FIG. 7B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 7C  shows one of the image sources  3  in more detail and  FIG. 7D  shows an image  31  projected by one of the image sources  3 . 
     The apparatus  1  in the examples of  FIGS. 7A to 7D  is similar to the apparatus  1  shown in the examples of  FIGS. 6A to 6D  and corresponding reference numerals are used for corresponding components. The apparatus  1  in the examples of  FIGS. 7A to 7D  differs from that of the examples of  FIGS. 6A to 6D  because in the example of  FIGS. 7A to 7D  the image sources  3  are provided at the outer edges of the apparatus  1  rather than in the center. The image sources  3  are positioned within the apparatus  1  so that when the user  33  is wearing the apparatus  1  the image sources  3  are provided towards the edge of the user&#39;s face. 
     The optical arrangement  5  is also arranged so that the first diffraction gratings  37  are provided towards the outer edges of the apparatus  1  rather than towards the centre to ensure that the light beams  17  from the image sources  3  are coupled into the exit pupil expanders  35 . 
     In the example of  FIG. 7B  the detectors  7  are shown. The detectors  7  may be arranged to detect the images that are projected onto the user&#39;s eye  11 . The detectors  7  may be positioned towards the centre of the apparatus  1  or in any other suitable locations. 
     In the example of  FIGS. 7A to 7D  the image sources  3  comprise displays  51  and the example image  31  is the same as the images provided in  FIGS. 5D and 6D . It is to be appreciated that the displays  51  could be arranged to provide different images in other examples of the disclosure. 
     It is also to be appreciated that other types of image source  3  could be used in other examples of the disclosure.  FIGS. 8A to 8D  illustrate another example embodiment in which a different type of image source  3  is used.  FIG. 8A  shows a front view of the apparatus  1 ,  FIG. 8B  shows a plan view of the apparatus  1  being worn by a user  33 ,  FIG. 8C  shows the image sources  3  in more detail and  FIG. 8D  shows an example image  31  which could be projected by the image sources  3 . 
     The optical arrangement  5  in the examples of  FIGS. 8A to 8D  is similar to the optical arrangement  5  shown in the examples of  FIGS. 7A to 7D  and corresponding reference numerals are used for corresponding features. The apparatus  1  in the examples of  FIGS. 8A to 8D  differs from that of the examples of  FIGS. 7A to 7D  because in the example of  FIGS. 8A to 8D  the image source  3  comprises a plurality of collimated light sources  34  rather than a display  51 . The plurality of collimated light sources  34  may be as described above. Other types of image sources  3  may be used in other examples of the disclosure. 
       FIGS. 9A to 9D  illustrate example projected images  31  that could be used in some examples of the disclosure. The example projected images  31  could be provided by a display  51  or by any other suitable image source  3 . 
     The example image  31  in  FIG. 9A  comprises an array  55  of objects. The array  55  may be arranged so that when the image  31  is projected onto the user&#39;s eye  11  the spacing between the objects within the array  55  as detected by the detector  7  may be compared to the spacing in the projected image  31  and used to determine if there has been any distortion in the image  31 . In the example of  FIG. 9A  the array  55  comprises a plurality of objects arranged in a plurality of columns and rows. In the example of  FIG. 9A  the columns are perpendicular to the rows. In the example of  FIG. 9A  the objects within the array  55  have a regular spacing so that the spacing between adjacent objects in a row is the same and the spacing between adjacent objects in a column is the same. Other arrangements for the array  55  may be used in other examples of the disclosure. The arrangement of the objects within the array  55  may depend on the optical arrangement  5  of the apparatus  1  or any other suitable factors. 
     The example image  31  in  FIG. 9B  shows a block of uniform colour. The colour could be any suitable colour. In some examples the colour could be blue. The blue block could be provided by a display  51 , or by a blue light source or by any other suitable means. 
     The example image  31  in  FIG. 9C  comprises structured light. The image  31  provided by the structured light comprises a light fringe  91  and two dark fringes  93 . The light fringe  91  provides a straight vertical line within the image  31 . The straight vertical line is provided towards the right hand side of the image  31 . Other arrangements for structured light may be used in other examples of the disclosure. For example a plurality of lines may be used in some examples. 
     The structured light may be provided by any suitable type of image source  3 . In some examples the structured light could be provided by a light source and a mask positioned in front of the light source. In other examples the structured light could be provided by a display  51 . Where the structured light is provided by a display  51  this may enable the display  51  to be controlled to change the image  31  provided by the structure light. 
       FIG. 9D  shows another example image  31  provided by structured light. In the example of  FIG. 9D  the image also comprises a light fringe  91  and two dark fringes  93 . However, in the example of  FIG. 9D  the light fringe  91  is thicker than in the example of  FIG. 9C . The thickness of the light fringe  91  could be controlled by controlling a display  51  or by using a different mask with a light source or by controlling the position of the mask relative to the light source or by any other suitable means. 
       FIGS. 10A and 10B  illustrate example images  31  projected onto a users eye  11 . In the examples of  FIGS. 10A and 10B  the image comprises an array  55  as shown in  FIG. 9A  in which the objects have a regular spacing. 
       FIG. 10A  shows the image  31  projected onto an eye  11  which has no irregularity. The spacing between the objects in the image  31  remains regular. The spacing between the objects can be detected by the detector  7  and the analysis means  9 . 
       FIG. 10B  shows the image  31  projected onto an eye  11  which has an irregularity. In the example of  FIG. 10B  the irregularity is a change in shape. The change in shape could be a change in the curvature of the cornea. This change in the shape of the eye  11  distorts the image  31  so that the spacing between the objects in the array  55  becomes irregular. That is, there is a larger spacing between adjacent objects in some rows and columns than there are in other rows and columns. The change in the spacing between the objects can be detected by the detector  7  and the analysis means  9 . 
     It is to be appreciated that other changes in the image  31  could be detected in other examples of the disclosure. For example, a change in colour of the image  31  could be indicative of an abrasion or other type of irregularity within the eye  11 . In such examples the image  31  that is used could-be a single block of colour as shown in  FIG. 9B . 
     If a change in shape, or other irregularity is detected then this information could be provided to the user  33 , or to a medical professional to indicate that an irregularity has been detected. This could prompt further examination and/or tests of the users eye  11 . 
     In examples of the disclosure it may be useful to enable the image  31  to be projected onto as large a surface of the users eye  11  as possible. For example, if the whole of the cornea can be covered by the image  31  then this may enable the whole of the cornea to be monitored. 
       FIG. 11  schematically illustrates incoming light beams  111  required for full coverage of the users cornea  113 . This shows that the incoming light beams  111  which provide the projected image  31  would need to come from a range of angles extending more than 90 degrees from the perpendicular on either side. The entire of the cornea  113  may be detected by the detector  7 . 
       FIG. 12  schematically illustrates the coverage of the user&#39;s cornea  113  that may be obtained with an apparatus  1  in some examples of the disclosure. This shows that where the image source  3  and optical arrangement  5  provide an output with a narrow angular range, only a portion of the user&#39;s cornea  113  can be covered by the image  31 . 
     In the example of  FIG. 12  the optical arrangement  5  provides an output beam of light with an angular coverage of 13 degrees either side of the perpendicular. It can be seen in  FIG. 12  that this only covers a small portion of the cornea  113  of the user&#39;s eye  11 . The portion of the cornea  113  that is covered in the example of  FIG. 12  may be smaller than the pupil of the eye  11 . 
     In some examples it may be sufficient for the image  31  provided by the image source  3  and optical arrangement  5  to only cover a small portion of the cornea  113  of the user&#39;s eye  11 . For example the apparatus  1  may be arrange to adjust the optical arrangement  5  so that the exit pupil  15  of the optical arrangement  5  is aligned with a particular part of the user&#39;s eye  11  which is of interest to the user  33  or a medical professional. In other examples the apparatus  1  may be arranged to increase the coverage of the cornea  113 .  FIGS. 13A to 13C  illustrate example apparatus  1  arranged to obtain extended coverage of the cornea  113  of the user&#39;s eye  11 . 
     In the example of  FIG. 13A  the coverage of the cornea  113  is increased by increasing the number of outputs provided by the optical arrangement  5 . In such examples the optical arrangement  5  may be arranged to comprise a plurality of exit pupils  15  and outputs maybe provided by each of the exit pupils  15 , or a subset of the exit pupils  15 . This therefore increases the overall size of the image  31  provided by the apparatus  1 . In some examples the outputs provided by the exit pupils  15  may be provided at different angles to ensure that the curved surface of the cornea  113  is covered by the image  31 . 
     In examples where the optical arrangement  5  comprises an exit pupil expander  35  the exit pupil expander  35  may comprise a plurality of second diffraction gratings  39  to provide the additional exit pupils  15 . The second diffraction gratings  39  are arranged to out-couple a beam of light from the substrate  36 . In such examples each of the second diffraction gratings  39  may provide an image so that the image  31  projected onto the user&#39;s eye  11  is a composite image comprised of the plurality of images from each of the second diffraction gratings  39 . In such examples the composite image may comprise the same image duplicated a plurality of times.  FIGS. 14A to 14C  illustrate example arrangements of second diffraction gratings  39  that may be used to provide a composite image in some examples of the disclosure. 
     In the example of  FIG. 13B  the coverage of the cornea  113  is increased by providing a plurality of detectors  7  at a plurality of different angles relative to the user&#39;s eye  11 . In the example of  FIG. 13B  a first detector  7  is provided straight in front of the user&#39;s eye  11 , a second detector  7  is provided offset at 45 degrees to the right and a third detector  7  is provided offset at 45 degrees to the left. This enables images from covering different sections of the cornea  113  to be detected by the detector  7 . 
     In the example of  FIG. 13C  the coverage of the cornea  113  is increased by moving the eye  11  relative to the apparatus  1 . This could be achieved by instructing the user  33  to move their eye  11  as required or by tracking the movement of the user&#39;s eye  11  while they are using the apparatus  1 . In other examples the movement could be achieved by moving the apparatus  1  or portions of the apparatus  1  relative to the users&#39; eye  11 . For example, in some embodiments, the optical arrangement  5  may be arranged to enable the exit pupil  15  to be moved relative to the user&#39;s eye  11 . 
     It is to be appreciated that other methods of obtaining the larger coverage could be used in other examples of the disclosure. For example, the optical arrangement  5  could be arranged to give an output with a larger angle of coverage. This could be achieved by using material with a high refractive index for the substrate  36  of the exit pupil expander  35  so as to achieve larger angles for the total internal reflection. 
       FIGS. 14A to 14C  show diffraction gratings that may be arranged within example exit pupil expanders  35  to obtain extended coverage of the user&#39;s cornea  113 . 
     In the example of  FIG. 14A  a first diffraction grating  37  is provided. The first diffraction grating  37  may be an in-coupling diffraction grating arranged to in-couple a beam of light  17  from the image source  3 . In the example of  FIG. 14A  the exit pupil expander  35  also comprises an intermediate diffraction grating  141  which is arranged to diffract the in-coupled beam of light towards the second diffraction grating  39 . 
     The second diffraction grating  39  provides the out-coupling grating and is arranged to out-couple the beam of light from the exit pupil expander  35 . In the example of  FIG. 14A  the second diffraction grating  39  is divided into a plurality of regions so that each region out-couples the beam of light from the exit pupil expander  35 . The different regions of the second diffraction grating  39  may have different specifications for the grating. For example the angle of the gratings may be different and/or the spacing of the grating may be different. 
     In the example of  FIG. 14A  each region the second diffraction grating  39  out-couples the same image or a similar image. This creates a larger out-coupled image. The larger out-coupled image is a composite image comprising a plurality of smaller images. The composite image may comprise duplications of the smaller image. The larger image provides for increased coverage of the cornea  113 . 
     In the example of  FIG. 14A  the second-diffraction grating  39  is divided into fifteen different regions to provide the composite image output. It is to be appreciated that other arrangements could be used in other examples of the disclosure. 
       FIG. 14B  illustrates another example in which the second diffraction grating  39  also provides a composite image. The example exit pupil expander  35  of  FIG. 14B  also comprises a first diffraction grating  37 , a second diffraction grating  39  and an intermediate diffraction grating  141 . In the example of  FIG. 14B  the intermediate diffraction grating  141  comprises different regions which diffract the beams of light at different angles. This helps to increase the number of images provided by the second-diffraction grating  39 . In the example of  FIG. 14B  the intermediate diffraction grating  141  comprises three different regions. The grating has different angular arrangement in the different regions. 
     In the example of  FIG. 14B  the second diffraction grating  39  only has five different regions. However, as the intermediate diffraction grating  141  has a plurality of different regions this still provides for a composite image comprising a plurality of output images. 
       FIG. 14C  illustrates another example in which the second diffraction grating  39  also provides a composite image. The example exit pupil expander  35  of  FIG. 14C  also comprises a first diffraction grating  37 , a second diffraction grating  39  and an intermediate diffraction grating  141 . In the example of  FIG. 14C  the second diffraction grating  37  comprises four different regions arranged in a square arrangement. Other types of arrangement may be used in other examples of the disclosure. 
       FIG. 15  illustrates an example method that may be implemented in some examples of the disclosure. The method may be implemented using example apparatus  1  as described in this application. 
     At block  151  the method comprises providing an image  31  from an image source  3 . At block  153  the method comprises conveying a beam of light  17  through an optical arrangement  5  to enable an image  31  from the image source  3  to be projected onto a user&#39;s eye  11  wherein the optical arrangement  5  comprises at least one light guide  13  and an exit pupil  15  where the exit pupil  15  is arranged to be positioned proximate to the user&#39;s eye  11  and the at least one light guide  13  is arranged to convey a beam of light  17  from the image source  3  to the exit pupil  15 . 
     The method also comprises, at block  155 , detecting the image  31  projected onto the user&#39;s eye  11  and, at block  157 , enabling the detected image to be analysed so as to identify changes in the projected image  31 . 
     It is to be appreciated that the method may comprise additional blocks that are not illustrated in  FIG. 15 . For instance, in some examples the method may comprise arranging the apparatus so that the exit pupil  15  of the optical arrangement  5  is aligned with the user&#39;s eye  11  or with a particular part of the user&#39;s eye  11 . This may enable the image  31  to be projected onto a selected part of the user&#39;s eye  11 . 
     Examples of the disclosure provide for an apparatus  1  and method of monitoring a user&#39;s eye  11 . The apparatus  1  uses a diffractive optical arrangement which does not shine light directly into a user&#39;s eye  11 . This may make the apparatus  1  more comfortable for the user  33  to use. 
     In some examples the apparatus  1  may be integrated into a wearable device  21 . For example, the apparatus  1  could be integrated into a near eye display. This wearable device  21  could be used for other uses such as augmented reality or virtual reality applications. This could make the apparatus  1  more convenient for the user  33  to use and may enable the monitoring to take place over extended periods of time. 
     The analysis of the image  31  projected on to the user&#39;s eye  11  can be performed by the detector  7  and the analysis means  9 . This may provide for digital analysis of the image  31  which could be performed by the apparatus  1  or by a remote device. The information could then be provided to a medical professional if needed. This therefore may enable the monitoring of the user&#39;s eye  11  to be performed without any medical professional or any special training. 
     In this description the term coupled means operationally coupled. It is to be appreciated that any number of components may be provided between coupled elements, including zero components. 
     The term “comprise” is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use “comprise” with an exclusive meaning then it will be made clear in the context by referring to “comprising only one . . . ” or by using “consisting”. 
     In this brief description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term “example” or “for example” or “may” in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus “example”, “for example” or “may” refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. 
     Features described in the preceding description may be used in combinations other than the combinations explicitly described. 
     Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. 
     Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 
     Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.