Head mounted display

Abstract

A head mounted display (HMD) has an optical assembly 310 configured to hold at least a first set 320 of one or more optical elements in place, wherein at least the first set of one or more optical ele

Description

HEAD MOUNTED DISPLAY Field of Invention The present invention relates to a head mounted display. Background Head mounted displays (HMDs) have become more affordable and versatile in recent years. An HMD may be used to provide virtual reality (VR) or augmented reality (AR) content to a user. However, whilst HMDs are becoming more affordable, it is still common for a group of users, such as a family or household, to share the use of one or more HMDs. Therefore, as each user may require an HMD to be customised to their individual needs, it is desirable to be able to easily customise one or more aspects of the HMD for each user. It is in this context that the present disclosure arises. Summary of the Invention In a first aspect, a head mounted display (HMD) is provided in claim 1. In another aspect, a a modifying optical element is provided in claim 15. Further respective aspects and features of the invention are defined in the appended claims. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 schematically illustrates an HMD worn by a user; Figure 2 is a schematic plan view of an HMD; Figure 3 schematically illustrates the formation of a virtual image by an HMD; Figure 4 schematically illustrates another type of display for use in an HMD; Figure 5 is a schematic plan of an HMD; Figure 6 is a schematic plan of an HMD comprising a sealed area in accordance with embodiments of the present disclosure; Figure 7 is a schematic plan of an HMD in accordance with embodiments of the present disclosure; Figures 8a-8e show an embodiment of replacing one or more optical elements with one or more modifying optical elements in accordance with embodiments of the present disclosure; Figures 9a-9¢ show an embodiment of replacing one or more optical elements with one or more modifying optical elements in accordance with embodiments of the present disclosure; Figure 10 is a schematic plan of an HMD comprising a humidity control unit in accordance with embodiments of the present disclosure; Figure 11 is a schematic plan of an optical element comprising a humidly control unit in accordance with embodiments of the present disclosure; Figure 12 is a schematic plan of an HMD with externally mounted modification lenses in accordance with embodiments of the present disclosure; Figure 13 is a schematic plan of an HMD with externally mounted modification lenses in accordance with embodiments of the present disclosure; and Figure 14 is a schematic plan of an HMD with externally mounted modification lenses. Description of the Embodiments In the following description, a number of specific details are presented in order to provide a thorough understanding of the embodiments of the present invention. It will be apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention. Conversely, specific details known to the person skilled in the art are omitted for the purposes of clarity where appropriate. Referring now to Figure 1, a user 10 is wearing an HMD 20 (as an example of a generic head- mountable apparatus — other examples including audio headphones or a head-mountable light source) on the user's head 30. The HMD comprises a frame 40, in this example formed of a rear strap and a top strap, and a display portion 50. As noted above, many gaze tracking arrangements may be considered particularly suitable for use in HMD systems; however, use with such an HMD system should not be considered essential. Note that the HMD of Figure 1 may comprise further features, to be described below in connection with other drawings, but which are not shown in Figure 1 for clarity of this initial explanation. The HMD of Figure 1 completely (or at least substantially completely) obscures the user's view of the surrounding environment. All that the user can see is the pair of images displayed within the HMD, as supplied by an external processing device such as a games console in many embodiments. Of course, in some embodiments images may instead (or additionally) be generated by a processor or obtained from memory located at the HMD itself. The HMD has associated headphone audio transducers or earpieces 60 which fit into the user's left and right ears 70. The earpieces 60 replay an audio signal provided from an external source, which may be the same as the video signal source which provides the video signal for display to the user's eyes. The combination of the fact that the user can see only what is displayed by the HMD and, subject to the limitations of the noise blocking or active cancellation properties of the earpieces and associated electronics, can hear only what is provided via the earpieces, mean that this HMD may be considered as a so-called “full immersion” HMD. Note however that in some embodiments the HMD is not a full immersion HMD, and may provide at least some facility for the user to see and / or hear the user's surroundings. This could be by providing some degree of transparency or partial transparency in the display arrangements, and / or by projecting a view of the outside (captured using a camera, for example a camera mounted on the HMD) via the HMD’s displays, and / or by allowing the transmission of ambient sound past the earpieces and / or by providing a microphone to generate an input sound signal (for transmission to the earpieces) dependent upon the ambient sound. A front-facing camera 122 may capture images to the front of the HMD, in use. Such images may be used for head tracking purposes, in some embodiments, while it may also be suitable for capturing images for an augmented reality (AR) style experience. A Bluetooth® antenna 124 may provide communication facilities or may simply be arranged as a directional antenna to allow a detection of the direction of a nearby Bluetooth transmitter. In operation, a video signal is provided for display by the HMD. This could be provided by an external video signal source 80 such as a video games machine or data processing apparatus (such as a personal computer), in which case the signals could be transmitted to the HMD by a wired or a wireless connection. Examples of suitable wireless connections include Bluetooth® connections. Audio signals for the earpieces 60 can be carried by the same connection. Similarly, any control signals passed from the HMD to the video (audio) signal source may be carried by the same connection. Furthermore, a power supply (including one or more batteries and / or being connectable to a mains power outlet) may be linked by a cable to the HMD. Note that the power supply and the video signal source 80 may be separate units or may be embodied as the same physical unit. There may be separate cables for power and video (and indeed for audio) signal supply, or these may be combined for carriage on a single cable (for example, using separate conductors, as in a USB cable, or in a similar way to a “power over Ethernet” arrangement in which data is carried as a balanced signal and power as direct current, over the same collection of physical wires). The video and / or audio signal may be carried by, for example, an optical fibre cable. In other embodiments, at least part of the functionality associated with generating image and / or audio signals for presentation to the user may be carried out by circuitry and / or processing forming part of the HMD itself. A power supply may be provided as part of the HMD itself. Some embodiments of the invention are applicable to an HMD having at least one electrical and / or optical cable linking the HMD to another device, such as a power supply and / or a video (and / or audio) signal source. So, embodiments of the invention can include, for example: (a) an HMD having its own power supply (as part of the HMD arrangement) but a cabled connection to a video and / or audio signal source; (b) an HMD having a cabled connection to a power supply and to a video and / or audio signal source, embodied as a single physical cable or more than one physical cable; (c) an HMD having its own video and / or audio signal source (as part of the HMD arrangement) and a cabled connection to a power supply; or (d) an HMD having a wireless connection to a video and / or audio signal source and a cabled connection to a power supply. If one or more cables are used, the physical position at which the cable enters or joins the HMD is not particularly important from a technical point of view. Aesthetically, and to avoid the cable(s) brushing the user's face in operation, it would normally be the case that the cable(s) would enter or join the HMD at the side or back of the HMD (relative to the orientation of the user’s head when worn in normal operation). Accordingly, the position of the cables relative to the HMD in Figure 1 should be treated merely as a schematic representation. Accordingly, the arrangement of Figure 1 provides an example of a head-mountable display system comprising a frame to be mounted onto an observer’s head, the frame defining one or two eye display positions which, in use, are positioned in front of a respective eye of the observer and a display element mounted with respect to each of the eye display positions, the display element providing a virtual image of a video display of a video signal from a video signal source to that eye of the observer. Figure 1 shows just one example of an HMD. Other formats are possible: for example an HMD could use a frame more similar to that associated with conventional eyeglasses, namely a substantially horizontal leg extending back from the display portion to the top rear of the user's ear, possibly curling down behind the ear. In other (not full immersion) examples, the user's view of the external environment may not in fact be entirely obscured; the displayed images could be arranged so as to be superposed (from the user's point of view) over the external environment. An example of such an arrangement will be described below with reference to Figure 4. In the example of Figure 1, a separate respective display is provided for each of the user's eyes. A schematic plan view of how this is achieved is provided as Figure 2, which illustrates the positions 100 of the user's eyes and the relative position 110 of the user's nose. The display portion 50, in schematic form, comprises an exterior shield 120 to mask ambient light from the user's eyes and an internal shield 130 which prevents one eye from seeing the display intended for the other eye. The combination of the user's face, the exterior shield 120 and the interior shield 130 form two compartments 140, one for each eye. In each of the compartments there is provided a display element 150 and one or more optical elements 160. The way in which the display element and the optical element(s) cooperate to provide a display to the user will be described with reference to Figure 3. Referring to Figure 3, the display element 150 generates a displayed image which is (in this example) refracted by the optical elements 160 (shown schematically as a convex lens but which could include compound lenses or other elements) so as to generate a virtual image 170 which appears to the user to be larger than and significantly further away than the real image generated by the display element 150. As an example, the virtual image may have an apparent image size (image diagonal) of more than 1 m and may be disposed at a distance of more than 1 m from the user's eye (or from the frame of the HMD). In general terms, depending on the purpose of the HMD, it is desirable to have the virtual image disposed a significant distance from the user. For example, if the HMD is for viewing movies or the like, it is desirable that the user's eyes are relaxed during such viewing, which requires a distance (to the virtual image) of at least several metres. In Figure 3, solid lines (such as the line 180) are used to denote real optical rays, whereas broken lines (such as the line 190) are used to denote virtual rays. An alternative arrangement is shown in Figure 4. This arrangement may be used where it is desired that the user's view of the external environment is not entirely obscured. However, it is also applicable to HMDs in which the user's external view is wholly obscured. In the arrangement of Figure 4, the display element 150 and optical elements 200 cooperate to provide an image which is projected onto a mirror 210, which deflects the image towards the user's eye position 220. The user perceives a virtual image to be located at a position 230 which is in front of the user and at a suitable distance from the user. In the case of an HMD in which the user's view of the external surroundings is entirely obscured, the mirror 210 can be a substantially 100% reflective mirror. The arrangement of Figure 4 then has the advantage that the display element and optical elements can be located closer to the centre of gravity of the user's head and to the side of the user's eyes, which can produce a less bulky HMD for the user to wear. Alternatively, if the HMD is designed not to completely obscure the user's view of the external environment, the mirror 210 can be made partially reflective so that the user sees the external environment, through the mirror 210, with the virtual image superposed over the real external environment. Different users may have different vision requirements that may require a user to wear glasses or contact lenses in order to correct the user's natural vision. However, some users may be unable to wear contact lenses and would therefore have to use an HMD either whilst wearing glasses, which may be difficult or uncomfortable, or using their uncorrected natural vision. Furthermore, it is likely that children may wear glasses instead of contact lenses, and may have particular difficulty in understanding how to make glasses comfortable whilst wearing an HMD, particularly as children’s glasses tend to have a more robust and bulky design; it is desirable to address or mitigate this problem. Additionally, some types of content that are configured to be displayed on an HMD may require the HMD to have optical characteristics that are different to the HMDs typical optical characteristics. For example, colour filters or fish-eye lenses. Therefore, it is desirable to provide an HMD that comprises easily modifiable optics that removably mount either on or in the HMD. Accordingly, turning now to figure 5, in an embodiment of the present description, a head mounted display (HMD) 300 comprises an optical assembly 310 configured to hold at least a first set of one or more optical elements 320 in place, wherein the at least the first set of one or more optical elements 320 are configured to be modified, by a user, with one or more modifying optical elements 330. The HMD 300 comprises a display element 340 that generates a displayed image which is refracted or reflected by at least the first set of one or more optical elements 320 (shown schematically as a convex lens but which could comprise compound lenses, mirrors, or other elements, including passive elements such as a flat transparent plate) in a respective optical assembly 310 so as to generate a virtual image for an eye of a user. In an embodiment of the present description, the respective optical assembly 310 may be modified, by the user, by replacing one or more of the one or more optical elements 320 with one or more modifying optical elements 330. Replacing one or more optical elements 320 of at least the first set of one or more optical elements can be advantageous, as the distance between a final optical element and the user’s face will remain unchanged. However, this may introduce issues that relate to keeping the first set of one or more optical elements clean and controlling the humidity inside the optical assembly 310; techniques that may address or mitigate these issues will be discussed in more detail later herein. Now referring to Figure 7, in an embodiment of the present description, the respective optical assembly 310 may be similarly modified, by the user, by adding one or more modifying optical elements 330 to the optical assembly 310. One or more modifying optical elements 330 may be added within the existing optical sequence of a set of optical elements, using similar insertion and removal approaches to that of replacing optical elements, as discussed later herein. Alternatively or in addition, one or more modifying optical elements 330 may be added at the user-facing end of the existing optical sequence, external to the existing set of optical elements. Adding the one or more modifying optical elements 330 to the optical assembly externally avoids the issue of breaking the seal on the set of one or more optical elements in the original optical assembly. However, adding one or more modifying optical elements to the optical assembly externally may introduce issues relating to holding the one or more modifying optical elements in place in the optical assembly, as discussed later herein. Each of these approaches is now discussed in turn. Turning now to figure 6, in an embodiment of the present description, the set of one or more optical elements are configured to be modified by replacing one or more optical elements 320 with one or more modifying optical elements 330. As noted previously, in an embodiment of the present description the HMD 300 comprises a display portion 340. Additionally, a sealed area 350 extends at least between the display portion 340 and the optical assembly 310, as shown in Figure 6. In an embodiment of the present description, the sealed area 350 may extend between the display portion 340 and a final optical element in the set of one or more optical elements 320, in which the final optical element faces the user. The sealed area 350 may prevent dirt and other debris from contaminating at least the first set one or more optical elements that are inside the HMD 300 and that may be difficult for a user to clean. Additionally, the sealed area 350 may have a different humidity, in comparison to the humidity outside of the sealed area, to prevent, for example, water vapour condensing on at least the first set of one or more optical elements, which may reduce the user’s visibility of the display portion 340. Turning now also to Figures 8a-8e, in an embodiment of the present description, the HMD comprises a resilient slit 360 for maintaining a seal for the sealed area 350 when the one or more optical elements 320 elements of the first set of one or more optical elements are replaced by the one or more modifying optical elements 330. For example, at least one of the one or more optical elements 320 may be in close proximity with at least one side of the optical assembly 310 that may comprise a respective rubber seal or resilient slit 360, as shown in figure 8a. Therefore, one of the one or more optical elements 320 may be removed as shown in Figure 8b through the rubber seal 360, which may deform around the removed one of the one or more optical elements 320, enabling the sealed area 350 to remain sealed. Figure 8c shows the optical assembly 310 after at least one of the one or more optical elements 320 of at least the first set of one or more optical elements has been removed. Then one of the one or more modifying optical elements 330 may be inserted into the optical assembly 310 through the rubber seal 360 as shown in Figures 8d and 8e, in which the rubber seal 360 may again deform around the one of the one or more modifying optical elements 330. Additionally, the HMD 300 may comprise one or more user operable switches 370, such as a lever or slide, that are accessible, by the user, from outside of the sealed area 350. When one of the one or more user operable switches 370 is engaged by the user, a respective one of one or more optical elements 320 is initially displaced so that at least a part of the respective one of one or more optical elements 320 is displaced through the rubber seal 360. Any similar suitable mount-release scheme may be considered, such as a push-release arrangement as commonly found for mounting and releasing memory cards from computers or phones. In this case, the user may push on the rubber seal, transmitting the push action to the optical element, causing it to partially eject through the seal for the user to remove. As another example, Figures 9a-9c show that at least one of the one or more optical elements 320 may be in close proximity with at least two opposing sides of the optical assembly 310 that may each comprise a respective rubber seal 360, as seen in Figure 9a. Therefore, one of the one or more modifying optical elements 330 may inserted into the optical assembly 310 through one of the at least two rubber seals 360 as shown in Figure 9b, which may deform around the one of the one or more modifying optical elements 330, and push one of the one or more optical elements 320 through another one of the at least two rubber seals 360 as shown in Figure 9b, until the optical element is pushed out and replaced, as shown in figure 9c. Turning now to figure 10, in an embodiment of the present description, the HMD comprises a humidity control unit 380 configured to control the humidity in the sealed area 350. For example, the humidity control unit 380 may be configured to detect the humidity inside the sealed area 350. For example, if the humidity control unit 380 detects that the humidity inside the sealed area 350 is too high, the humidity control unit 380 may vent the excess moisture via the use of an extractor fan or the like. As another example, the humidity control unit 380 may comprise a desiccant, such as silica gel, that can reduce the humidity inside the sealed area. Turning now to figure 11, in an embodiment of the present description, the one or more optical elements 320 and the one or more modifying optical elements 330 may comprise the humidity control unit 380. For example, if the humidity control unit 380 comprises a desiccant, such as silica gel, it may be advantageous for this to be attached directly to an optical element 320. Therefore, the desiccant may be easily replaced or dried out for re-use by a user whilst the respective optical element 320 is not currently inserted into the optical assembly 310, as the desiccant may become saturated if it is not replaced or dried out. It will be appreciated that the figures illustrate the optical elements mounted within square frames. However, this is a non-limiting example. Depending on how the lenses are mounted within the HMD, any shape of frame may be considered, including an incomplete frame, or the optical element may comprise an integral component used for mounting or gripping purposes such as one or more tabs or flanges, or may have no frame and / or no integral component at all. Referring now to figure 12, in an embodiment of the present description, a final optical element 390 in the set of one or more optical elements faces the user (typically a final lens, or a passive protective flat plate, as shown); and the set of one or more optical elements are configured to be modified by adding one or more modifying optical elements 330 externally between the final optical element and the user. Typically this may be achieved by removably mounting the one or more modifying optical elements 330 either to the user facing outside end of the optical assembly 310, or to the body of the HMD at a location placing the one or more modifying optical elements 330 between the end of the optical assembly 310 and the user, whether in direct contact with the optical assembly 310 or not. In an embodiment of the present description, the HMD 300 comprises a resilient covering 302 adjacent to the final optical element. In an embodiment of the present description, the resilient covering comprises one or more recesses (e.g. a fold or slit) in which a modifying optical element 330 may be removably mounted between the final optical element of the existing set of one or more optical elements and the user. For example, the one or more recesses may be made of a material, such as rubber, that provides a force sufficient to hold the modifying optical element 330 in place during operation of the HMD 300, but still enables a user to easily remove the modifying optical element 330. In an embodiment of the description, two opposing recesses may be used to hold the modifying optical element 330 in a similar manner to how a person holds a monocle in front of their eye. Optionally, three, four or more recesses may be used to provide further security of retention, or the recess may be annular. The modifying optical element may comprise a frame, flanges, tabs or other extensions (as shown in Figure 12) to facilitate being held by a recess. The location of the recesses, and extensions on the modifying optical element, may be used to specify an orientation of the modifying optical element where this is important (for example if the optical element corrects astigmatism in the user). Referring now to figure 13, in an embodiment of the present description, the HMD 300 comprises one or more magnets 304 positioned to hold the modifying optical element 330 in a mounted position. For example, the or each magnet may be a permeant magnet that provides a force sufficient to hold the modifying optical element in place during the operation of the HMD 300, or may be an electromagnet that may be deactivated when a user wants to remove the modifying optical element. The or each magnet may be used with or without the above mentioned recesses 302 to retain the modifying optical element. The modifying optical element may comprise a metal frame or metal flanges (for example bonded to the edge of the optical element) to facilitate magnetic attachment. Again, the location of the magnet or magnets, and corresponding points of attachment on the optical element, may be used to specify an orientation of the modifying optical element where this is important (for example if the modifying optical element corrects astigmatism in the user). Referring now to figure 14, in an embodiment of the present description, the HMD 300 comprises a screwthread mount 306 surrounding the final optical element, to which a modifying optical element 330 may be removably mounted between the final optical element and the user using a frame or one or more lens flanges comprising a corresponding screwthread. For example, the modifying optical element 330 may be screwed onto the screwthread mount to add the modifying optical element 330 to the optical assembly 310. If a user desires to remove the modifying optical element 330 the user may then unscrew the modifying optical element from the screwthread mount. Again, the orientation of the modifying optical element can be specified by the relative orientation of the modifying optical element in a screwthread frame, and a predetermined net rotation of the modifying optical element caused by screwing it fully into the screwthread mount. In an embodiment of the present description, the HMD 300 comprises a clip mount (not shown) surrounding the final optical element, to which a modifying optical 330 element may be removably mounted between the final optical element and the user. For example, the modifying optical element 330 may be clipped onto the clip mount to add the modifying optical element 330 to the optical assembly 310. If a user desires to remove the modifying optical element 330 the user may then unclip the modifying optical element 330 from the clip mount. One or more magnets or recesses may be used as described above in conjunction with one or more clip mounts, for example to restrain a free (unclipped) side of the modifying optical element from moving. Again, the orientation of the modifying optical element can be specified by the relative positions of the clip or clips and any extension to the modifying optical element or frame thereof used as a point of attachment. In an embodiment of the present description, the HMD 300 comprises a detector that may detect one or more optical properties of the set of one or more optical elements held by the optical assembly. For example, the detector may directly determine the one or more optical properties of the set of one or more optical elements by using, for example, an image sensor. Alternatively, or additionally, each optical element may comprise a marker, which is read by the detector, that provides the detector with information that identifies the one or more optical properties of the respective optical element. Additionally, the HMD may change the configuration of displayed content in response to the identification of the optical properties of the set of one or more optical elements held by the optical assembly. For example, an application may require a modifying optical element that blocks blue light. Therefore, the HMD may detect if the set of one or more optical elements comprises an optical element that has an optical property that blocks blue light. In an embodiment of the present description, the HMD 300 comprises a left eye set of one or more optical elements and a right eye set of one or more optical elements. Therefore, in an embodiment, the user may modify the left eye set of one or more optical elements differently to the right eye set of one or more optical elements if, for example, the user has different vision requirements for each of the user’s respective eyes. In an embodiment of the present description, the left eye set of one or more optical elements and the right eye set of one or more optical elements are configured to be modified with one or more selected from the list consisting ofthe same one or more modifying optical elements, and different one or more modifying optical elements, or alternatively only one of the left eye set of one or more optical elements and the right eye set of one or more optical elements are modified by the one or more modifying optical elements. Therefore, both the left eye set and the right eye set of the one or more optical elements may be modified with the same one or more modifying optical element if, for example, a user has the same vision requirements for each of the user's respective eyes, or with different one or more modifying optical elements if, for example, the user has different vision requirements for each of the user's respective eyes, or only one of the left eye set and the right eye set of the one or more optical elements may be modified by one or more modifying optical elements if, for example, the user has vision requirements, which are different to the default vision requirements of the HMD, in only one eye. Hence the left and right eye sets of one or more optical elements can be modified independently, which may result in similar or differing modifications, or modification of only one set. It will be appreciated however that in principle a conjoined pair of modifying optical elements for left and right eye sets may be provided for replacing or adding to the optical elements of these sets simultaneously and in parallel, with the joining structure shaped to fit the HMD and allow the requisite motion of the modifying optical elements to engage on or in the HMD to modify the optical element sets. For example with respect to Figures 8a-e, a conjoined frame may be used to insert a pair of lenses 330 into the optical assembly, for example either from above or below. Similarly for example with respect to Figures 12 or 13, a conjoined frame may be used to position lenses in the respective recesses and / or in line with the respective magnets on the HMD. Additionally, in an embodiment of the present description, one or both sets of one or more optical elements may be modified by both adding one or more modifying optical elements within the optical assembly 310 and / or externally to it and also replacing one or more optical elements within the optical assembly 310 with one or more modifying optical elements. Summary Embodiment In a summary embodiment of the present description, a head mounted display (HMD) comprises an optical assembly configured to hold at least a first set of one or more optical elements in place, wherein at least the first set of one or more optical elements are configured to be modified, by a user, with one or more modifying optical elements, as described elsewhere herein. In an instance of the summary embodiment, a final optical element in the first set of one or more optical elements faces the user; and optionally at least the first set of one or more optical elements are configured to be modified by adding the one or more modifying optical elements between the final optical element and the user, as described elsewhere herein. In this instance, optionally the HMD comprises a resilient covering adjacent to the final optical element, as described elsewhere herein. In this case, optionally the resilient covering comprises one or more recesses in which a modifying optical element may be removably mounted between the final optical element and the user, as described elsewhere herein. Similarly in this case, optionally the HMD comprises a clip mount surrounding the final optical element, to which a modifying optical element may be removably mounted between the final optical element and the user, as described elsewhere herein. Further optionally in this instance, a magnet may be positioned to hold the modifying optical element in a mounted position, as described elsewhere herein. Again similarly in this case, optionally the HMD comprises a screwthread mount for example surrounding the final optical element or positioned as a protruding rim or similar extension in a facia comprising the final optical element, to which a modifying optical element may be removably mounted between the final optical element and the user, as described elsewhere herein. In an instance of the summary embodiment, at least the first set of one or more optical elements are configured to be modified by replacing one or more optical elements with one or more modifying optical elements, as described elsewhere herein. In an instance of the summary embodiment, optionally the HMD comprises a display portion, and a sealed area that extends at least between the display portion and the optical assembly, as described elsewhere herein. In this instance, optionally the HMD comprises a resilient slit for maintaining a seal for the sealed area when the one or more optical elements of the first set of one or more optical elements are replaced by the one or more modifying optical elements, as described elsewhere herein. Further optionally, the HMD comprises a humidity control unit configured to control the humidity in the sealed area, as described elsewhere herein. In this case, optionally the HMD comprises a humidity control unit configured to control the humidity in the sealed area, as described elsewhere herein. Further optionally, at least one of the one or more optical elements of the first set of one or more optical elements, or the one or more modifying optical elements comprise the humidity control unit, as described elsewhere herein. In an instance of the summary embodiment, optionally the HMD comprises a detector configured to detect one or more optical properties of the set of one or more optical elements held by the optical assembly, as described elsewhere herein. In an instance of the summary embodiment, optionally the HMD comprises a left eye set of one or more optical elements and a right eye set of one or more optical elements, as described elsewhere herein, and optionally the left eye set of one or more optical elements and the right eye set of one or more optical elements are configured to be modified with one or more selected from the list consisting of: the same one or more modifying optical elements, and different one or more modifying optical elements. Optionally only one of the left eye set of one or more optical elements and the right eye set of one or more optical elements are modified by the one or more modifying optical elements, as described elsewhere herein. In this instance, optionally a pair of modifying optical elements are mounted on a shared frame, as described elsewhere herein. In a summary embodiment of the present description, a modifying optical element is adapted to be removably mounted in or on an HMD having corresponding mounting means, as described herein. For example the modifying optical element may be mounted in a frame, or have extensions, flanges, tabs or the like adapted to enable it to be inserted into the optical assembly, for example through a resilient slit, and optionally to push an existing optical element out, or adapted to enable it to be mounted external to the optical assembly using one or more of at least a first recess, magnet, or clip, or a screw thread. The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

Claims

CLAIMS iL A head mounted display ‘HMD’ comprising: an optical assembly configured to hold at least a first set of one or more optical elements in place, wherein at least the first set of one or more optical elements are configured to be modified, by a user, with one or more modifying optical elements.

2. An HMD according to claim 1, in which at least the first set of one or more optical elements are configured to be modified by replacing one or more optical elements with one or more modifying optical elements.

3. An HMD according to claim 1 or claim 2, in which a final optical element in the first set of one or more optical elements faces the user; and at least the first set of one or more optical elements are configured to be modified by adding the one or more modifying optical elements between the final optical element and the user.

4. An HMD according to claim 3, comprising a resilient covering adjacent to the final optical element, and wherein the resilient covering comprises one or more recesses in which a modifying optical element may be removably mounted between the final optical element and the user.

5. An HMD according to any one of claims 3 to 4, comprising: a clip mount surrounding the final optical element, to which a modifying optical element may be removably mounted between the final optical element and the user.

6. An HMD according to any one of claims 3 to 5, comprising: a magnet positioned to hold the modifying optical element in a mounted position.

7. An HMD according to claim 3, comprising: a screw thread mount, to which a modifying optical element may be removably mounted between the final optical element and the user.

8. An HMD according to claim 1, comprising: a display portion; and a sealed area that extends at least between the display portion and the optical assembly.

9. An HMD according to claim 8, comprising a resilient slit for maintaining a seal for the sealed area when one or more optical elements of the first set of one or more optical elements are replaced by the one or more modifying optical elements.

10. An HMD according to claim 8 or 9, comprising: a humidity control unit configured to control the humidity in the sealed area.

11. An HMD according to claim 10, wherein at least one of the one or more optical elements of the first set of one or more optical elements or the one or more modifying optical elements comprise the humidity control unit.

12. An HMD according to claim 1, comprising: a detector configured to detect one or more optical properties of the set of one or more optical elements held by the optical assembly.

13. An HMD according to claim 1, comprising: a left eye set of one or more optical elements and a right eye set of one or more optical elements, wherein the left eye set of one or more optical elements and the right eye set of one or more optical elements are configured to be modified with one or more selected from the list consisting of: i. the same one or more modifying optical elements; and ii. different one or more modifying optical elements.

14. An HMD according to claim 13, wherein: a pair of modifying optical elements are mounted on a shared frame.

15. A modifying optical element adapted to be removably mounted in or on an HMD having corresponding mounting means.

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