Head mounted display apparatus and method of illuminating a scene therewith

Abstract

An illumination module for use with a head mounted display (HMD) device and a method of illuminating a scene with the HMD equipped with the illumination module are disclosed. The illumination comprises at least one light source, a housing having an aperture distal the light source and an internal surface intermediate the light source and the aperture, wherein the internal surface is configured as a light guide, having maximal internal reflection. Light guides advantageously focus the initial illumination without little to no loss relative to conventional mirror-like configurations, whereby they are accordingly more energy efficient, advantageously for autonomous HMD devices powered with an internal battery. The illumination module emits light frontally of the HMD towards a target in a scene. An imaging sensor captures light re-emitted by a target in the scene, in consequence of its illumination by the emitted light, as image data.

Description

HEAD MOUNTED DISPLAY APPARATUS AND METHOD OF ILLUMINATING A SCENE THEREWITHFIELD OF INVENTION

[0001] The invention belongs to the field of head mounted display (‘HMD’) apparatuses and more particularly, but not exclusively, to HMD apparatuses operating and / or configured with sources of illumination.BACKGROUND TO INVENTION

[0002] Head mounted display (‘HMD’) devices have gained popularity in surgical procedures, for their capacity to provide the wearer with surgical context-relevant digital information within the wearer’s field of view during a procedure. HMDs are digital display devices with some onboard data processing capacity, which project digital video content in a user’s direct field of view, either in superposition to the user’s real physical environment in the case of see-through augmented reality (‘AR’) glasses, or as computer-generated imagery in replacement of that real physical environment in the case of virtual reality (‘VR’) headsets or still, lately, as a combination of captured physical environment and computer-generated imagery in the case of mixed reality (‘MR’) headsets.

[0003] Functional capacities of HMD devices are in a state of constant evolution, as improvements to miniature electronic components open opportunities to accommodate new fields of application. A specific example well suited to HMDs intended for medical procedures, is fluorescence-based imaging, which has gained comparable popularity as intra-operative imaging technology. This technique involves injecting a fluorophore or contrast agent, for instance indocyanine green (ICG), in a patient to allow Near-Infrared (NIR)-based detection and visualization of target tissue intended for observation or surgical removal. In use, the ICG absorbs light emitted by a light source, typically with a wavelength of e.g. 780 nm and is excited into fluorescence, whereby the light is re-emitted at a slightly higher wavelength, e.g. in the range 830 to 850 nm. A NIR-sensitive imaging sensor configured with suitable cut-off and band-pass filters detects the re-emitted light and generates corresponding NIR image data.

[0004] Fluorescence-based imaging systems based upon the above principles use a NIR illuminator and camera arrangement tethered to a remote processing unit, outputting captured image data inclusive of fluorescent tissue to a remote display unit. Such NIR arrangements of the prior art are typically either a static or semi-static device mounted to an adjustable cantilevered arm,or a mobile wand-like device, in either case held at a predetermined distance of the target tissue, wherein the NIR light source illuminates the tissue with a predetermined intensity of at least 1 mW per cm2across the illuminated area, the camera captures excited tissue at that predetermined distance and outputs it to the remote processing unit, which processes it for output to the remote display unit.

[0005] Given the ergonomic disadvantages of such fluorescence-based imaging systems, notably the transient occlusion of some or all of the light source by a surgeon’s or nurse’s head or hand during a procedure, and the requirement for a surgeon or nurse to hold a wand-like NIR device in situ throughout the procedure, equipping an HMD with a NIR illuminator and camera arrangement appears desirable, moreover to prevent any spatial disconnect for the surgeon when periodically shifting their field of vision between the surgery site with the actual tissue to work upon, and the remote display unit showing the fluorescent tissue.

[0006] However, given the non-trivial duration of many surgical procedures, there is a dual ergonomic imperative both to maintain the weight of a HMD device as low as possible and to sustain the HMD device in self-powered operation as long as possible. This imperative mitigates against conventional light sources, e.g. with light emitting diodes (‘LED’), the standard rate of illumination of which is known to be insufficient for providing the requisite level of illumination at a surgery site: this approach would require both a substantial illumination module combining very many LEDs, together with a correspondingly large and heavy battery for sustained use. This dual imperative likewise mitigates against a laser-based lighting solution, which would require a similarly-large large and heavy battery.

[0007] Accordingly, there is a requirement for an improved lighting solution for conventional HMD devices.SUMMARY OF INVENTION

[0008] Aspects of the invention are set out in the accompanying claims, respectively aimed at various embodiments of an illumination module for use with a head mounted display (HMD) device, and various embodiments of a method of illuminating a scene with an HMD device incorporating the illumination module.

[0009] In a first aspect therefore, the present invention provides an illumination module for use with a HMD device, comprising at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, wherein the or each internal surface is configured as a light guide havingmaximal internal reflection. Light guides advantageously focus the initial illumination without little to no loss relative to conventional mirror-like configurations, whereby they are accordingly more energy efficient, advantageously for autonomous HMD devices powered with an internal battery. Embodiments are considered wherein the illumination module may comprise a plurality of light sources with respective light guides, combining energy efficiency with simplicity of modulation through the selective switching and / or dimming of individual light sources to effect the adjustment required by the modulation.

[0010] Various embodiments of the light guide configuration are considered. The light guide may be configured as a hollow reflector, with the internal surface having a root mean square (RMS) height (Sq) in the range of 50 picometers (pm) to 5 micrometers (pm), through finishing the entire internal surface of the housing intermediate the light source and the aperture uniformly to the requisite measure of smoothness, analogous to polished glass; alternatively, through the application of a coating with similar properties to the entire internal surface.

[0011] Alternatively, or additionally, the light guide may comprise one or more lens-like members, i.e. the volume bounded by the entire internal surface of the housing intermediate the light source and the aperture may be filled with a solid member, made of glass, polycarbonate or other fully transparent material, effectively embodying a homogeneous lens-like member, having a surface boundary with the requisite degree of smoothness. Such an embodiment may be further enhanced with configuring the immediately adjacent internal surface of the housing with a high-reflection coating. Embodiments of the lens member should preferably be made of a fully transparent material, having a smoothness of surface boundary with a root mean square (RMS) height (Sq) in the range 50 pm to 5 pm.

[0012] In embodiments of the illumination module, the or each light source preferably emits light in a specific wavelength in the range 350 to 1 ,100 nanometers (nm). Where an effect of light emitted by the illumination module is to be re-emitted by a target, e.g. the tissue of a patient, under a process by which a target absorbs light, of any type including visible, infrared, and ultraviolet light, and then emits it back out in a different direction, suitably, the light source may comprise at least one near infrared (‘NIR’) light source emitting light at a specific NIR wavelength. The HMD imaging sensor may thus be a NIR imaging sensor configured to output NIR image data, with the target or portion thereof comprising tissue excited by, absorbing, or reflecting the emitted NIR light.

[0013] Alternatively or complementarily, the light source may also, or instead, comprise at least one visible light source emitting light at a specific wavelength. A wavelength of the re-emittedlight may be in the same or a different range than the specific wavelength. For example, light absorbed by tissue, such as veins during vascularization analysis, may be re-emitted or reflected in substantially the same specific wavelength as the emitted light, whereas NIR light absorbed by tissues injected with a fluorophore may be re-emitted in a wavelength different to that of the emitted NIR light, or reflected to a varying extent by surrounding tissue.

[0014] Variant embodiments may comprise at least a first light source emitting light in a specific wavelength outside the visible spectrum and at least a second light source emitting light in a specific wavelength within the visible spectrum, wherein the first and second light sources have substantially the same collimation as each other and the second light source is configured to implement a visible edge at the periphery of the area of illumination. This particular configuration advantageously provides the HMD user with a visible reference frame within which to observe tissue excited by, absorbing, or reflecting the emitted non-visible, e.g. NIR, light.

[0015] Variant embodiments are considered for maximizing the usefulness of a low powered, highly-focused, lightweight illumination module according to the invention. In embodiments of the illumination module with an adjustable rate of illumination or intensity, light emission by the or each light source may be adjusted with light intensity modulating commands generated by the HMD device. The illumination module may be integral with the HMD device, or releasably securable thereto. In a variant, the illumination module and the HMD device may each comprise a respective modular interface for releasably securing one to the other. The illumination module may comprise a power source supplying the or each light source. Alternatively, the or each light source of the illumination module may be supplied by an electrical circuit of the HMD device including, in the case of an illumination module secured the HMD via a modular interface, through interconnects of the modular interface.

[0016] In embodiments of the illumination module, a shape of the or each aperture may be circular and a shape of the volume surrounded by the respective inner surface may be frusto- conical, having an apex section at or proximate the respective light source. Alternatively, a shape of the or each aperture may be oblong and a shape of the volume surrounded by the respective inner surface may be a trapezoidal prism. This alternative embodiment is particularly useful with embodiments of the illumination module comprising a combination of visible and non-visible light sources, configured to implement a visible edge at the periphery of the area of illumination. Alternatively still, a shape of the or each aperture may be oblong and a shape of the volume surrounded by the respective inner surface may comprise a trapezoidal prism adjacent the aperture transitioning into a frusto-cone adjacent the light source, accordingly wherein edges of the prismcorresponding to corners of the oblong aperture are progressively rounded along the inner surface towards the respective light source.

[0017] In a further aspect, the present invention provides a head mounted display (HMD) device comprising an illumination module comprising at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, wherein the internal surface is configured as a light guide and the module emits light frontally of the HMD towards a target, an imaging sensor capable of capturing light, re-emitted by at least a portion of the target in consequence of its illumination by the emitted light as image data, display means and data processing means operably connected to the illumination module, the imaging sensor and the display means, configured to output processed image data to the display means.

[0018] Embodiments of the HMD device may comprise variant embodiments of the illumination module as previously introduced and further described herein. In particular, in certain embodiments of the HMD apparatus, the illumination module may be either integral with the HMD device, or releasably secured thereto, for example as an illumination module. Accordingly embodiments of the HMD apparatus may further comprise a modular interface, wherein the illumination module is releasably secured to the HMD apparatus via the modular interface for powering and receiving light intensity modulating commands from the data processing means. This feature advantageously increases the range of illumination types for use with a given HMD, wherein illumination modules of varying types may be configured with distinct light sources of specific wavelengths, e.g. for use with different fluorophores, and / or in combination with monochromatic illumination, e.g. white.

[0019] In a further aspect, the present invention provides a method of illuminating a scene with an illumination module of a head mounted display (HMD) device, comprising the steps of providing an illumination module comprising at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, configuring the or each internal surface as a light guide, combining the illumination module with a head mounted display (HMD) device comprising display means, an imaging sensor capable of capturing light re-emitted by at least a portion of the target in consequence of its illumination by the emitted light as image data, and data processing means operably connected to the illumination module, the imaging sensor and the display means, configured to output processed image data to the display means, and emitting light frontally of the HMD towards a target with the an illumination module.

[0020] In embodiments of the method, the step of configuring may further comprise configuring the or each internal surface as a hollow reflector, having a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers. Alternatively, or additionally, the step of configuring may further comprise locating at least one lens member in the housing intermediate the or each light source and the respective aperture. Optionally, this may further involve making the lens member in a fully transparent material, having a smoothness of surface boundary with a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers.

[0021] With embodiments of the illumination module comprising at least one near infrared (‘NIR’) light source, the step of emitting light preferably comprises emitting NIR light with the NIR light source at a specific NIR wavelength.

[0022] With embodiments of the illumination module, or HMD device comprising same, wherein the illumination module comprises at least a first light source emitting light in a specific wavelength outside the visible spectrum, and at least a second light source emitting light in a specific wavelength within the visible spectrum, the step of emitting light further comprises emitting light outside the visible spectrum with the first light source and emitting light within the visible spectrum with the second light source, and the method may comprise the further steps of arranging the first and second light sources to have substantially the same collimation as each other, and configuring the second light source to implement a visible edge with visible light peripherally of an area of illumination of the first light source.

[0023] With embodiments of the HMD comprising a modular interface, the step of combining may comprise the further steps of releasably securing the illumination module to the HMD device via the modular interface. In variants, this may further involve powering the light sources of the illumination module via the modular interface

[0024] Other aspects of the invention are set out in the accompanying claims.BRIEF DESCRIPTION OF DRAWINGS

[0025] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: -Figure 1 illustrates a head mounted display (HMD) device according to the invention, comprising an illumination module as a source of illumination in a first embodiment of the invention, an imaging sensor and an optional ranging sensor.Figure 2 is a block diagram of a hardware architecture for the HMD device shown in Figure 1 , including data processing means, a memory and displays.Figure 3 shows a user wearing the HMD of Figures 1 and 2 and illustrates the illumination range of the illumination module and the respective fields of view of the camera and optional ranging sensor.Figure 4 details data processing steps performed by the HMD device of Figures 1 to 3 when the HMD includes a ranging sensor.Figure 5 illustrates the contents of the memory of Figure 2 at runtime when performing the steps of Figure 4.Figure 6 illustrates a further embodiment of the illumination module shown in Figures 1 to 4, with a point source of light wherein the housing is integral with the HMD and configured as a hollow reflector.Figure 7 illustrates a further embodiment of the illumination module shown in Figures 1 to 4B, with several point sources of light and respective homogeneous lens-like members within the housing, configured with a modular interface for releasable securing to the HMD.Figure 8 in a lateral view of the internal volume of a light guide shown in Figures 6 and 7, including a removeable homogenous lens-like member.Figure 9 illustrates a further embodiment of the illumination module shown in Figures 1 to 4B, with several point sources of light of varying types and respective homogeneous lens-like members.Figure 10 shows an imaging sensor view of incoherent irradiance generated with the illumination module embodiment of Figure 9 against a target surface.Figure 11 is a graph plotting the incoherent irradiance shown in Figure 10 against the target surface area, showing an illumination boundary.Figure 12 is a lateral view of an alternative embodiment of the internal volume of a light guide shown in Figures 6 to 9, including a removeable homogenous lens-like member.DETAILED DESCRIPTION OF DRAWINGS

[0026] There will now be described by way of example specific modes contemplated by the inventor. In the following description and accompanying figures, numerous specific details are set forth in order to provide a thorough understanding, wherein like reference numerals designate like features. It will be readily apparent to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail, to avoid obscuring the description unnecessarily.

[0027] With reference to Figures 1 to 5 initially, wherein like numerals reference like features, an embodiment of a head mounted display (‘HMD’) device is shown, in the example anaugmented reality (‘AR’) HMD device 10. The AR HMD device comprises a wearer visor 20, which includes optical combiners to optically combine computer-generated content with the real view of the world observable through a main see-though portion 22. The virtual content is rendered on eye-respective micro video displays 24A, 24B located equidistantly of a central bridge portion overlying a wearer’s nose in use, that are placed outside the wearer’s field of view.

[0028] Each micro video display portion 24A, 24B consists of a respective video display unit, in the example a micro OLED panel with a minimum 60 Hz frame refresh rate and a resolution of 1920x1080 pixels, located proximate a lower edge of the visor so as to leave the see-though portion 22 extending above it and up to its upper edge. Collimation lenses are placed between each micro display 24A, 24B and optical combiner to focus the virtual 2D image whereby the display portions 24A, 24B implement, perceptually, a single video display 24 occupying a subset of the front aspect of the HMD device, wherein the wearer can observe both the ambient physical environment across the see-though portion 22 and the computer-generated content at a predefined and comfortable viewing distance on a virtual image plane, namely the display focal plane.

[0029] The HMD 10 comprises at least one illumination module 30 capable of emitting light 32 of a specific wavelength towards a target in the environment frontally of the HMD, e.g. a surgery room wherein an HMD wearer 36 faces a patient 38 lying atop an examination table in the example shown in Figure 3. The illumination module 30 comprises at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, wherein the internal surface is configured as a light guide having maximal internal reflection, collimating the emitted light according to the optical principle of Total Internal Reflection (‘TIR’) for defining an area of illumination at the target having a sharp peripheral edge between adjacent lit and unlit surfaces.

[0030] Total internal reflection occurs when light within a transparent volume, for example a homogenous bloc of glass or polycarbonate, encounters a boundary with a material having a lower index of refraction, such as air, at an angle greater than the critical angle. According to Snell’s Law, the angle at which light emerges from the material depends upon both the angle at which the light hits the boundary of the material, and the difference in the refractive index of the material: the higher the index difference and the greater the angle of incidence, the greater the bending. Thus, for certain shapes with linear planes or boundaries, such as rectangular plates, pipes and cylinders, it is possible for light to become trapped inside the guide until it reaches edges or an end of the guide opposed to the light source, wherein it is transported with minimal losses.

[0031] In the first embodiment of the HMD shown and described, the illumination module is a near infrared (‘NIR’) light source 30, which emits NIR light 32 in a specific wavelength selected in the range 700 to 1 ,100 nm, for example 780 nm. The NIR light source 30 of the example is selected for fluorescence-based imaging and may be focused for energetic efficiency. Accordingly, upon administering a contrast agent to tissue of the patient 38, all or a portion of the patient tissue 39 that is targeted for observation, surgical removal or other purpose, re-emits the received light in a different wavelength, for example in the range 830 to 850 nm. Skilled readers will appreciate that the principle disclosed herein readily extends to any other type of target, including non-biological such as markers, for example a passive NIR marker of the Aruco™ type releasably secured to a location of the patient 38 selected for an orthopaedic procedure ; likewise to other types of illumination, wherein further embodiments described herein incorporate one or more visible light source(s) which emit light in a specific wavelength selected in the range 350 to 700 nm.

[0032] The HMD device 10 further comprises at least one imaging sensor 40 capable of capturing light re-emitted by the target tissue or portion thereof 39 in consequence of its illumination by the illumination module 30. Adverting to the source of NIR illumination of the example, accordingly the imaging sensor 40 is a high-resolution or full-resolution (‘NIR’) camera 40, with a field of view (‘FoV’) 42 of typically 70 to 90 degrees and which captures light in a wavelength range of typically 350 to 1100 nm. An alternative embodiment contemplates at least a pair of cameras 40 arranged in a stereoscopic configuration, shown in dotted lines in Figure 1. The (or each) NIR camera 40 outputs the captured re-emitted light as image data 340, namely a sequence of image frames 442, each at a resolution of 1280x1024 pixels or higher, and at a rate of 60 frames per second or more, wherein only light that is re-emitted by the target tissue or portion thereof 39 is encoded, illustrated as a solid grey color in Figure 3 for contrast relative to the patient and operating table content shown in dotted lines to denote their absence in each image frame 340.

[0033] Optionally, the HMD device 10 further comprises an environmental sensor 50 capable of detecting at least a portion of a target frontally of the HMD device and outputting sensor data representative of the target or portion thereof. In the embodiment shown and described, the environmental sensor 50 is a ranging sensor, for example a low-power single-zone or multi-zone time of flight (ToF) sensor, which does not require any specific computing unit, as even a microcontroller can process its output data, including under a relatively high capture or polling rate of 60 Hz or more. The optional environmental sensor 50 has a respective FoV 52 similar to the FoV 42 of the imaging sensor 40, comprising a single detection zone or cell or several suchzones e.g. arranged in a matrix, having a detection range d. The environmental sensor continuously polls the environment frontally of the HMD device 10 for targets 39 located in the range d within its FoV 52, as illustrated in Figure 3, for instance in the range 200 to 700 centimetres wherein targets of interest are expected to appear, for example tissue 39 of the patient 38 injected with a contrast agent and fluorescing under NIR illumination.

[0034] The skilled reader will understand that the present technique may be practiced with various types of environmental sensors which, subject to their characteristics and capacities, may allow a detection of target according to type and / or operational distance. By way of non-limitative example, the inventive concept disclosed herein is considered readily implementable by the skilled person with stereoscopic sensors, ToF sensors and matrix array sensors, with which sensor data can be selected as one or more of e.g. distance data and color data ; likewise depth- capable (3D perceptive) sensors with which different distances can be measured for different parts of the target tissue when the line of sight of the HMD device 10 is not substantially orthogonal to the target.

[0035] The HMD device 10 further comprises data processing means operably connected to the (or each) illumination module 30, the (or each) imaging sensor 40, the optional environmental sensor 50 and the displays 24A, 24B and, optionally, a data connectivity capacity. In particular, the HMD device 10 includes a data processing unit (‘CPU’) 201 , which is a general-purpose microprocessor, for instance according to the Cortex™ architecture manufactured by ARM™, acting as the main controller of the HMD device 10. The CPU 201 may further include a dedicated image signal processing (‘ISP’) unit or module to receive and pre-process image data generated by the imaging sensor 40 before outputting the corresponding image data to the CPU 201. When present, this ISP unit is either integral or coexists with the CPU 201 , that is programmed to perform other data processing tasks described hereafter. The CPU 201 is coupled with memory means 202, comprising volatile random-access memory (RAM), non-volatile random-access memory (NVRAM) or a combination thereof. The CPU 201 and memory 202 communicate data bilaterally over a data input / output bus 203, to which the other components of the HMD 10 are similarly connected, in order to provide headset functionality and receive user commands.

[0036] The data connection between the (or each) imaging sensor 40 and the CPU 201 , via the bus 203 or another, is a high-frequency data communication interface which is sensitive to external electromagnetic interference (EMI) and must be shielded accordingly. The respective data connections between the CPU 201 and the illumination module 30 and between the CPU 201 and the optional environmental sensor 50, via the bus 203 or another, are implemented as low-frequency data communication interfaces, for instance according to the l2C protocol, wherein any EMI over that interface is negligible considering the integer data type and volume output by the ranging sensor.

[0037] User input data may be received directly from a physical input interface 204, which may be one or more buttons, including at least an on / off switch, and / or a portion of the HMD casing configured for haptic interaction with a wearer’s touch. User input data may also be received as analogue sound wave data by a microphone 205, for which the CPU 201 (or a DSP unit or module, not shown) implements an analogue-to-digital converting function, and processed audio data is output to a speaker unit 206.

[0038] Power is supplied to all components by an electrical circuit 207, which is interfaced with an internal battery module 208, wherein the battery is periodically recharged by an electrical converter 209. Power is supplied to the above components by an electrical circuit 207, which is interfaced with an internal battery module 208, wherein the battery is periodically recharged by an electrical converter 209.

[0039] Embodiments of the HMD device may further include networking means 210, shown in dotted line in the figure as a wireless network interface card or module (WNIC) also connected to the data input / output bus 203 and the electrical circuit 207, apt to interface the HMD with a wireless local area network (‘WLAN’) generated by a local wireless router. Alternative or additional wireless data communication functionality may be provided by the same or another module, for example implementing a short-range data communication according to the Bluetooth™ and / or Near Field Communication (NFC) interoperability and data communication protocol.

[0040] Data processing configuration and basic functionality of the HMD device 10 of Figures 1 to 3 is now described by reference to Figure 4, which shows an example logic implementable by a HMD device 10 comprising an illumination module 30, wherein data structures stored in the memory 202 and processed by the CPU 201 are shown in Figure 5.

[0041] When powering up the HMD 10, an operating system (‘OS’) 501 is initially loaded at step 401 , for governing basic data processing, interdependence and interoperability of HMD components 201 to 209, including the WNIC 210 when present. The HMD OS may be based on Android™ distributed by Google™ of Mountain View, California, United States. The OS 501 includes subroutines for reading and processing input and output data, optionally including subroutines 502 to configure the HMD device 10 for bilateral network communication with remote terminals via the WNIC 210 interfacing with a network router device. Still at step 401 , a set ofinstructions 503 embodying a controlling application for the illumination module 30 is loaded, either as a subroutine of the OS 501 or in a higher computational layer. When loaded in a higher computational layer, the controlling application 503 is interfaced with the illumination module 30 and the ranging sensor 50 through the OS 501 via one or more Application Programmer Interfaces (API) 504.

[0042] At any specific time at runtime, data stored and circulating within the example architecture shown in Figure 2 according to the principles described herein includes the image data 340 output by the imaging sensor(s) 40, light source-modulating command data 530 generated by the CPU 201 , calibration data 533 representative of an intensity of the light emitted by the illumination module 30, then sensor data 550 generated by the environmental sensor 50 specifically, and also image data processed by the CPU 201 and output to the display units 24A, 24B plus, optionally, processed audio data output to the speaker unit 206.

[0043] The HMD 10 then initializes the environmental sensor 50 at step 402, whereby the environmental sensor 50 loads a discrete set of operating instructions 505, e.g. a sensor firmware, with which it initializes its detecting cell(s) for detecting one or more environmental characteristics : whenever a target 39 is present within its respective FoV 52 within the distance interval d, the ranging sensor 50 of the example generates a value representative of the environmental characteristic dependent upon the detected target 38, in the example a distance between the ranging sensor 50 and a surface of the detected target within its FoV 52, comprising target tissue 39 injected with the contrast agent. By reference to the example scene depicted in Figure 3, the patient tissue 38 is located approximately 60 centimetres from the sensor 50 wherein the or each sensor cell corresponding to same within the FoV 52 would be assigned a distance value of e.g. ‘60’ by way of sensor data.

[0044] In a computing context, the processing required for target detection as described typically resides in higher computing layers, e.g. at the application level, which is advantageous for processing operations involving complex models, but requires significant computational resources and power draw. The technique presented herein improves this approach by displacing the operational requirement of detecting targets to a lower computing layer, at the OS-kernel level, with significant relief on computing and power resources, at the expense of an additional component 50. This is made possible by the low-frequency interface of the environmental sensor 50 to the CPU 201 , recalling that even so simple a data processing unit as a microcontroller can process sensor values and output same to the CPU across that interface.

[0045] The HMD 10 may optionally load an application program 506 for additional data processing functionality, for example post-processing and rendering local and / or remote data to a user interface 507 which is initialized at step 403 and output to the VDlls 24A, 24B. In the absence of an optional application program 506, the HMD 10 still initializes a user interface 507 for the OS at said step 403, which is output to the VDlls 24A, 24B.

[0046] At step 404, the illumination controlling application 503 invokes a calibration routine for the illumination module 30, in order to adapt its intensity currently at the default level to specifics of the situational and environmental circumstances. By reference to illumination rates deemed optimal for ICG-based NIR applications, the routine can require the HMD wearer to locate the front of the HMD device 10 at a predetermined distance from the target tissue 39, wherein user adjustment instructions are relayed in the user interface 507 and are updated in quasi real-time by processed distance data 540, or substantially in real-time by sensor data 550 from the ranging sensor 50, alternatively at a predetermined distance from an external calibration reference, for example a physical reference pattern wherein adjustments are made until the position of image data 340 of the reference pattern in the GUI 507 is matched to the wearer’s view of the physical reference pattern through the see-through portion 22, over a predetermined period of time.The predetermined distance is preferably set as a function of the default intensity of the illumination module 30, for instance by reference to a stored distance value known to achieve the desired illumination rate with the default intensity at the target tissue 39. The predetermined period of time can be set as a function of the known time-to-complete of the initialization routine of the ranging sensor 50. The calibration routine establishes coincidence between the stored distance and the actual distance (SD) measured by the ranging sensor 50 through the target detection, over the predetermined period deemed to provide sufficient operational accuracy from the ranging sensor 50 since starting the HMD 10 and which, when achieved, is set by the illumination controlling application 503 as a situational datum embodying an illumination constraint (IC) 533 at step 405.

[0047] Upon completing the calibrating routine, image data 340 received by the CPU 201 from the imaging sensor 40 is processed by the CPU configured by the OS 501 at step 406 for out- putting to the user interface 507 at step 407, wherein the processed image data is optionally augmented with the application program 506 when present.

[0048] In parallel with this sequence, as the ranging sensor 50 remains operative as soon as initialised independently of the CPU 201 activity and tasks whilever the HMD device 10 remains in use, target tissue 39 present within the FoV 52 of the ranging sensor 50 continuously triggersits detection zone(s), representative of a detection event, in real time. The ranging sensor 50 processes each detection event with determining a value for the one or more zone(s) involved by that respective detection event, in the example the distance between itself and the surface of the target tissue 39, and thus continuously outputs distance data (SD) as environmental sensor data 550 to the CPU 201 at step 408. In this embodiment, the processing of image data 340 at step 406 advantageously excludes the computation (extrapolation) of distance data, accordingly relieving the data processing load placed upon the CPU 201.

[0049] At step 409, the CPU 201 configured by the illumination control application 503 compares the real-time distance data (SD) 550 of step 408B, with the illumination constraint (IC) distance data 533 of step 405 to compute a positive or negative difference relative to the situational datum, wherein a positive difference is representative of movement of the HMD 10 away from the target tissue 39, requiring an increase in the intensity of the illumination module 30 for maintaining the desired illumination rate at the target tissue 39 and, reciprocally, a negative difference is representative of the reverse situation, requiring an decrease in the intensity of the illumination module.

[0050] At step 410, the CPU 201 configured by the illumination control application 503 computes command data 530 for modulating the light intensity of the illumination module 30 according to the comparison result of step 409, wherein the command data encodes an intensity increase in case of a positive difference, respectively an intensity decrease in case of a negative difference, and wherein the value for the intensity increase or decrease is computed in proportion to the scale of the difference relative to the normative reference that is the illumination constraint 533. The computed command data 530 is then output to the illumination module 30 for modulating its intensity at step 411.

[0051] Control thereafter returns to a next iteration of step 406, wherein the next image data 340 (N+1) received from the imaging sensor 40 is processed, with reference to Figure 4A, for extracting the next processed distance data 540 from the next sequence of image frames 340 and outputting to the user interface 507 at a next iteration of step 407.

[0052] This next image data is captured by the imaging sensor 40 in optimal lighting conditions irrespective of a change in the distance between the HMD 10 and the target tissue 39 since the previous image data 340 was captured, processed and output, as a result of the real-time adjustment of the illumination module intensity per steps 409 to 411 , which maintains the rate of illumination of the target tissue 39 substantially at the rate embodied by the illumination constraint 533.

[0053] The environmental sensor 50 remains operative independently of the CPU 201 activity and tasks whilever the HMD device 10 remains in use, and its firmware may implement additional functionality, notably a switching between active and idle states, inclusive of the illumination module 30 and / or imaging sensor(s) 40, according to preset periods of non-detection, for enhanced power conservation.

[0054] With reference to Figures 6 to 9 now, wherein like numerals reference like features, specific embodiments of the illumination module 30 are contemplated by the inventors, which are optimized for minimal draw on the battery 208 by combining a low-power light source with the light guide technique described hereinbefore.

[0055] A further embodiment of the illumination module 30 shown both in a front aspect and a side elevation in Figure 6 comprises an illumination module 30 embodied as a permanent feature of the HMD front fascia. The module 30 comprises a housing 610 internally of the HMD device 10, wherein a point source of light 620, for instance a light emitting diode (‘LED’) or an arrangement or array of closely-adjacent LEDs, is contained within the housing proximate a first closed extremity thereof, distal the HMD device front surface. The LED 620 is supplied with power by the electrical circuit 207 of the HMD device 10 and interfaced with the CPU 201 through the data bus 203. Lasers are considered another suitable example of point source of light compatible with the principles and embodiments presently described herein, which may be preferred for certain applications but at the cost of a higher power consumption relative to LEDs and the like.

[0056] The housing 610 comprises an aperture 630 at a second extremity opposed to the first, accordingly distal the LED 620 and opening frontally of the HMD device 10 towards potential target(s) 39. The LED 620 has a smaller dimension than the aperture 630 and the internal volume 640 of the housing extending therebetween is substantially circular, accordingly a shape of the internal volume 640 is generally frusto-conical, with the LED 620 located substantially at its apex and wherein an inner surface 650 of the internal volume tapers outward, towards and up to the aperture 630.

[0057] The internal volume 640 is configured as a light guide, to collimate substantially all of the light emitted by the LED 620 frontally of the HMD device 10 towards potential target(s) 39. Depending on the illumination wavelength of the LED 620 and the intended application, the internal volume 640 may be hollow, wherein the inner surface 650 is preferably straight with high smoothness e.g. with a root mean square (RMS) height (Sq) in the range 50 pm to 5 pm, or by applying a coating to the surface having corresponding properties: materials configured into light guidescan bend and take on more complex shapes, but light leakage occurs most at any bends, dots and / or other surface asperities, at which light trapped by TIR encounters the guide boundary at less than the critical angle and thus escapes the light guide.

[0058] A known formula for the total fraction of scattered light (Bennett and Porteus, 01-Feb- 1961 , Journal of the Optical Society of America) is:wherein TISBP is the total integrated scatter, Ro is the nominal reflection of the surface (assumed 1.0 for TIR), 0j is the angle of incidence, Rqis the surface roughness and A is the wavelength. Using this formula requires that the units of Rq and A must be the same, e.g. both nanometres, microns, or others. Although the formula is exponential, it is effectively linear when (Rq A) < Angles of incidence within the internal volume 640 range from about 45 degrees to more than 80 degrees, whereby an angle of 1 radian is typical. At this angle of incidence for e.g. NIR light with a wavelength of 780nm, a surface roughness of 1nm gives a TIS value under 1 %. However, as most of this scatter is forward scatter, i.e. light that is scattered only a limited angle of deviation away from the specular beam, this light is not lost, because it will still land within the illumination field, even though it may blur marginally the edge of that field.

[0059] The interface between the LED 620 and the light guide 640 is designed to minimize Fresnel loss. Suitably the LED 620 may be a flat-type LED rather than a lens-type LED, affixed centrally of a substantially planar entrance surface 660 of the light guide, i.e. co-axially with a main axis A of the internal volume 640, for an optimal combination of light extraction properties with a simple and compact design. A front aspect or surface 670 of the light guide 640 at the level of the aperture 630 and orthogonal to the entrance surface 660 is designed as a diffusor.

[0060] Accordingly, light rays emitted by the LED 620 strike the input face 660 at any angle and are bent at the interface such that they experience TIR at the walls 650 of the light guide. Once light rays have entered the light guide, they bounce about and between the curved surfaces 650 and keep travelling until they reach the diffusor 670 at the far end of the light guide, wherein any light passing through the light guide entrance 660 face undergoes TIR inside the guide.

[0061] A further embodiment of the illumination module 30 shown in a front aspect and side elevation in Figure 7 comprises an illumination module 700 configured as a removable feature of the HMD device 10. For this embodiment, the HMD device comprises a modular interface (not shown) adapted to releasably secure the module 700 thereto, and to transmit both power andlight intensity commands to the module in use, thus interfaced with both the electrical circuit 207 and the CPU 201 through the data bus 203.

[0062] In this embodiment, the illumination module 700 again comprises a housing 610, an underside of which is configured with a module interface 710 designed to engage with and releas- ably secure to the HMD device’s modular interface, for instance with a tongue-and-groove configuration 720 comprising suitable power and data interconnects, such as spring-loaded (or ‘pogo’) pins, and a releasable clip or magnetic mechanism. In this embodiment still, the module 700 comprises a plurality of point sources of light 620I-N, for instance several light emitting diodes (‘LED’) of lower individual power relative to the unique LED of the first embodiment, arranged in a matrix, contained within the housing proximate a first lateral extremity thereof, and supplied with power and interfaced with the CPU 201 through the mated modular interfaces.

[0063] The housing 700 comprises a plurality of light guides, one respective for each point light source 620I-N, each having an aperture 630N at a second lateral extremity opposed to the first, thus distal the corresponding point light source 620Nand opening frontally of the illumination module 700 towards potential target(s) 39 frontally of the HMD device 10. The internal surface 640 of each light guide extending peripherally of each point light source 620I.Ntowards and up to the corresponding and respective aperture 630Nis again configured for Total Internal Reflection (‘TIR’), as a hollow reflector in order to collimate light emitted by the point light source 620.

[0064] Skilled readers will appreciate that the principles disclosed herein contemplate sources of illumination 30 distinct from the HMD device 10, but operatively connected therewith for at least data communication. The principles disclosed herein also contemplate sources of illumination 30 configured with an internal power source for supplying the light source(s), in order to relieve the draw on the HMD device’s battery 208, preferably factoring in ergonomic requirements, in particular the total weight of an illumination module with its own battery when designed for removable attachment to and frontally of an HMD device 10.

[0065] With reference to Figure 8, rather than be configured as a hollow reflector, the or each internal volume 640 may be occupied by a bloc 800 of solid material with a high refractive index, for instance an optically transparent plastic or polycarbonate material, in the example having a frusto-conical outer shape matching the frusto-conical shape of the inner surface 650 of each light guide, and provided with its high reflection characteristic by suitable surface finishing of the bloc’s outer surface to the requisite degree of smoothness, or by applying a suitable coating thereto, i.e. intermediate the outer longitudinal surface of the bloc 800 and the housing inner curved surface 650. The configuration using several point light sources with respective lensmembers is preferable to a single, much larger LED, the optical member for collimating which would be correspondingly large and heavy, and accordingly unergonomic, and also much more powerful in order to generate comparable level of illumination, for example over 1 mW per cm2at a 50 cm distance, a common context for surgical applications.

[0066] The skilled person will readily understand that the principles of a light guide implementing TIR and a modular interface described by reference to the illumination module 30 may be extended to other types of illuminators and sensors, advantageously accommodating a variety of illuminators with respective wavelengths of illumination, for example for illuminating and detecting contrast agents with different fluorophore wavelengths, and extending to visible light sources and combinations of multiple light types, including laser light sources. For example, for illuminators comprising light source(s) with a specific wavelength outside the visible spectrum, the addition of at least one visible light source having substantially the same collimation as that of the non-visible light-source(s), and highly focused to implement a visible edge at the periphery of the juxtaposed area of illumination, advantageously informs the user about the area of illumination of the non-visible light source(s).

[0067] A variant with multiple type light sources is shown in Figure 9, wherein like numerals still reference like features. A further embodiment of the illumination module 30 is shown in a front aspect, which comprises a removable illumination module 900 with a housing 910, having an underside configured with a module interface 710 as previously described. The module 900 also comprises a plurality of point sources of light, in this embodiment combining five NIR light LEDs 620 and a visible light LED 920, e.g. a white light, arranged in a matrix, with the visible light LED mounted centrally of a top row of three point light sources, the five NIR light LEDs forming a U- shape about the white light LED. The removable illumination module 900 further comprises a cooling system, mounted at the rear of the LEDs.

[0068] The housing 910 again comprises a plurality of light guides, one respective for each point light source 620, 920, wherein the internal surface 940 of each light guide extending peripherally of each point light source 620, 920 towards and up to the corresponding aperture is again configured for Total Internal Reflection (‘TIR’), either as a hollow reflector or with a lens-like bloc 800, in order to collimate light emitted by the point light source 620, 920. In this embodiment, each aperture has an oblong or square cross-section, whereby a shape of the internal volume between the corresponding aperture and the respective LED, likewise a shape of each bloc 800 if used, is generally a trapezoidal prism matching the trapezoidal prism shape of the inner surface.

[0069] The apertures, and optional lens members 800 when present, have substantially the same collimation as each other, i.e. each light guide is collimated with the imaging sensor 40, and the oblong or square cross-section of each aperture has an aspect ratio in proportion with the image(-capturing) aspect ratio of the imaging sensor 40. Accordingly, the area respectively illuminated by each of the visible (920) and non-visible (620) light sources is suitably oblong itself, suitably matching the aspect ratio of the image data 340 captured by NIR camera 40 and wherein the white light generated by the visible light LED 920 defines a sharp edge which remains visible peripherally of the total area of illumination of the module 900, itself illuminated in a homogenous manner.

[0070] A dual advantage of this configuration is that, as the NIR surface illumination is matched and effectively bounded by the visible illumination area, wherein any illumination beyond that periphery is minimal and drop offs sharply, the HMD wearer 36 remains visually aware at all times, of which portion of the target 39 within their FoV is NIR illuminated, moreover the NIR surface illumination remains matched with the registration area of the NIR camera, inclusive of any zooming in or out. This is illustrated in Figure 10, which shows incoherent irradiance generated with the illumination module embodiment presently described against a target 39, as captured by an imaging sensor 40, and in Figure 11 , which maps the incoherent irradiance of Figure 10 against the target surface area.

[0071] The advantage is particularly noticeable relative to a substantially circular illuminated area resulting from circular apertures, wherein much of the effective illumination is lost at the periphery due to the aspect ratio mismatch with the oblong or square camera aspect ratio, presenting an operational risk since the HMD wearer 36 is unable to distinguish at such peripheral locations, whether the tissue 39 is NIR illuminated or not.

[0072] Light transmission and distribution within the illuminated area can be further enhanced still, with optimized geometries of the internal surface 650 of the light guide. With reference to Figure 12, an alternative embodiment of a light guide 640 according to the invention combines the advantages associated with an oblong aperture 630, with the optimally-homogenous capture and distribution of light emitted by the light source 620, 920 associated with a conical shape.

[0073] In the light guide embodiment shown in Figure 12, a shape of the aperture 630 is suitably oblong, and a shape of the inner surface 650 immediately adjacent the light source 620, 920 is suitably circular, wherein a diameter of the inner surface 650 immediately adjacent the light source 620, 920 is substantially smaller than a diagonal dimension of the aperture 630, thus wherein the volume of the light guide bounded by the inner surface 650 tapers outward from thelight source towards the aperture. The inner surface 650 defining four orthogonal sides immediately adjacent the aperture transitions gradually and smoothly into a conical shape immediately adjacent the light source, whereby a first portion 1010 of the light guide volume is shaped as a trapezoidal prism, effectively transitioning into a second portion 1020 of the light guide volume shaped as a frusto-cone.

[0074] Irrespective of illumination module embodiment, likewise irrespective of whether the illumination module embodiment is integral with the HMD device 10 as shown in Figures 1 to 3, or releasably secured thereto as shown in Figures 7 and 9, methods of illuminating a scene with the illumination module 30 involve at least configuring the or each internal surface of the module as a light guide 640, combining the illumination module 30 with the HMD device 10 and emitting light frontally of the HMD 10 towards the target 39 with the illumination module 30.

[0075] Given the modularity of design for the illumination module 30 and the multiplicity of light types implementable therein, the inventors contemplate alternative embodiments of the method of illumination control. By way of non-limitative example, for variants sources of illumination 30 incorporating a plurality of light sources, e.g. point light sources 620, of a same illumination type, e.g. NIR of a specific wavelength, the controlling command may be computed at step 410, and the command data 530 may encode accordingly, to selectively switch one or more discrete light sources 620I.Non or off according to the adjustment in light intensity determined from the comparison of step 409. A less ergonomic but simpler manual switching of the discrete light sources 620I-Nis considered within the scope of the inventive principle disclosed herein.

[0076] Alternatively, for variant illumination modules 30 incorporating a plurality of light sources with respective illumination types, e.g. wherein one or more point light sources are NIR illuminators with a distinct specific wavelength relative to others, the method may comprise an initial step of selecting the light source(s) suitable for the situational and environmental circumstances amongst all others, for example by manually switching on discrete light sources 620I-N deemed suitable, at or prior to the calibration and setting of steps 404, 405, wherein the controlling command is computed at step 410, and the command data 530 encode accordingly, to selectively modulate only the selected light source(s) 620I-N according to the adjustment in light intensity determined from the comparison of step 409.

[0077] Alternatively still, variant sources of illumination 30 incorporating one or more light source(s) may implement respective illumination types with an optical filtration arrangement modifying the specific wavelength of the one or more light source(s) according to the HMD wearer’s specific requirement during use. Alternatively still, or additionally, embodiments of themethod may implement a safety protocol for illumination module 30 incorporating highly-focused light source(s), such as the module 900.

[0078] The combination of an illumination module 30 with a HMD device 10 and methods described herein accordingly optimises user-machine interaction, providing an illumination technique apt to enhance the visual perception of an HMD wearer onto targets in their line of sight in the environment, that are targeted for interaction such as a surgical procedure, e.g. breast cancer nodules. Medical applications generally expected to benefit from all embodiments of the HMD device with one or more sources of illumination modulated according to the technique disclosed herein include, non-exhaustively, those based upon auto-fluorescence of veins (e.g. taking blood, blood perfusion and vascularization analysis in any surgery), fluorophore detection (oncology, lymph nodes, blood perfusion and vascularization analysis in any surgery) and target markers (i.e. orthopaedics, oncology, robotics).

[0079] In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa. The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims

CLAIMS1. An illumination module for use with a HMD device, comprising at least one light source ; a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, wherein the or each internal surface is configured as a light guide.

2. The illumination module according to claim 1 , wherein the or each internal surface is configured as a hollow reflector, having a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers.

3. The illumination module according to claim 1 or 2, further comprising at least one lens member located within the housing intermediate the or each light source and the respective aperture ; optionally wherein the lens member is homogeneous and made of a fully transparent material, having a smoothness of surface boundary with a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers.

4. The illumination module according to any of claims 1 to 3, wherein the or each light source emits light in a specific wavelength in the range 350 to 1,100 nanometers.

5. The illumination module according to claim 4, wherein the light source comprises at least one near infrared (‘NIR’) light source emitting light at a specific NIR wavelength.

6. The illumination module to claim 4 or 5, wherein at least a first light source emits light in a specific wavelength outside the visible spectrum and at least a second light source emits light in a specific wavelength within the visible spectrum, wherein the first and second light sources have substantially the same collimation as each other and the second light source is configured to implement a visible edge at the periphery of the area of illumination.

7. The illumination module to any of claims 1 to 6, wherein the or each light source is configured with an adjustable rate of illumination, and wherein a rate of illumination of the or each light source is commanded by the HMD device.

8. The illumination module according to any of claims 1 to 7, wherein the illumination module is integral with the HMD device ; or wherein the illumination module is releasably securable to the HMD device ; andoptionally wherein the illumination module and the HMD device each comprises a respective modular interface for releasably securing one to the other.

9. The illumination module according to any of claims 1 to 8, wherein the illumination module comprises a power source supplying the or each light source ; or wherein the or each light source of the illumination module is supplied by an electrical circuit of the HMD device.

10. The illumination module according to any of claims 1 to 9, wherein a shape of the or each aperture is circular and a shape of the volume surrounded by the respective inner surface is frusto-conical ; or wherein a shape of the or each aperture is oblong and a shape of the volume surrounded by the respective inner surface is a trapezoidal prism; or wherein a shape of the or each aperture is oblong and a shape of the volume surrounded by the respective inner surface comprises a trapezoidal prism adjacent the aperture transitioning into a frusto-cone adjacent the light source.

11. A head mounted display (HMD) device comprising- an illumination module comprising at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture, wherein the internal surface is configured as a light guide and the module emits light frontally of the HMD towards a target; an imaging sensor capable of capturing light, re-emitted by at least a portion of the target in consequence of its illumination by the emitted light, as image data; display means ; and data processing means operably connected to the illumination module, the imaging sensor and the display means, configured to output processed image data to the display means.

12. The HMD device according to claim 11 , wherein the illumination module comprises at least one near infrared (‘NIR’) light source emitting light at a specific NIR wavelength, the imaging sensor is a NIR imaging sensor configured to output NIR image data, and the target or portion thereof comprises tissue excited by, absorbing and / or reflecting emitted NIR light.

13. The HMD device according to claim 11 or 12, wherein the illumination module comprises at least one visible light source emitting light at a specific wavelength, and the target or portion thereof comprises tissue excited by, absorbing and / or reflecting the emitted light.

14. The HMD device according to claim 12 or 13, wherein the illumination module comprises at least a first light source emitting light in a specific wavelength outside the visible spectrum, at least a second light source emitting light in a specific wavelength within the visible spectrum, the first and second light sources having substantially the same collimation as each other and the second light source being configured to implement a visible edge with visible light peripherally of an area of illumination of the first light source.

15. The HMD device according to any of claims 11 to 14, wherein a shape of the or each aperture has an aspect ratio in proportion with an image-capturing ratio of the imaging sensor and wherein the or each light guide is collimated with the imaging sensor.

16. The HMD device according to any of claims 11 to 15, wherein illumination module is integral with the HMD device or releasably secured thereto.

17. The HMD device according to claim 16, further comprising a modular interface for releasably securing the illumination module thereto; and optionally wherein the or each light source of the illumination module is supplied by an electrical circuit of the HMD device through the modular interface.

18. A method of illuminating a scene with an illumination module of a head mounted display (HMD) device, comprising the steps of- providing an illumination module comprising at least one light source, a housing having an aperture distal the or each light source and an internal surface intermediate the or each light source and the respective aperture ; configuring the or each internal surface as a light guide ; combining the illumination module with a head mounted display (HMD) device comprising display means, an imaging sensor capable of capturing light re-emitted by at least a portion of the target in consequence of its illumination by the emitted light as image data, and data processing means operably connected to the illumination module, the imaging sensor and the display means, configured to output processed image data to the display means ; and emitting light frontally of the HMD towards the target with the illumination module.

19. The method according to claim 18, wherein the step of configuring further comprises configuring the or each internal surface as a hollow reflector, having a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers.

20. The method according to claim 18 or 19, wherein the step of configuring further comprises locating at least one lens member in the housing intermediate the or each light source and the respective aperture ; and optionally making the lens member in a fully transparent material, having a smoothness of surface boundary with a root mean square (RMS) height (Sq) in the range 50 picometers to 5 micrometers.

21. The method according to any of claims 18 to 20, wherein the illumination module comprises at least one near infrared (‘NIR’) light source, and the step of emitting light further comprises emitting NIR light with the NIR light source at a specific NIR wavelength.

22. The method according to any of claims 18 to 21 , wherein the illumination module comprises at least a first light source emitting light in a specific wavelength outside the visible spectrum, and at least a second light source emitting light in a specific wavelength within the visible spectrum, the step of emitting light further comprises emitting light outside the visible spectrum with the first light source and emitting light within the visible spectrum with the second light source, the method comprising the further steps of- arranging the first and second light sources to have substantially the same collimation as each other, and configuring the second light source to implement a visible edge with visible light peripherally of an area of illumination of the first light source.

23. The method according to any of claims 18 to 22, wherein the step of providing further comprises providing the or each aperture with an oblong or square shape ; and optionally, wherein the step of configuring further comprises configuring a shape of the volume surrounded by the respective inner surface to comprise a trapezoidal prism adjacent the aperture transitioning into a frusto-cone adjacent the light source.

24. The method according to any claims 18 to 23, wherein a shape of the or each aperture has an aspect ratio in proportion with an image-capturing ratio of the imaging sensor and wherein the step of combining further comprises collimating the or each light guide with the imaging sensor.

25. The method according to any of claims 18 to 23, wherein the HMD device comprises a modular interface, the step of combining comprising the further steps of- releasably securing the illumination module to the HMD device via the modular interface ; and optionallypowering the source of illumination via the modular interface.

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