Patent Description:
Some displays, such as head-mounted displays (HMDs), head-worn displays (HWDs) or Heads Up Displays (HUD) are targeted to be as small and lightweight as possible. An example HMD <NUM> is illustrated in <FIG>.

HMDs, such as HMD <NUM>, are wearable by a user by means of an appropriate support <NUM>. The support includes one or more optical elements <NUM> which can be viewed by one of both eyes of the user. Although not shown in detail, the optical elements <NUM> include a substantially transparent display medium. The user is able to view the exterior environment through the optical elements <NUM>. The user is also able to view images relayed to the eye of the user in use via the HMD.

In conventional systems, images are relayed to the eye of the user in use using lens trains or folded optical designs. Lens trains or folded optical designs are incorporated into the HMD <NUM>. Traditionally, lens trains or folded optical designs are incorporated within the support <NUM> of the HMD <NUM>.

Traditional optical lens trains are linear and non-folded for simplicity. Multiple elements are usually used to achieve the performance required. For this reason, they are not particularly suitable for use in modern HMDs that are required to be compact, lightweight, and optimised for anthropometric data.

Traditional folded optical designs can be more compact, but can also introduce light loss mechanisms, reducing system efficiency. One of the simplest folded optical designs consists of an optical arrangement <NUM> as shown in <FIG>.

The optical arrangement <NUM> comprises a beamsplitter <NUM> and a spherical combiner <NUM>. In use, images are directed from a display source <NUM> or relay lens onto the beamsplitter <NUM>. The beamsplitter <NUM> partially reflects the images onto the concave surface of the spherical combiner <NUM>. It can be understood that the surface is concave relative to the input light. The spherical combiner <NUM> reflects a collimated exit pupil through the beamsplitter <NUM> towards the user's eye <NUM>.

However, if used in a HMD the optical arrangement <NUM> has to be adapted to enable the user to view the exterior environment <NUM>. To do this, the beamsplitter <NUM> and spherical combiner <NUM> must be at least semi-transparent. As a result, some image source light is lost upon interaction with each element, as light is lost when only partial reflection occurs. Therefore the image presented to the eye is dimmer than desired. In addition, the light must pass twice through the beamsplitter <NUM>, and this also increases the losses and can introduce ghost images.

A further disadvantage of optical arrangements <NUM> is that they are often not sufficiently lightweight and compact. The geometry of the beamsplitter <NUM> and spherical combiner <NUM> have to be matched to the exit pupil requirement, and so have to be sufficiently large to cater for the required anthropometric range, thereby increasing the size of the optical geometry.

An improvement to the optical arrangement is provided in <CIT> and in expired <CIT> and <CIT>. These patents show folded optical arrangements having multi-part folded eyepiece and relay lens assemblies.

An example of a known folded optical arrangement <NUM> found in these patents is shown in <FIG>. The optical arrangement <NUM> has a compact eyepiece in an off-axis solid arrangement. The arrangement <NUM> comprises a wedge <NUM>, prism <NUM>, and cemented makeup piece <NUM>. The prism <NUM> receives an image from a relay lens <NUM> or display source. The wedge <NUM> and prism <NUM> together may form a collimated image for viewing by the user's eye <NUM>. The makeup piece <NUM> optically manipulates light to counteract the effects of the prism <NUM> and wedge <NUM> so that the user can correctly view the exterior environment <NUM>. The makeup piece <NUM> is typically bonded to the prism <NUM> through means of an optical adhesive.

As can be seen in <FIG>, the prism makes use of a transmission surface and a conventional curved combiner surface. Similarly, the wedge has two transmission surfaces. The make-up piece also has a surface matched to the combiner surface of the prism.

While the folded optical arrangement <NUM> solves some of the problems identified with the optical arrangement <NUM>, new issues can be introduced. For example, the optical arrangement <NUM> of <FIG> contains off-axis components of simple surface form, for example spherical and cylindrical surfaces, which cause residual aberrations in the viewed image such as astigmatism and distortion.

Additionally the optical arrangement <NUM> of <FIG> is ideally suited for use with an image source of controlled numerical aperture (NA). In such arrangements, a relay lens with internal hard-stop is used to vignette unwanted light/rays to control the size of the resultant system exit pupil.

However, if such an optical arrangement is paired with a flat panel display, emissive display or direct image source with uncontrolled NA (excluding the use of a relay lens), the exit pupil size may not be controlled and unwanted light can propagate through the optical system resulting in a larger exit pupil which may not be fully corrected to remove aberrations. In <FIG> the unwanted light is shown by reference numeral <NUM>. In this scenario, if the user's pupil is axially aligned to the centre of the exit pupil the display appears well corrected. Disadvantageously, movement of the eye or optical arrangement results in the user viewing areas of the exit pupil with mainly uncorrected light. In these areas, the image may appear blurry, distorted, or incorrect, and this is a clear disadvantage in a high performance conformal display.

Accordingly, one object of the present invention is to overcome the problems of existing folded optical arrangements for use in HMDs.

According to a first aspect of the present invention there is provided a folded optical arrangement for use in a view-through display to transmit an image from an image source to a user's eye, the arrangement providing a folded optical transmission path and comprising: an optical system having a first optical element comprising a first plurality of optically powered surfaces; and a second optical element comprising at least one optically powered surface, the optical system configured to receive light forming the image from the image source, and configured to present a virtual image of the image source to the user with an apparent focus between a predetermined distance and optical infinity; and a compensator element located between the first optical element and an external view to receive the external view for combination with the image output from the optical system, wherein the first plurality of optically powered surfaces and the at least one optically powered surface of the second optical element are arranged to define a plurality of interfaces along the folded optical path and wherein a refractive index change at each interface is predetermined to control the direction of light passing through the interface; wherein one surface of the first optical element and one surface of the second optical element are adjacent to one another and each define an angle with a respective other surface of the relevant optical element at opposing ends of the adjacent surfaces and wherein the opposing angles are not equal; and wherein the adjacent surfaces are non-complementary or non-sympathetic such that there is no way to place the surfaces together without leaving a gap.

Preferably, the compensator element is adapted to minimise refractive errors induced on the external view by either the first optical element or the second optical element.

Preferably, the compensator element is optically bonded to the first optical element.

Preferably, one surface of the compensator element is matched to a surface of the first optical element.

Preferably, the first optical element comprises at least three optically powered surfaces.

Preferably, the first optical element comprises a single-piece three-sided element having an elongate, substantially triangular cross-section.

Preferably, the second optical element comprises at least two optically powered surfaces.

Preferably, the second optical element is a wedge.

Preferably, the first optical element and the second optical element are arranged in juxtaposition with one another so that the adjacent surfaces of the first and second optical elements are substantially aligned.

Preferably, the adjacent surfaces of the first and second optical elements are separated by a gap.

Preferably, the first optical element comprises a substantially concave surface.

Preferably, at least one of the optically powered surfaces of the first optical element is described by a multiple order polynomial.

Preferably, one of the angles is less than <NUM>°.

According to a second aspect of the present invention there is provided a display comprising a folded optical arrangement according to the first aspect of the invention.

Preferably, the display is in the form of at least one of a head mounted display, a head worn display and a heads up display.

In general, the present invention relates to improvement in or relating to optical improvements for displays such as for example a head mounted or head worn display (HMD, HWD respectively) or a heads up display (HUD). In particular, the present invention relates to a folded optical arrangement for use in a display to transmit images from an image plane to a user's eye, and to a display incorporating the folded optical arrangement.

An exemplary display such as for example a HMD or HWD according to the present invention comprises a folded optical arrangement, such as the optical arrangement of <FIG> (which will be described later), in order to overcome the problems of existing arrangements in the art.

<FIG> shows a cross-sectional of a folded optical arrangement <NUM> according to an embodiment of the invention. <FIG> shows the path of light rays through the optical arrangement <NUM> from an image source <NUM> to a user's eye <NUM> via an optical field lens <NUM> and focusing optic <NUM>. In addition, light passes from the exterior environment to the user's eye <NUM> via the focusing optics <NUM> and a compensator element <NUM>.

The optical arrangement <NUM> comprises an optical system <NUM>, also described as focusing optics. Light forming an image from the image source <NUM> is directed towards the optical system <NUM>. The optical system <NUM> receives the light forming the image. The light is focused by the optical system <NUM> to create a virtual image at an apparent focal distance. The virtual image is output from the optical system <NUM> and transmitted towards the location of the user's eye <NUM>. In addition light passes through the focusing optics from the external view towards the eye. Therefore the user is able to view the virtual image simultaneously with the external view of the outside world. The virtual image would typically be focussed at a predetermined apparent distance of between for example about <NUM> and optical infinity.

For the purposes of the figures, it will be assumed that the user's eye <NUM> is in the location shown, and references to the user's eye should be interpreted to mean that the typical use case is being described. However, it will be appreciated that the user's eye is not required for the invention to operate according to the principles set out herein. The optical arrangements described below ultimately generate exit pupils in the direction of an assumed position of the user's eye when the device is in use, regardless of where the user's eye actually is.

An enlarged representation of the optical system <NUM> is shown in <FIG>. The optical system <NUM> comprises a first optically powered optical element <NUM> (hereinafter referred to as a prism element) and a second optically powered element <NUM> (hereinafter referred to as a wedge element). The prism element <NUM> and wedge element <NUM> operate as an optical lens system, to focus the light for output towards the eye <NUM>. The prism element <NUM> and wedge element <NUM> may also be configured to optimise or counteract unwanted optical aberrations that are typically introduced by optical lens arrangements. For example, surface features of the prism element <NUM> and/or wedge element <NUM> may be configured to reduce aberrations and/or correct any other optical defects. The use of the two elements, whilst maintaining a space, such as an air space, between the elements allows the collimating element to operate as an spaced optical doublet to improve chromatic correction. Furthermore a third optical element, such as a field lens <NUM> as shown in <FIG>, may be added between the prism element <NUM> and image source to provide additional optical correction.

<FIG> shows a simple representation of the prism element <NUM> and the wedge element <NUM> to show example angular orientations. It should be noted that in <FIG> the surfaces of the elements are represented as flat for convenience, however the surfaces may be curved as is described with reference to <FIG> and <FIG>. <FIG> shows a normal <NUM> relative to a wedge surface <NUM>, from which the light exits the focussing element. The normal is located at the centre of surface <NUM> and crosses surface <NUM> at point K. The normal extends to a point L where it intersects surface <NUM>; to a point M where it intersects surface <NUM>; and to a point N where it intersects surface <NUM>. In addition, two angles are indicated <NUM> for the prism element and <NUM> for the wedge element at point J. Angle <NUM> is defined by surfaces <NUM> and <NUM> whilst angle <NUM> is defined by surfaces <NUM> and <NUM>. The angle <NUM> can be determined based on the tangent of its angle. The tangent of angle <NUM> being: <MAT>.

Surfaces <NUM> and <NUM> are referred to herein as the adjacent surfaces of the respective wedge element and prism element as these are adjacent to one another in the normal orientation of the focussing elements. One of the angles (<NUM>, <NUM>) is defined at one end of the adjacent surfaces and the other is defined at the other end of the adjacent surfaces. In other words, the angles are at opposing end of the adjacent surfaces. The interface between the adjacent surfaces is referred to herein as the adjacent interface.

The two angles can be varied to obtain an optimal orientation of the two elements which in turn give rise to optimal optical properties for the focussing element. The angular difference between the two angles is predetermined and the angles are not equal and angle <NUM> is less than angle <NUM>. The specific angles are not essential but the angular difference is determined to ensure the correct path for the light coming from respective sources. By way of example angle <NUM> could be for example <<NUM>°.

In the present invention the prism element as referred to throughout is used for ease and is not intended to be a limitation as to the form and shape of the element <NUM>. The prism element is thus intended to include any optical element comprising a plurality of optically powered surfaces. This could be a three surface prism or may have two or more surfaces. The preferred form is a three surface prism, but other shapes and forms are equally applicable. Similarly the wedge element is used of ease of reference but could be different shapes and forms. The wedge element is thus intended to include any optical element comprising one or more optically powered surfaces. At least one of the optically powered surfaces on the prism element or wedge element may be described by a multiple order polynomial.

The combination of the prism element and the wedge element define a plurality of interfaces between the optically powered surfaces of each element. As light passes through the combination (also referred to as the focussing element) and as a result of the interfaces there is a change in refractive index, which leads to a change in direction of the light passing through the interface. This enables light beams to be directed by the combination. Due to the relative positioning of the prism and wedge there are different interfaces and different shapes of interface this helps define a "folded path" through the focussing element.

The prism and wedge elements may be in direct contact or have a gap between their surfaces. The gap can be an air gap or may comprise other material such as glue etc. The addition of the air gaps adds a further interface (for example, from the prism to air; from air to the wedge and so on). The further interfaces have effect of further directing the light. The overall light direction will be described in greater detail below. The result of the combination is that due to the multiple interfaces light can be guided in a very controlled manner and in a component that is compact and light as is the optimum for head mounted optics. The juxtaposition of the first and second optically powered elements (with or without a gap) define a compact folded path through which light can be directed.

The optical system <NUM> produces an exit pupil that is well corrected whilst maintaining a low volume and size. As used herein, 'well-corrected' is intended to mean that defects of the light, such as aberrations, artefacts, chromatic distortion, are minimised in order to provide a predefined standard of optical performance which may be different for different applications.

The prism <NUM>, a 3D representation of which is also shown in <FIG> and <FIG>, is a single-piece three-sided element having by way of example an elongate, substantially triangular cross-section. The prism <NUM> therefore has two three-edged bases (only one of which is shown in Figure <NUM>) <NUM> and three surfaces <NUM>, <NUM>, <NUM> joining corresponding edges of the two bases <NUM>. Any number of the surfaces <NUM>, <NUM>, <NUM> of the prism <NUM> are optically powered and the surface powers may be different from one surface to the next. According to the claimed invention, the prism element <NUM> and wedge element are separated by a gap <NUM> such as for example an air gap. The gap <NUM>, wedge <NUM> and prism <NUM> may be of any combination of materials and as each may be different the interfaces between the three can provide a difference in refractive index from one element to the next. This can be exploited in the present invention to control the direction of light passing through the combination of elements. The gap may be formed from any type of spacing material having a different refractive index than the other elements. In <FIG> example, the prism <NUM> is surrounded by air, which has a lower refractive index than the other elements. Many other different combinations of material can be used.

The operation of the prism element <NUM> alone and in combination with the wedge element <NUM> and the gap <NUM> will now be described. Light from the image source <NUM> enters the prism element <NUM> at a first, receiving surface <NUM> via the field lens. The receiving surface <NUM> is optically powered, and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape.

The light travels through the prism element <NUM> and undergoes Total Internal reflection (TIR) at a second surface <NUM>. The TIR occurs because the surrounding material, air, has a lower refractive index than the material of the prism <NUM> and because the angle of incidence of the light is greater than the critical angle for the interface at the surface <NUM> between the prism element <NUM> and the air. The surface <NUM> is also optically powered and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. It should be noted that partial internal reflections or a reflection due to a reflective coating may be used instead of TIR, however these may be less efficient.

The surface <NUM> is tilted relative to a central axis A of the prism <NUM>. Tilting the surface <NUM> relative to the normal axis can help to reduce TIR breakdown that would occur if the transmission surface were aligned closer to the normal axis. Tilting the surface <NUM> in this way beneficially enables the image plane to be oriented at a shallower angle relative to the normal axis, permitting a more compact arrangement.

By virtue of having undergone TIR at the surface <NUM>, the light is reflected towards the surface <NUM>. Surface <NUM> is partially reflective, as it is also required to be transmissive in order for the user to view the outside world. The coating may be a simple partially reflective coating, or a more tailored coating designed specifically for defined wavelengths of light. The surface <NUM> is also optically powered and may be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. The surface <NUM> is optically coated to reflect light. Light reflected within the prism <NUM> towards the surface <NUM> therefore experiences the surface <NUM> as a second surface mirror. The surface <NUM> may be not tilted or minimally tilted relative to the normal axis A to reduce off axis aberration. The light reflected by the coating applied to the surface <NUM> returns toward the surface <NUM>.

The light reflects from the surface <NUM> and is re-incident on the surface <NUM> at an angle that is less than the critical angle for the interface at the surface <NUM> between the prism element <NUM> and the air, so the light is transmitted through the surface <NUM> and exits the prism element <NUM>. The light exits the prism element <NUM> and travels into the gap <NUM> between the prism element <NUM> and the wedge element <NUM>.

The light travels through the gap <NUM> and enters the wedge element <NUM>. The refractive index of the air in the gap <NUM> is lower than the refractive index of the material of the wedge element <NUM>. The wedge element <NUM> is for example, an elongate element having quadrilateral cross-section, and so has two surfaces <NUM>, <NUM> connected by upper and lower ends <NUM>, <NUM>. The surfaces <NUM>, <NUM> and ends <NUM>, <NUM> extend between bases (not shown in <FIG>). The wedge element <NUM> may also take a substantially triangular cross section, without an upper end <NUM>.

The wedge element <NUM> receives light from the gap <NUM> at the first, input surface <NUM>. The light leaves the wedge <NUM> at the second, output surface <NUM>.

In the embodiment of <FIG>, the input surface <NUM> of the wedge <NUM> may also be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. The light is transmitted through the wedge <NUM> from the input surface <NUM> to the output surface <NUM>.

The output surface <NUM> is typically planar but may also be described by a spherical, aspherical, cylindrical, toroidal or multiple order polynomial surface shape. At the output surface <NUM>, the light exits the wedge element <NUM> because the light is incident on the output surface <NUM> at an angle that is less than the critical angle for the surface. The light that exits the wedge element <NUM> is now collimated and forms a well corrected exit pupil.

The shape of surfaces that may be a polynomial or extended polynomial shape as mentioned above may be modelled by determining parameters of the lens. One parameter that is used is a determination of the surface sag. The surface sag for the surfaces that use this surface form could (for example) be described by the following equation, which perturbs a conic aspheric surface by adding additional polynomial terms. <MAT> Where.

It will be appreciated that this is just one example of modelling the surfaces; other may equally well be used.

In the embodiment of <FIG>, the prism element <NUM>, gap <NUM>, the wedge element <NUM> and the compensator element provide a number of degrees of freedom that are used to manipulate the light in order to result in collimated, well-corrected exit pupil. The collimated light exits the optical system <NUM> at the wedge element <NUM>. In some embodiments, further components may be incorporated to increase the number of degrees of freedom of the arrangement and collimated light may enter or exit the optical system <NUM> via a different component. For example, a further optical element, a field lens, may be incorporated into the optical system <NUM> to further reduce residual aberrations such as distortion and/or field curvature. In some embodiments, a further corrective element may be incorporated in the optical system <NUM> to chromatically correct the light. These elements could include additional refractive, reflective, holographic or diffractive optical components to further manipulate the source light for the purpose of improving or enhancing optical performance in conjunction with the prism and wedge optical elements.

Moreover, in the embodiment of <FIG>, any of the three surfaces <NUM>, <NUM>, <NUM> of the prism element <NUM> and either of the two surfaces <NUM>, <NUM> of the wedge <NUM> may have varied optical power and surface forms. In some examples which are not part of the claimed invention, the optical properties of the surface <NUM> of the prism element <NUM> and the input surface <NUM> of the wedge element <NUM> may be matched or designed to be complementary for specific applications. With regards to the compensator element this has a matching/complementary surface form that corresponds to prism combiner surface.

The materials of the prism element <NUM>, wedge element <NUM> and compensator element <NUM> may be the same or may be different to take advantage of the optical characteristics such as refractive indices that different materials have. Similarly, the surrounding material and/or spacing material of the gap <NUM> are variable to optimise the optical characteristics of the optical system <NUM>. Materials for the optical elements can be of any appropriate nature. For example, the materials may comprise one or more of optical glasses, polymers and plastics of varying refractive index and abbe number may be used, such as: N-BK7 (low index glass), N-SF6 (high index glass), 7980_0F (low index fused silica), PMMA (low index polymer) and E48R (low index polymer). It may be advantageous to have both optical elements made from different materials so as to combine materials with different indices and different dispersion characteristics. For example, the optical wedge may be manufactured from a material with low dispersion to mitigate chromatic splitting of light during the interaction with the powered surface.

Returning to <FIG> or <FIG>, the compensator element <NUM> is located such as to be sympathetic with the surface <NUM> of the prism element. The compensator element <NUM> is shown in greater detail in figure <NUM> below. The compensator element <NUM> is an optical element which enables external light from the outside world to be combined with light from the image source prior to the combined image being presented to the user. The first surface of the compensator element <NUM> is designed to be sympathetic to or to match with the surface <NUM> such that the two can be optically cemented or bonded through the use of an optical adhesive <NUM>. The thickness and surface form of the second surface of the compensator element <NUM> can be optimised to minimise the introduction of refractive errors, such as distortion, into the user's view of the outside world. These refractive errors would otherwise be apparent without the use of a compensator element, thus degrading the user's view of the outside world.

Surface <NUM> of the prism element and surface <NUM> of the wedge element (the adjacent surfaces) are shown in <FIG> and <FIG> as appearing to be matching or sympathetic to each other in part due to the two dimensional nature of the drawings, however it should be noted that the surfaces are not matching or sympathetic. For example, surface <NUM> of the wedge element may have optical power and/or be curved and surface <NUM> may have no optical power and/or be linear, and therefore the adjacent surfaces will not be matched or sympathetic. According to the claimed invention, the adjacent surfaces are dissimilar and not complementary such that the adjacent surfaces are not sympathetic to each other and/or matching. A non-complementary shape or non-sympathetic shape refers to a shape such that when placing the shapes together, there is always a gap between the surfaces.

A non-complementary shape or dissimilar shape allows an additional optical surface form for correction of aberrations.

Referring to <FIG> and <FIG> if the display is an HMD or HWD it is wearable by means of an appropriate support (not shown). The support may contain one or more optical elements which can be viewed by one or both eyes <NUM> of the user. The HMD may further include a control system. The optical element <NUM> of <FIG> or <FIG> may be located relative to the arc or shape of the head. In <FIG> a view from above is shown and in <FIG> a side view is shown. It will be appreciated there are many alternatives to the arrangements shown and that the scale of the drawings is for readability, and is not limiting.

For use with or as the invention, the HMD can be of any appropriate type including googles, glasses, a helmet or helmet visor suitable for use in multiple fields. Ideally, the device is portable or adapted to be portable by means of the support. Although not shown in detail the support may include a support adapted to support the optical elements in front of the eye. The support may include: frames; side arms and supports for goggles and glasses; a helmet or visor; a headband; a neck or shoulder worn support; a gaming headset; or any other support that could be worn to hold the optical elements in the desired position.

The control system is variable depending on the use of the HMD. The control unit may be in situ or remote from the HMD. The control device may include a communications module for communicating with the optical elements and with other modules either on the HMD or remote therefrom. The communications may be wireless and/or wired. The control module may include different modules for carrying out different functions. These functions are not limited in any way but may include imaging, tracking, scene generation, processing, storage, power supply, audio etc..

To display images to the user via the optical arrangement, the HMD also incorporates an image source corresponding to the optical arrangement. The image source may have a controlled numerical aperture or an uncontrolled numerical aperture and may comprise a flat panel display, emissive display, a reflective display, a projection optic, a relay lens or any other type of display source, image or light generation unit.

<FIG> show respectively a 2D view of the combination of the prism, wedge and compensator with the ray path shown and a 3D view of the combination of the prism, wedge and compensator with optically powered surfaces shown.

In the claims, the term 'comprising' does not exclude the presence of other elements or steps.

Claim 1:
A folded optical arrangement (<NUM>) for use in a view-through display to transmit an image from an image source (<NUM>) to a user's eye (<NUM>), the arrangement providing a folded optical transmission path and comprising:
an optical system (<NUM>) having a first optical element (<NUM>) comprising a first plurality of optically powered surfaces (<NUM>, <NUM>, <NUM>), and a second optical element (<NUM>) comprising at least one optically powered surface (<NUM>, <NUM>), the optical system configured to receive light forming the image from the image source,
and configured to present a virtual image of the image source to the user with an apparent focus between a predetermined distance and optical infinity; and
a compensator element (<NUM>) located between the first optical element (<NUM>) and an external view to receive the external view for combination with the image output from the optical system (<NUM>),
wherein the first plurality of optically powered surfaces and the at least one optically powered surface of the second optical element are arranged to define a plurality of interfaces along the folded optical path and wherein a refractive index change at each interface is predetermined to control the direction of light passing through the interface; and
wherein one surface (<NUM>) of the first optical element and one surface (<NUM>) of the second optical element are adjacent to one another and each define an angle (<NUM>, <NUM>) with a respective other surface (<NUM>, <NUM>) of the relevant optical element at opposing ends of the adjacent surfaces and wherein the opposing angles are not equal, characterised in that the adjacent surfaces (<NUM>, <NUM>) are non-complementary or non-sympathetic such that there is no way to place the surfaces (<NUM>, <NUM>) together without leaving a gap (<NUM>).