Streamlined optical system

Abstract

A streamlined optical system is disclosed which provides a window through an opaque boundary structure of an object, for example, an object exposed to or immersed in a fluid flow. The optical system comprises a refractive element spanning both sides of the boundary structure. The refractive element is configured to form a refractive window through the opaque boundary structure via which light passes through at least one refractive surface of the refractive window, the light following a path at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure along which, for example, fluid is flowing. The refractive element is configured to project from the surface of the second side of the boundary structure. The projection of the refractive element forms a streamlined convex portion including one or more planar surfaces inclined at a non-zero angle A from the surface of the second side of the boundary structure. Light following a path at a grazing or near grazing angle of incidence along the surface of a second side of the boundary structure passes via at least one of the one or more planar surfaces formed in the streamlined convex portion to provide a non-distorted field of view, FoV which includes a portion of the surface of the second side of the boundary structure, thus allowing the second side of the boundary structure to be monitored for example using an imaging system such as a camera or the like.

Description

STREAMLINED OPTICAL SYSTEM

[0001] The disclosed technology relates to a streamlined optical system, in particular but not exclusively to a streamlined optical system including a refractive element with a streamlined surface configured for observing a fluid flow exposed or immersed surface. Observation of a surface by an optical system embedded in the surface requires the optical system to protrude from the surface in order to obtain the required optical geometry. Where a fluid is flowing along a surface, however, the more an optical system protrudes, the more the fluid flow is disrupted. Where the fluid flow includes suspended particles such as dust and debris, disruption induces turbulence which increases the rate at which any particles suspended in the fluid flow are deposited on the optical system and surface in the vicinity of the optical system. Such deposition increases the rate at which the optical system requires cleaning in order to maintain a clear FoV of the boundary surface.

[0002] In particular, but not exclusively, the disclosed technology relates to an optical system configured as a self-cleaning optical system comprising a refractive window through a boundary structure of an object, such as a boundary structure, which allows light passing through the refractive element to follow a path at a grazing or near grazing angle along at least one surface of the boundary structure. The optical system comprises a refractive element which has a substantially streamlined configuration protruding through the boundary structure. The streamlined configuration may be provided at one end by a convex portion which protrudes above the surface of the boundary structure at one end into a fluid flowing along the surface. The convex portion is truncated by one or more planar surfaces which form the refractive window and is configured to allow laminar fluid flow along the boundary surface to be maintained over a refractive window. This way the laminar fluid flow provides a cleaning function over the refractive window.

[0003] In particular, but not exclusively, the disclosed technology relates to an optical system or assembly, for example, a self-cleaning optical system, which is configured to provide a field of view, FoV, to an imaging apparatus such as a camera through a window in a boundary structure where the FoV includes light at a grazing angle of incidence or at a near grazing angle of incidence along the far surface of that boundary structure. Alternatively, or in addition, in particular but not exclusively, the optical system may be used to illuminate the surface of the boundary structure by providing an optical source on one side of the boundary structure and using a refractive window to direct light to follow a grazing or near grazing angle along a surface on the opposite side of the boundary structure.

[0004] Such a boundary may form or comprise an otherwise opaque boundary structure. For example, a boundary structure may be a rigid structure and as such comprise a wall or housing of an object. This allows a FoV to be captured using a camera system located within the housing of the outside surface of the housing or vice versa. The light forming the FoV may be from any suitable and desired part of the electromagnetic spectrum, for example, it may comprise near-infra-red, NIR, or ultra-violet, UV light.

[0005] The refractive element which provides the window through the boundary structure presents a refractive surface which projects out at an angle on one side of the boundary structure via which a FoV can be obtained along the boundary structure which includes light at a grazing or near grazing angle of incidence to the boundary structure. This also allows the FoV to include light at a grazing or near grazing angle of incidence which is incident at or extremely close to the point at which the light passes through the refractive element. This allows a FoV of one side of a boundary structure to be made available to a viewing or imaging apparatus such as a camera disposed on the other side of the boundary structure.

[0006] The boundary structure may act as a boundary between two different environments. For example, for objects such as aircraft, vehicles, vessels, pipelines or other types of apparatus or equipment, or buildings or another type of infrastructure or objects, the environment on the outside, or inside, of the boundary structure may comprise a flowing fluid, for example, flowing water or air, and / or have other hazardous characteristics compared to the environment on the other side. For objects used at depth in water or in a vacuum such as space, there may be differences in pressure or different levels of exposure to hazardous radiation between the two environments separated by the boundary structure.

[0007] It is often desirable to obtain a clear, distortion-free, image along a surface of an object when it is moving. It may also be advantageous if the window or aperture through a boundary structure is concealed and / or protected as much as possible. It is also advantageous in some situations to have an optical system which has a low projection from the surface of the object to reduce the chance of turbulence in any fluid flow along the surface of the object.

[0008] Prior art systems may be configured to reduce the height profile above the boundary surface, for example, by locating a camera or the like within a recess in the boundary. However, this will provide at best only a limited FoV along the surface of the outer housing due to the lip of the recess creating a blind spot.

[0009] Finally, shielding a camera may be useful in some environments.SUMMARY STATEMENTS

[0010] The disclosed technology may comprise the aspects and embodiments described herein and any apparent combination or modification thereof to someone of ordinary skill in the art. The disclosed invention is as set out by the accompanying claims.

[0011] The disclosed technology generally relates to an optical system for mounting on or in an object which provides a refractive window through a boundary structure, for example a boundary structure such as a housing or casing for an object, for example, a housing for or forming part of the object, which may be a moving object. A boundary structure such as an object housing may not completely encompass or contain the object in some embodiments, however, in other embodiments, the boundary structure may form an enclosure or container for the object. All references herein to a boundary structure should be interpreted as a reference toa boundary structure of an object, for example, a wall or housing of the object.

[0012] Incident light at a grazing or near grazing angle to the surface of the boundary structure or housing of the object may be captured at or close to the edge or lip or corner of the aperture formed by the refractive window through the boundary structure and pass via the refractive window to within the boundary structure to form a FoV at an imaging apparatus such as a camera or the like. The optical system accordingly may allow, in some embodiments, what can be considered a view around the corner formed by the walls of the aperture and a surface or surfaces of the boundary structure or housing of the object to be obtained.

[0013] In this respect, although herein reference is made to obtaining a FoV including light at grazing or near grazing incidence to a surface of a boundary structure such as an boundary structure or shell and to an imaging apparatus located inside the object, it will be apparent that the disclosed technology may equally be applied to monitor an internal surface of a boundary structure such as an boundary structure wall or shell where the imaging apparatus is located on the other, external, side of the boundary structure, for example, the other side of the boundary structure wall or shell. For example, light may be injected within a pipeline along its length and light at a near-grazing angle of incidence to the internal pipeline housing later picked up by an imaging apparatus such as a camera system mounted on the exterior of the pipeline using an embodiment of the disclosed technology.

[0014] References to light herein may refer to visible or optical light or to electromagnetic radiation lying in a different part of the electromagnetic spectrum, for example, the term light may refer to infra-red, IR, near-infra-red, NIR, ultra-violet, UV, light, or other frequencies of radiation.

[0015] The term imaging apparatus accordingly refers to any suitable imaging apparatus for the type of radiation which passes through the boundary structure or any other type of object housing, which is to be imaged on one side by an imaging system located on the other side of the boundary structure or object housing.

[0016] A first aspect of the disclosed technology comprises an optical system, for example, a streamlined optical system, for providing a window through a boundary structure, for example a boundary structure of an object, which may be a moving object, where the refractive element spans the first side A to a second side B, in other words spans both sides, of the boundary structure and is configured to form a refractive window through the boundary structure via which light incident at a grazing or near grazing angle of incidence, e, relative to a surface of a second side of the boundary structure passes to form a field of view, FoV, at an imaging apparatus.

[0017] In some embodiments, the refractive element is configured to project from the surface of the second side of the boundary structure, the projection of the refractive element forming a streamlined portion, which may further provide a curved or convex streamlined portion either alone or in combination with a separate streamlined hood element. The streamlinedportion, including one or more planar surfaces inclined at a non-zero angle A from the surface of the second side of the boundary structure, where the light following a path at a grazing or near grazing angle of incidence, e, along the surface of the second side of the boundary structure passes via at least one of the one or more planar surfaces of the streamlined portion to form a non-distorted field of view, FoV, of an area on the surface of the second side of the boundary structure.

[0018] In some embodiments, the configuration of the streamlined portion and the one or more planar surfaces forming the refractive window form a streamlined profile in the fluid flow but may not be streamlined in directions other than the fluid flow. The streamlined profile may result from a circular or elliptical truncated cone configuration of the optical system above the boundary surface, where the truncation comprises a planar section inclined at a non-zero angle to the boundary surface. The streamlined profile may also result from a frustoconical configuration which protrudes from the boundary surface in addition to the one or more planar sections which are inclined at a non-zero angle to the boundary surface. In some embodiments, the planar sections comprise one or more circular, oval, or rectangular planar sections.

[0019] In some embodiments, the streamlined portion projects into a fluid flow along the surface, wherein the configuration of the stream-lined portion and one or more planar surfaces forming the refractive window are configured to allow non-turbulent, laminar, fluid flow across the refractive window. The laminar flow may wash or clean the refractive window or any transparent film or surface protecting the refractive window.

[0020] In some embodiments, the imaging apparatus is located on first side A of the boundary structure.

[0021] In some embodiments, the imaging apparatus is located on the first side A in a recess within the boundary structure such that the light does not pass completely through the boundary structure before forming the FoV at the imaging apparatus.

[0022] The term laminar or non-turbulent fluid flow a used herein refers to a fluid flow having sufficiently low or no degree of turbulence to result in there being no material increase in the drag force resulting from fluid flow on the protrusion and the laminar or non-turbulent fluid flow cleans the refractive window free from debris which might otherwise settle on the refractive window due to turbulence.

[0023] In some embodiments, the optical system is an optical system embedded in or extending through an opaque boundary structure, where the refractive element is configured to have a portion having a streamlined curved surface which protrudes into the fluid flow along the surface of opaque boundary structure and the refractive window comprises two or more planar surface, at least one planar surface being inclined at a non-zero angle, A, to the surface of the boundary structure along which fluid is flowing to capture a field of view, FoV, of the surface which includes light which has following a grazing or near grazing angle of incidence along the surface, and wherein the streamlined convex portion and the refractive window are configuredto allow non-turbulent, laminar, fluid flow across the refractive window.

[0024] In some embodiments, the optical system is a self-cleaning optical system comprising a refractive element configured to be covered in part by a streamlined convex portion protruding into the fluid flow, wherein the streamlined convex portion and the refractive window are configured so that the laminar fluid flow across the refractive window cleans the refractive window.

[0025] In some embodiments, the non-zero angle of inclination, A, of at least one of the planar surfaces forming the refractive window, is less than 20 degrees from the plane of the observed boundary surface and the optical system is configured to provide a non-distorted field-of-view, FoV, of the surface to an imaging system.

[0026] In some embodiments, the refractive window comprises a planar surface, alternatively it may comprise more than one planar surface and / or one or more curved surfaces, wherein at least one surface of the refractive window is configured to capture light at the grazing or near grazing angle of incidence relative to the surface on the second side of the boundary structure.

[0027] In some embodiments, the boundary structure comprises an object. In some embodiments, the boundary structure comprises a part of an object. In some embodiments, the boundary structure comprises a shell or housing or another form of boundary structure for an object.

[0028] In some embodiments, the object comprises one of: a vehicle, an aircraft, a vessel, or a pipeline or conduit for conveying a fluid and the boundary structure comprises a boundary structure for housing the object.

[0029] In some embodiments, the optical system is configured to project less than 7mm above the surface of the boundary structure, for example, less than 7mm from a surface of the boundary structure. A projection of 7mm thereabouts may be provided in some embodiments for a refractive window measuring 50mm or thereabouts in cross-section, for example, a refractive window of approximately 50mm x 50mm.

[0030] In some embodiments, the optical system further comprises mounting means and / or fixing means to mount or locate the optical system in or onto the object or a boundary structure at least partially containing the object.

[0031] In some embodiments, the refractive element is configured to form the refractive window by passing through an aperture in the boundary structure in order to present at least one planar refractive surface disposed at an angle to the plane of the surface on the second side of the boundary structure, wherein the disposition of at least one of the refractive surfaces of the refractive element at the second surface is configured to capture a FoV including incident light at a grazing or near-grazing angle of incidence along the plane of the second surface of the boundary structure and guide light along the body of the refractive element towards the imagingapparatus.

[0032] The higher the optical system projects from, in other words presents a raised profile above, the surface of the boundary structure, the more it is likely to disrupt any fluid flowing over that surface. This disruption may also cause turbulence in the flow. Turbulent flow may result in debris accumulating at or along the raised profile or projection provided by the optical system. As debris and dirt accumulate over the window provided by the refractive element, the desired FoV of the surface of the object obtained via the refractive element will become more and more obscured. Accordingly, the extent of the projection of the refractive element and / or the optical system above the boundary structure surface should ideally be as low as possible, for example, 7mm or less, to reduce potential debris deposition or at least the rate of debris deposition over the refractive element window. This may also reduce the rate at which the visibility of the desired FoV deteriorates and also allow the interval between maintaining the refractive window clean to be extended.

[0033] Another benefit of having a reduced raised profile above the surface of the boundary structure of the object is that it will reduce a drag factor for the object relative to any fluid flow. This may cause various beneficial effects, for example, reducing the drag of an object such as a vehicle or aircraft in air or a vessel in water may reduce the amount of fuel consumed by the propulsion system of the object.

[0034] Accordingly, it is advantageous that an optical system having a refractive window through a boundary structure or housing of an object as disclosed herein allows the surface projection of the optical system above a surface of the boundary structure to be reduced or minimized when obtaining a FoV including light at grazing or near grazing angles of incidence along the boundary structure surface. In some embodiments of the optical system according to the first aspect or the technology generally disclosed herein, the boundary structure comprises an boundary structure, and the optical system is configured to provide a field of view, FoV around a lip of an aperture located in the boundary structure of the object, where the FoV includes light at a grazing or near-grazing angle of incidence along the surface of the second side of the boundary structure, where the optical system comprises a refractive element configured to pass through the aperture in order to present at least one planar refractive surface disposed at a non-zero angle of inclination, A, to the plane of the second surface of the boundary structure, where the disposition of the refractive surface at the surface on the second side of the boundary structure is configured to capture a FoV including incident light at a grazing or near-grazing angle of incidence along the plane of that surface of the boundary structure and guide light along the body of the refractive element, and where the imaging apparatus is configured to focus received light refracted via the refractive element comprising the FoV including the incident light at the grazing or near grazing angle of incidence.

[0035] In some embodiments, the refractive element comprises additional planar or curved surfaces to the planar refractive surface.

[0036] In some embodiments, the refractive element is protected by a hood element which projects in the form of a truncated sphere or cone from the surface of the second side of the boundary structure.

[0037] In some embodiments, the refractive element comprises a prism.

[0038] In some embodiments, additional reflective elements such as mirrors may be provided.

[0039] In some embodiments, one or more reflective elements may be provided as coatings on the refractive element.

[0040] In some embodiments, the window of the refractive element may be provided with a coating to reduce reflection and / or enhance transmission of light at the grazing or near grazing angle of incidence.

[0041] In some embodiments, the first side of the boundary structure is inside an object, and the second side is outside the boundary structure or object, and imaging apparatus is located inside the boundary structure or object.

[0042] In some embodiments of the optical system disclosed herein, the optical system is mounted on or in an object and the boundary structure of the object and the refractive window of the optical system seal the inside of the boundary structure from an external environment.

[0043] In some embodiments, the external environment is a fluid environment.

[0044] In some embodiments, a non-turbulent fluid flows over the refractive window in the fluid environment.

[0045] In some embodiments, the refractive element is configured to reflect refracted light towards the imaging apparatus in the opposite direction to the direction light is incident on the refractive window.

[0046] In some embodiments, the refractive element is a composite or non-homogenous refractive element comprising at least two refractive regions having different refractive properties, where the refractive regions share at least a partially transparent interface allowing light to pass through from the first refractive region to at least a second refractive region.

[0047] In some embodiments of such a refractive element comprising at least two refractive regions, a first refractive region is configured to passthrough the aperture in order to present the refractive window at an angle to the plane of the surface of the second side of the boundary structure which allows the refractive element to capture incident light at a grazing or near-grazing angle of incidence along that surface and guide the captured light along the body of the first refractive region to the second refractive region, wherein the second refractive region is configured with an inner reflective surface configured to reflect received light towards the imaging apparatus.

[0048] In some embodiments, the imaging apparatus is configured to capture an image or a series of images or a video feed of images, each providing a FoV or a series of FoVs along the first surface of the boundary structure.

[0049] In some embodiments, the imaging apparatus comprises one or more of a camera, a stereoscopic camera, a depth camera system, and a charge-coupled device, CCD, optical system. The camera may be an event-based camera in some embodiments.

[0050] In some embodiments, the imaging apparatus comprises a compound lens arrangement configured to provide an external pupil for forming the FOV.

[0051] In some embodiments, the external pupil is located within or adjacent to the refractive element.

[0052] As used herein, the term grazing angle of incidence is an angle of incidence relative to the surface of the second side of the boundary structure where it would be considered completely planar to the surface of the boundary structure if the angle of incidence was 0 degrees. In some embodiments, accordingly, the term near-grazing angle of incidence may be used instead to refer to a suitably low angle of incidence close to zero, for example, less than 5 degrees, or less than 3 degrees or less than 1 degrees of angle of incidence. However, both grazing and near-grazing terms used herein may be considered to refer to any angle of incidence suitably close to being co-planar with the surface of the boundary structure which is to be included in the FOV. This means in some embodiments the grazing angle, zero degrees of angle of incidence, may instead be a near-grazing angle which is slightly above zero degrees of incidence. For example, the FOV may include light which is incident below one of: 5, 2, 1 or 0.5 or 0.25 or 0.1 degrees relative to the surface in some embodiments. It will be apparent to anyone of ordinary skill in the art that the lower reach of the FoV includes the grazing or near grazing angle of incidence and that the extent of the FoV captured may include far higher angles of incidence, particularly if, for example, the refractive element and imaging apparatus form part of an optical geometry arrangement which uses the Scheimpflug principle to align the optical object plane more closely to the target object.

[0053] In some embodiments, the grazing or near-grazing angle of incidence captured by the FoV of the optical system allows the optical system to be used to monitor a surface of an object.

[0054] Another, second, aspect of the disclosed technology comprises a kit of parts for assembling an optical system according to any of the aspects and embodiments disclosed herein.

[0055] Another, third, aspect of the disclosed technology comprises a refractive element configured to form a refractive window in an boundary structure of an object, the refractive element being configured to present a field of view including light at grazing or near grazing angles of incidence along a surface of the housing wall to an imaging apparatus located within the boundary structure.

[0056] In some embodiments, the refractive window comprises one or more planar or curved surfaces.

[0057] In some embodiments, the refractive element may be a homogeneous orinhomogeneous refractive element.

[0058] In some embodiments of the disclosed technology, the refractive element comprises a composite refractive element, for example, two refractive elements having different refractive or other optical properties may be placed in close proximity with an airgap or sharing a common surface boundary.

[0059] In some embodiments of the disclosed technology, the refractive element, which may comprise a homogenous or a composite or non-homogenous refractive element, has one or more coatings which provide internally reflective surfaces.

[0060] In some embodiments, alternatively, or in addition, one or more external reflective surfaces such as mirrors coats may be provided.

[0061] In some embodiments, there is an air gap between at least one refractive element and the imaging apparatus.

[0062] In some embodiments, the composite or inhomogeneous refractive element comprises at least two refractive regions sharing an adjacent transparent or partially transparent surface, wherein a first of the at least two refractive regions is configured to receive light at grazing or near grazing angles of incidence along a surface of the housing wall and refract the light to a second refractive element of the at two refractive elements, wherein the second refractive element is configured to reflect and refract light received via the first refractive element to the imaging apparatus.

[0063] Another, fourth aspect of the disclosed technology comprises an object having an boundary structure or located in an boundary structure, where the boundary structure configured to protect an interior of the object and an optical system according to the first aspect and any of its embodiments disclosed herein where the boundary structure is provided with at least one aperture via which the or a refractive element of the optical system passes to present a refractive window on one side of the boundary structure, wherein the refractive element or a window of the refractive element and a hood over the refractive element form a seal over the aperture in the boundary structure.

[0064] In some embodiments, the first side of the boundary structure is the interior of the boundary structure, and the second side of the boundary structure is the exterior of the boundary structure.

[0065] In some embodiments, the first side of the boundary structure is the exterior of the boundary structure, and the second side of the boundary structure is the interior of the boundary structure.

[0066] In some embodiments, the object comprises one of a conduit or pipeline for conveying fluid, a vehicle or vehicle accessory, a train engine or carriage, a submersible, submersed, or surface sea-craft, a robotic system or robot, a space craft or launch vehicle; and an aircraft.

[0067] Another, fifth, aspect of the disclosed technology relates to a surface monitoringsystem for monitoring a surface of an object, the surface monitoring system comprising at least one optical system according to the first aspect or any one of its embodiments disclosed herein. The surface monitored may be an external surface or an internal surface.

[0068] In some embodiments of the disclosed optical systems, to assist with keeping the refractive surface clear of debris, a coating to repel any surface debris build-up may be applied to the refractive window. For example, a hydrophobic or oleophobic coating may be applied, or similar type of dirt and particular repelling coating that is impervious in the environment in which the optical system is disposed. Such coats may be protective and / or easier to clean and allow a cleaner refractive window surface to be maintained for longer than an untreated surface.

[0069] In some of the embodiments of the disclosed optical systems, to assist with grazing light being refracted, one or more surfaces of the window provided by the refractive element may have a special coating applied in some embodiments. For example, an antireflection coating may be applied in some embodiments.

[0070] Another, sixth, aspect of the disclosed technology relates to an illumination system for illuminating an area on one side of a boundary structure using light provided via a refractive element spanning the boundary structure from a light source located on the far side of the boundary structure.

[0071] In some embodiments, the illumination system comprises any one of the disclosed optical system aspects or their embodiments for imaging a FoV along a surface of a boundary structure with the imaging apparatus replaced with a suitably configured light source so that the FoV is instead illuminated by light from the light source.

[0072] In some embodiments, the optical light source of the illumination system may comprise a collimated or uncollimated light source.

[0073] In some embodiments, the illumination system may comprise a collimated or uncollimated light source and a lens arrangement configured to provide a point light source within the refractive element.

[0074] Advantageously, this may allow a more compact illumination system to illuminate a larger surface area on the other side of any boundary structure which the refractive element bridges.

[0075] For example, in some embodiments, the optical system comprises or further comprises: a window through a boundary structure of an object, a light source located on a first side of the boundary structure of the object, and a refractive element spanning both sides of the boundary structure, the refractive element being configured to form a refractive window through the boundary structure via which light from the light source illuminates, at a grazing or near grazing angle of incidence, a surface of a second side of the boundary structure, opposite the first side.

[0076] In some embodiments, the light may be polarized light.

[0077] In some embodiments, the light may be unpolarized light.

[0078] In some embodiments, one or more surfaces of the refractive element is provided with a suitable coating to improve the transmission of light from the light source through the one or more surfaces.

[0079] Some embodiments of the optical systems disclosed herein may comprise one or more optical system according to the first aspect for imaging a FoV including light at a grazing or near grazing angle of incidence along a boundary surface and one or more optical systems according to the fifth aspect for illuminating the FoV.

[0080] The disclosed aspects and embodiments may be combined with each other in any suitable manner which would be apparent to someone of ordinary skill in the art.BRIEF DESCRIPTION OF THE DRAWINGS

[0081] The disclosed technology will now be described in more detail with reference to the accompanying drawings, which are byway of example only, and in which:FIGS. 1 A and 1 B show schematically side views of example prior art optical systems; FIG. 2A shows schematically a side view of an optical system according to an example embodiment of the disclosed technology;FIG. 2B shows schematically a side view of an optical system according to another example embodiment;FIG. 2C shows schematically a side view of a refractive element of an optical system according to another example embodiment;FIG. 2D shows schematically an overhead plan view of the refractive element of the optical system of FIG. 2C.FIG. 2E shows schematically an overhead plan view of another example embodiment of a refractive element according to the disclosed technology;FIG. 2F shows schematically a perspective view of an optical system including the refractive element of FIG. 2E;FIG. 3 shows schematically an imaging apparatus according to an example embodiment of the disclosed technology;FIG. 4 shows schematically an optical system according to another example embodiment of the disclosed technology;FIG. 5 shows schematically an optical system according to yet another example embodiment of the disclosed technology;FIG. 6 shows schematically an optical system having an imaging apparatus according to an example of the embodiment disclosed technology configured to capture an enlarged FoV using an external pupil;FIG. 7 shows schematically FIG. an imaging apparatus having an internal pupil;FIG. 8 shows schematically an optical system configured as an illumination system according to an example embodiment of the disclosed technology;FIG. 9 shows schematically another example of an optical system configured as anillumination system according to an example embodiment of the disclosed technology;FIG. 10 shows schematically another example of an optical system configured as an illumination system according to an example embodiment of the disclosed technology; and FIG. 11 shows schematically yet another example of an optical system configured as an illumination system according to an example embodiment of the disclosed technology.DETAILED DESCRIPTION

[0082] Some example aspects and embodiments of the present disclosure are described in detail below with reference to the accompanying drawings, which may include the best mode of implementing the disclosed technology currently contemplated. The disclosed technology may, however, be realized in many different forms and should not be construed as being limited to the sets of features described below. Features which are apparent to anyone of ordinary skill in the art may be omitted where their inclusion is implicit for the sake of brevity or if not essential to the disclosed embodiments. Like numbers in the drawings refer to like elements throughout, but the same element may be differently numbered in some drawings. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Certain features whose presence or use is apparent to anyone of ordinary skill in the art may be omitted from the description and / or drawings for the sake of clarity.

[0083] The drawings illustrate various embodiments of optical systems 100, 400, 500, 600, 700, 800, 900, 1000, 1100, according to the disclosed technology. Not shown in the accompanying figures for the sake of clarity are any mounting mechanisms for mounting or otherwise fixing the optical system directly or indirectly on or into an anchor point of the object 108 or inner side of the boundary structure 106. In embodiments which refer to an imaging apparatus 102 being used to observe or monitor a field of view, FoV of a surface, it is implicit that this will be fixed in position in or on the boundary structure relative to the refractive element 122 using a suitable mounting or fixing system. An example of a suitable mounting or fixing system comprises a vibrational damping mechanism in some embodiments to improve the quality of any images captured and recording using the optical system.

[0084] All references herein to a boundary surface refer to a surface such as a that which may be found on one side of a boundary structure forming an object housing or case. References to a boundary structure accordingly also refer to an object housing or case, unless the context clearly requires otherwise. An example of a boundary structure for an object such as a vehicle or other form of land craft, e.g. a train, aircraft, or space craft comprises the vehicle body or fuselage of the aircraft a as a particular embodiment of such a boundary structure, which may form a wall or building infrastructure feature where the object is a building or similar form of infrastructure.

[0085] FIGS. 1 A and 1 B illustrate prior art examples of optical systems which demonstrate how boundary surface geometry and optical system geometry affect obtaining aFoV including the boundary surface and how to obtain such a FoV, fluid flow over the boundary surface is affected.

[0086] FIG. 1 A shows schematically an example prior art periscope type optical system 1 providing an observation point 2 with a field of view, FOV, 3 which passes through sides 4a, b, of a planar boundary structure 5. Light forming the FoV 3 enters a channel 6 through the boundary structure 5 via observation window 7 and then follows a path 8 from side 4b, to side 4a by being reflected off mirrors 7a and 7b. Optical system 1, however, can only provide a FoV 3 outwards from side 4b of the boundary structure 4 due to the geometry of the optical system 1 as the observation window 6 is co-planar with a surface 11 of boundary structure 4, and so cannot include the surface 11 as there is nothing to deflect light following a grazing angle of incidence along surface 11 into the window 6. As shown, fluid 10 flowing along surface 11 in the direction of the arrows shown will accordingly flow over a sealed window 7 provided on side B. If, over time, the fluid experiences any turbulence over the surface 11 or sealed window 7, any dirt or debris carried by the fluid may be deposited and accumulate on the window. This accumulation of dirt or debris may result in the observable FoV 3 obtained through the window 7 becoming increasingly obscured overtime. If, however, the fluid flow is non-turbulent, and the window 7 is co-planar with the surface 11 , then the fluid flow may not be disrupted over the window 7 reducing the rate at which FoV is likely to degrade due to dirt and debris accumulation.

[0087] FIG. 1 B retains the number system of FIG. 1 A,. FIG. 1 B shows another prior art example of a periscope type of optical system where the geometry of optical system 1 is different from that shown in FIG. 1A as the boundary 5 how has a surface comprising two levels 11a, 11b. The optical window 7 is normal to the lower level 11 b but co-planar with an edge 11 c of the upper boundary surface level 11a. This geometry allows a FoV 3 to include an area of the lower surface level 11b. The geometry of the surface 11 however results in turbulent flow 12 over the edge 11 c, which then passes over window 7. This turbulence increases the rate at which dirt and debris suspended in the fluid flow accumulates and obscures FoV 3.

[0088] Fluids such as air or water often include suspended particles such as pollen, dust and other forms of debris. It is accordingly highly desirable if turbulence over the window 7 is kept to a minimum so as to maintain a clear FoV for as long as possible.

[0089] The embodiments of the disclosed optical systems are configured to protrude from a surface of a boundary structure such as an object or object housing into a fluid flow along the surface. By configuring the optical system to have a streamlined geometry which provides laminar fluid flow over an observation window through the boundary structure, it is possible to maintain the FoV clear for longer than if the geometry introduces non-laminar, that is to say turbulent, fluid flow over the refractive surface. The boundary surface into which the optical system is embedded is preferably planar or near planar at least in the vicinity where the optical system protrudes from the boundary surface.

[0090] By embedding the optical system in the boundary surface, a FoV can be obtained of the area of the boundary structure in the vicinity of the optical system, and laminar flow is maintained over the observation window of the optical system by configuring the optical system to protrude through an aperture 104 in the boundary surface with a streamlined geometry which is configured to allow a substantially laminar fluid flow to be maintained over the refractive observation window 116 via which the FoV 112 is captured.

[0091] Advantageously, the disclosed optical systems permit a FoV to include light which has followed a grazing or near grazing optical path along the surface where fluid is flowing but, by not introducing non-laminar flow, the frequency at which the window capturing such light needs cleaning is not likely to be increased. In particular, in some embodiments, the laminar fluid flow over the observation window may be configured to provide a self-cleaning optical system, similar to an air-wash. Air-wash and liquid wash systems are well known. For example, an air-wash may be used to maintain a window of a wood stove clean. One or more additional coatings may be provided on one or more surfaces of the optical system exposed to the fluid flow in some embodiments to reduce drag on the exposed optical system and / or to reduce reflection from the window via which the FoV is captured. In some embodiments, the coatings may also enhance the self-cleaning effect of the optical system. Advantageously, if the boundary structure is part of an object being propelled through a static or flowing fluid, for example if the object comprises a land craft such as a train or vehicle, a watercraft, for example, a marine or submarine vessel, an aircraft, or a spacecraft travelling through a fluid, reducing turbulence and may reduce fuel consumption. Other forms of boundary structures may be located on objects such as pipelines and conduits through which fluid is flowing, in which case reducing drag is beneficial as less power is needed to propel the liquid through the object over long distances.

[0092] FIG. 2A shows an optical system 100 according to an example embodiment of the disclosed technology. In FIG. 2A, the optical system 100 is shown configured with an imaging system 102 in the form of a camera, however, as would be apparent to any one of ordinary skill in the art, the imaging system 102 may comprise a human eye in some embodiments. The optical system 100 comprises a refractive element 122 which forms an observation window 116 through a boundary structure 106 of an object 108. The refractive element 122 spans both sides, shown as side A and side B, of the boundary structure 106 in the illustrated embodiment. The refractive element 122 is configured to protrude out of surface 110 of the boundary structure 106 to form a refractive window 116 through the boundary structure 106 via which light 118a,c passes, where the light 118a,c including light 118c following a path at a grazing or near grazing angle of incidence, e, relative to a surface of a second side, side B of the boundary structure.

[0093] In the embodiment of FIG. 2A, the optical system 100 is configured with an imaging system 102 which allows the optical system 100 to function as a monitoring system forthe surface 110 which forms side B of the boundary structure 106 along which a fluid may flow in the direction of the arrow shown. To assist in streamlining the part of the refractive element 122 that protrudes into the fluid flow, as shown in the example embodiment of FIG. 2A, the optical system 100 includes a streamlined component presenting a streamlined curved surface 124 to the fluid flowing along boundary surface 110. In the embodiment shown in FIG. 2Athe streamlined component comprises a hood, preferably an opaque hood, which may be formed from metal or another suitable material, which protrudes into the fluid flow adjacent the refractive element 122 and which may extend partially over part of the refractive element 122.

[0094] In FIG. 2A, the optical system 100 is configured to allow rays 118a, 118c of light 118, for example, visible light, from one side to pass from one side B of a boundary structure such as an opaque boundary structure 106 to the opposite side A of the boundary structure 106 and form a FOV 112 comprising light rays 118a, 118c where light 118c comprises light captured at a grazing or near grazing angle of incidence e by planar surface 116 of the refractive element 122. The boundary structure 106 is opaque in at least the vicinity of the optical system 100.

[0095] The optical system 100 preferably comprises an imaging apparatus 102 having an external pupil 120 (not shown in FIG. 2A, see FIG. 2B for example) as this may help with providing a more laterally compact optical design than if an internal exit pupil is provided. In some embodiments, this means that where the optical system is installed in a cavity within the boundary structure, the cavity size can be kept laterally much smaller than would be possible if a camera with an internal exit pupil was provided. The optical system 100 shown schematically in FIG. 2B has a refractive element 122 which is installed in an aperture 104 of width W in the boundary structure 106 of the object 108.

[0096] In some embodiments, the optical system 100 provides an observation window through an opaque boundary structure which enables monitoring of the surface 110 of side B of the boundary structure! 06 when it is part of or forms a boundary structure of an object 108 which is configured to be exposed to or immersed in a fluid flow. It will be appreciated that in some embodiments, the optical system may be exposed to a fluid flow which is only intermittent in the direction shown in the FIGS, and / or that at times there may be no fluid flow or fluid flowing in a different direction. The configuration of the disclosed optical systems may be streamlined for fluid flows in just one direction or in multiple different directions in some embodiments.

[0097] The imaging apparatus 102 is shown located on first side A of the boundary structure 106 of the object 108 in FIG. 2A however, it may be embedded within the boundary structure 106 of an object in other embodiments, in which case side A may be considered to be located within the structure of the boundary. In all embodiments, however, the geometry of at least one planar surface forming refractive window 116 of the refractive element 122 permits the observed or monitored FoV 112 captured by the imaging apparatus 102 to include light 118c which has followed a path at a grazing or near grazing angle of incidence along the surface of side B of the boundary structure 106.

[0098] In some embodiments of the disclosed technology, FIG. the first side A of the boundary structure 106 may comprise the interior of an boundary structure 106, however, in some embodiments, side A may be provided on the exterior of a boundary structure 106, for example, if the object is a pipeline or conduit within which fluid is flowing.

[0099] In FIG. 2A refractive element 122 is disposed on both sides of the boundary structure 106. The refractive element 122 is configured to form a refractive window 116 through the boundary structure 106 which protrudes into the fluid flow with a height H. The refractive window 116 is inclined to the surface 110 on side B of the boundary structure at an angle A (not shown in FIG. 2A, see FIG. 2B for an example illustrating angle A). Light 118c incident at a grazing or near grazing angle of incidence e relative to the surface of the second side B of the boundary structure 106 is refracted towards the imaging apparatus 102 to allow FoV 112 which includes light ray118cto be captured by the imaging apparatus 102.[000100] The refractive window 116 may comprise one or more streamlined planar surfaces and may form part of or be adjacent to one or more streamlined surfaces 124 in some embodiments. At least one planar surface of which provides a refractive surface via which light travelling along at a grazing or near grazing angle of incidence is captured and refracted towards imaging apparatus 102 to form an undistorted view of the boundary surface 110 along which fluid is flowing. The at least one or more curved surfaces 124 may be provided by the optical system 100 including a curved or convex hood or the refractive element 122 may be provided with a curved or convex portion which protrudes above the boundary surface 110. The convex portion of the hood or refractive element may result in an elliptical or circular perimeter 126 being formed at the base on the convex portion with boundary surface 110, examples of which are shown schematically in FIG. 2D and 2E described later below. The one or more planar surfaces which form refractive window 116 may be provided as one or more slices through the convex portion in some embodiments. At least one of the one or more planar surfaces are inclined at a suitable angle A to the boundary surface 110 which is to be observed to capture a FoV 112 which includes light at a grazing or near grazing angle of incidence e to the plane of the boundary surface 110. The planar surface which captures light at a grazing or near grazing angle of incidence e to the plane of the boundary surface 110 may intersect with the perimeter 126 of the refractive element where it emerges through the aperture 104 in the boundary surface 110 in some embodiments. Alternatively, in other embodiments such as those shown in FIGS. 2E and 2F, a small region may be provided between the perimeter 126 and the lower edge planar surface. This small region is also referred to herein as a lip.[000101] Advantageously, in some embodiments, when assembled in the boundary structure or boundary structure, the optical system is configured to project a sufficiently low distance so that even if protected by a hood forming curved surface 124, providing the hood in combination with the refractive window is suitably streamlined in profile, any turbulence in the fluid flow along the boundary structure does not increase over the curved surface 124, and if thefluid flow is non-turbulent, laminar flow is maintained over the curved surface 124 and the refractive window. In some embodiments, the optical system may comprise a refractive window which presents an area of approximately 50mm2(or 50mm square, although the planar surface area may be elliptical or circular in cross-section) which extends less than 7mm above the surface of the boundary structure or the boundary structure 106.[000102] In some embodiments of the optical system 100 shown in FIGS. 2Ato 2F , the refractive element is configured to form the refractive window by passing through an aperture in the boundary structure to present a portion through which one or more streamlined planar slices are provided to form the refractive window 116. In other words, above the boundary surface, the optical system is configured to have a truncated cone or truncated dome-like shape, wherein the truncation is formed by one or more planar surfaces, at least one planar surface being inclined at a non-zero angle of inclination A to the boundary surface along which fluid is flowing. The refractive element is streamlined due to the configuration of the optical system forming a protruding convex portion in some embodiments which is truncated to present a planar refractive surface disposed at an angle A to the plane of the surface 110 on the second side, side B, of the boundary structure 106.[000103] The disposition of at least one planar window of the refractive surface 116 at the surface 110 is at an angle A which is greater than zero but less that approximately 20 degrees to the plane of the boundary surface so that the refractive window is suitably configured to capture a FoV 112 including incident light at a grazing or near-grazing angle of incidence along the plane of the boundary surface 110 of the boundary structure 106 and guide the light along the body of the refractive element 122 towards the imaging apparatus 102. In other words, the disposition of the refractive element 122 spanning the first side A and second side B of the boundary structure 106 allows a FoV 112 which includes incident light 118c which has travelled at a grazing or near-grazing angle of incidence e along the plane of the surface 110 of the second side B of the boundary structure 106 to be guided along the body of the refractive element 122 towards the imaging apparatus 102. An example of a grazing angle of incidence to surface 110 is illustrated by e schematically in the accompanying FIGS. .[000104] In some embodiments of the optical system 100, for example, as illustrated schematically in FIG. 2E and 2F described later below, the optical system comprises an imaging apparatus 102 for obtaining a field of view, FoV, 112 visible around a lip formed at the perimeter 126 of the convex portion of the refractive element 122 with aperture 104 located in boundary structure 106 of an object 108, the FoV including light at a grazing or near-grazing angle of incidence along surface 110 of the boundary structure 106.[000105] In FIGS. 2B-2F, the streamlined optical system 100 has a much lower raised profile height H as the refractive element 122 is configured with a streamlined convex curved surface 124. This height H may be much lower than would be provided if a separate hood was used to provide a streamlined surface over the refractive element 122 such as FIG. 2A showsschematically.[000106] Advantageously, light incident at a grazing or near grazing angle e is located at the periphery of the FoV 112 formed at the imaging apparatus 102 in the embodiments shown in the Figures, and not more centrally, which allows the FoV 112 captured to extend further (or higher) away from the plane of the surface 110 of the boundary structure 106. The extension of the FoV 112 formed at the imaging apparatus 102 is shown schematically as HF in the side view shown in FIG. 6 described later below.[000107] The optical systems 100 shown in FIGS. 2Ato 2F may be configured to allow the boundary surface 110 to be observed by an imaging system such as a human eye, in the manner of a periscope for example, or use an electronic or optical imaging system to capture the FoV 112 as still image(s) or as a video stream, which may be recorded and stored in some embodiments. By configuring a lens system of the imaging system 102 with an external pupil 120, the optical system 100 may be made laterally more compact than is possible if the lens system has an internal exit pupil.[000108] In some embodiments, the optical system 100 may reside within the boundary structure 106 within a recess rather than be placed on the other side of the boundary structure and comprises an imaging apparatus 102 which has a lens arrangement providing an external pupil 120 (see also the description of FIG. 3 below) which is provided within the recess.[000109] The imaging apparatus 102 described in FIG. 3 may be used with any of the disclosed optical systems having a refractive element 122, 402, 502 for example, such as are illustrated in the optical systems shown in FIGS. 2A-2F, 4, 5, 6, 7, and 9 to obtain a FoV 112 FIG. which includes light 118a, 118b, 118c where light 118c comprises light incident at a grazing or near grazing angle e along the surface 110 of the boundary structure 106.[000110] The term light 118 (or component light rays 118a,b,c) refers to any suitable type of electromagnetic radiation, for example, visible light, which can be imaged by a suitable imaging apparatus 102. As shown in FIGS. 2A-2F, 4, 5, 6, 7, 8, 9, 10, and 11 described herein, light 118 comprises a number of rays 118a,b,c. Light 118c represents schematically light which has followed a path at or near a grazing angle of incidence e along surface 110. The imaging apparatus 102 of FIG. 3 may comprise any suitable type for the desired imaging and may comprise a depth camera system or an event camera system in some embodiments.[000111] The FoV formed by imaging apparatus 102 in FIG. 1 B includes light 118c and has been captured without requiring a wider aperture than the refractive element 122 needs to pass through. Whilst this depends on the dimensions of the imaging apparatus 102 used, and possibly also whether the imaging apparatus uses a tilt mechanism, the diameter of the aperture via which the refractive element 122 passes, W, may be as little as less than 7mm in some embodiments.[000112] The arrangements of the imaging apparatus 102 and the embodiments of the refractive element 122 shown in FIG. 2Ato 2F provide a better view around the lip or corner ofaperture 104 of the boundary structure 106 and along the surface 110 of the boundary structure 106 than, for example, a fish-eye lens-based solution would, as a less distorted FoV is obtained via the one or more planar surfaces which are provided as slices through the streamlined convex surface portion of the refractive element 122 to form the refractive window 116.Moreover, in some embodiments, a tilt-lens mechanism which implements the Scheimpflug principle may be used within the imaging apparatus 102 to extend the reach HF (see FIG. 6 described below) of the FoV 112 normal to the plane of the surface 110.[000113] Returning to FIG. 2A, which illustrates schematically an embodiment where the refractive element 122 is shielded on one side by a streamlined hood, the hood extends by a distance H from the surface 110 of the object 108 into the fluid flow. However, in FIG. 2B-2F where the curved surface 124 of the refractive element 122 acts as the hood and forms a seal with the boundary surface 110 along a perimeter 126, the height H to which the optical system 100 protrudes into the fluid flowing along surface 110 can be further reduced than if a separate hood is provided. By using the convex portion, for example, a hood, in combination with the refractive element, or a protective window over the refractive element, to form the seal with the boundary structure, a lower protrusion into the fluid flow along the boundary surface can be achieved, meaning the refractive window is likely to experience a slower or no loss of clarity due to any buildup of debris suspended in or carried by a fluid such as air or water experiencing an increase in turbulence over the refractive window 116. In other words, any fluid flowing along the boundary surface 110 may experience less turbulence if the refractive element itself comprises a convex portion onto which a protective opaque coating is provided than the flowing fluid would experience if an additional protective element such as a hood is provided over or adjacent to the refractive element 122. Accordingly, in some embodiments, it may be advantageous if the refractive element 122 is configured with an opaque coating on a convex portion which, even with the opaque coating, will only protrude into the fluid flow by a very small amount H. In some embodiments, where an opaque or other form of protective coating or a hood forming from a different material is used over the refractive element, H can be made is sufficiently low for the anticipated environmental fluid flow conditions for there to be non-turbulent, that is to say laminar flow, over the window 116 as well as along the surface 110 of the boundary structure 106 whilst still allowing a refractive window to present an inclined planar surface to allow imaging of the surface along which the fluid is flowing. Typical examples of the height that may be obtained using the convex portion as a hood are around 6 or 7mm. In some embodiments, this represents a reduction by a factor of between 4 and 5, for example, around 4.5 in the raised profile presented by prior art non-streamlined optical systems.[000114] In some embodiments, to assist with keeping the refractive window 116 clear of debris a hydrophobic coating or other type of repellent coating may be applied to the planar surface and / or to the curved surface 124 of the convex portion of the refractive element 122. In addition, the configuration of the optical system may be designed to result in the optical systemself-cleaning the refractive window using the laminar fluid flow that results when the optical system is inserted into a fluid flow. Moreover, to assist with increasing the amount of grazing light 118c being refracted towards the imaging apparatus 102, the refractive window 116 may also or instead have additional special coatings applied in some embodiments. For example, an anti-reflection coating may be applied in some embodiments.[000115] As shown in the embodiment illustrated schematically in FIG. 2B, optical system 100 has a particular configuration in which the proximity of the near edge of refractive element 122 to the imaging apparatus 102 is reduced as much as possible by truncating the refractive element 122 in the bottom left corner. This truncation of the refractive element 122 together with the lens arrangement providing an external pupil 120, allows the optical system 100 to have a more compact form factor within the boundary structure 106 of object 108.[000116] Also shown in the example embodiment of FIG. 2B is the replacement of the streamlined hood shown in the example embodiment of FIG. 2A with a streamlined straight surface 124 forming a streamlined portion of refractive element 122 whereas the example embodiment of FIG. 2C shows the refractive element being provided with a streamlined curved surface 124, for example, a streamlined curved surface which may form a convex portion. In a similar manner to that shown in the example embodiment of FIG. 2A, the observation window 116 shown in FIG. 2B comprises a planar surface which is inclined at angle A to the boundary surface 110 which is being observed. Angle A is preferably greater than 0 but less than, for example, 20 degrees. The planar surface of the refractive window 116 shown in FIG. 2B also provides an undistorted FoV 112 to an observer or imaging system 102. The angle A at which the planar surface is disposed relative to the surface 110 of side B of the boundary structure 106 may be dependent on the boundary structure geometry and is designed to ensure FoV 112 captures sufficient light which has travelled at a grazing or near grazing angle of incidence e along the surface 110 of side B of the boundary structure to provide a useful area of observation of the surface 110.[000117] FIGS. 2C and 2D show respectively schematically side and overhead plan views of the embodiment illustrated schematically in FIGS. 2A and 2B in which the curved surface 124 provided by the hood shown adjacent to the refractive element 122 in FIG. 2A is replaced by a convex portion covering at least in part one end of the refractive element 122 which also presents a streamlined curved surface 124 which protrudes into the fluid flow along surface 110 is shown. In FIG. 2C, one or both of the refractive element and the adjacent covering convex portion, which may be provided by a hood, form a truncated convex portion which presents a curved surface 124 above the boundary surface 110 with the truncation being provided as an inclined planar surface which forms refractive window 116. FIG. 2D a plan view of the optical system of FIG. 2C which illustrates more clearly how the streamlined convex surface 124 includes a planar section which comprises the refractive window 116. In FIGS. 2C and 2D, the planar section is inclined to the boundary surface 110 and extends to the perimeter 126 formedby the surface 124 of the convex portion of the refractive element and the edge of the aperture 104 through which the refractive element passes. As shown in FIG. 2D, the perimeter defines an ellipse and the planar surface forming the refractive window 116 is also in the form of an ellipse, however, it will be obvious to anyone of ordinary skill in the art that other shapes and configurations of the convex portion and the planar surface may be have similar streamlined properties in other example embodiments of the disclosed technology.[000118] For example, if refractive window is provided in a conical convex portion defining a circular perimeter then the optical system 100 is streamlined and may support laminar fluid flow over the refractive window even when the fluid flow is not in the direction illustrated relative to the planar surface of the refractive window 116.[000119] The refractive window 116 may comprise more than one planar surface in some embodiments with at least one of the more than one planar surfaces forming or forming part of the refractive window 116 which provides a surface via which light travelling along at a grazing or near grazing angle of incidence e is captured and refracted towards imaging apparatus 102. In some embodiments where more than one planar surface is configured to capture light at a grazing or near grazing angle of incidence, more than one FoV of the surface 110 may be captured at the same time by the imaging apparatus 102. FIGS. 2E and 2F illustrate examples of embodiments where the refractive element 122 comprises a refractive window 116 formed from two planar surfaces in its convex portion.[000120] FIG. 2E shows schematically a plan view of the boundary structure side B in which an embodiment of the refractive element 122 is shown having have a streamlined and substantially cylindrical cross-section such that a circular or elliptical perimeter 126 with the surface 110 is formed by a convex portion of the refractive element 122 which protrudes above the surface 110 of the side B of the boundary structure 106 along which fluid is flowing. The convex portion presents a curved, smooth, and streamlined surface 124 as shown in the perspective view of FIG. 2F within the substantially laminar flow lines 200 illustrate substantially non-turbulent fluid flows over the curved surface 124 of the convex portion and the one or more planar surfaces which truncate the convex portion to form refractive window 116.[000121] The height H that the curved surface 124 of the convex portion of the refractive element 122 protrudes above the surface 110 of the boundary structure in combination with the streamlined configuration of convex curved surface 124 is designed so that if fluid flowing along the surface 110 of the boundary structure is non-turbulent then non-turbulent laminar flow will be maintained over both the curved surface 124 and the one or more planar slices which form the refractive window 116 in the curved surface.[000122] In the configuration shown in FIGS. 2A-2D, a single planar refractive surface in the convex portion of the optical system 100 provides a refractive window 116 which is inclined at an angle A to the surface 110 of the boundary structure and extends to the perimeter 126 with the surface 110. In contrast, in the embodiment illustrated in FIGS. 2E and 2F, therefractive window 116 is formed from two planar refractive surfaces. In FIGS. 2E and 2F, the more central planar refractive surface is shown as a circular slice in the convex surface 124. The planar refractive surface shown to the left-hand side of the refractive element 122 is configured so that its planar elliptical surface is inclined at angle A to the planar boundary surface 110 but does not touch the surface 110 in FIGS. 2E and 2F. This permits the FoV to include a view along the boundary surface 110 but does not include the perimeter 126 which forms the boundary at the base of the refractive element which is adjacent to the boundary surface 110.[000123] FIG. 2F shows the embodiment of the refractive element 122 of FIG. 2E in a perspective view. FIG. 2F represents the laminar flow as a plurality of streamlines or flow lines 200. In FIG. 2F, the refractive element 122 has a small lip region between the edge of the refractive window 116 and the perimeter 126 with the surface of boundary structure 106. A small opaque lip is shown in FIG. 2E and 2F between the portion of the refractive window 116 and the circular or elliptical perimeter 126 formed by the optical system 100 with the boundary structure surface 110. In FIG. 2F, the second, more central, planar surface is visible under the laminar flow lines 200 which , in which case a FoV 112 can be composed at an observation point on side A of the boundary structure 106 which includes an undistorted FoV 112 along the boundary structure surface 110 in the close vicinity to the circular or elliptical perimeter 126 and may also include another undistorted view of the environment around the boundary structure obtained via the more central planar surface shown in FIGS. 2E and 2F. The different areas of FoV 112 may be obtained selectively in some embodiments and / or be provided as a composite FoV at the imaging apparatus 102 in some embodiments.[000124] The optical system of the disclosed technology as illustrated by the example embodiments shown in FIGS. 2Ato 2F supports laminar, in other words, non-turbulent, fluid flow over the refractive window 116 formed by the planar surfaces which slice through a streamlined curved surface portion provided over or adjacent to a refractive element 122 above the boundary surface. By ensuring the convex surface is sufficiently streamlined and only protrudes a small amount H above the boundary surface 110, dirt and debris are less likely to be deposited from the fluid flowing over the refractive window 116 which reduces the need to clean the window in order to maintain a clear FoV 112 of the boundary surface 110.[000125] Another benefit of H being small and only a streamlined convex portion of the optical system 100 protruding into a fluid flowing along boundary surface 110 is that the amount of drag exerted on the boundary surface 110 by the fluid flowing around and over the refractive element is reduced.[000126] To reduce H to as low a value as possible, the curved surface 124 of the streamlined portion of the optical system is configured in some embodiments to form a seal along perimeter 126 with the boundary surface 110. In some embodiments, this streamlined portion is provided by a separate hood element with a streamlined curved surface 124. Thiscurved surface 124 may cover part or all of the refractive element. In some embodiments, in addition or instead of the hood, an additional protective planar screen or window may be provided which conforms to the one or more planar surfaces that form the refractive window 116 provided by the refractive element.[000127] As has been mentioned earlier, in all of the example embodiments disclosed herein, where the boundary surface 106 or object 108 is exposed to or immersed in a fluid flow, the fluid may flow in other directions in addition to or instead of the direction of fluid flow shown in FIG. 2A, however, the degree to which flow is laminar over the refractive window may be dependent on the direction of fluid flow relative to the angle of inclination of the planar surface(s) which form the refractive window..[000128] FIG. 3 shows schematically an enlarged cutaway side view of an example embodiment of an imaging apparatus 102 of an optical system 100 according to the disclosed technology such as that shown in FIGS. 2A-2F, 4, 5, 6,7, and 9. The imaging apparatus 102 may comprise any suitable imaging apparatus, for example, a camera system which is configured to form images using visible light in some embodiments.[000129] The camera or imaging apparatus 102 may be configured to capture one or more or a series images or a data feed comprising image data, for example, a video sequence may be captured. The embodiment of the example imaging apparatus 102 having an external pupil 120 shown schematically in FIG. 3 may be adapted for use in any of the other embodiments of the optical system shown in the other FIGS. 2A-2F, 4, 5, 6, 7 and 9. It will be apparent to anyone of ordinary skill in the art that the principles described herein for a camera or visible light imaging apparatus 102 may be readily adapted to capture images of electromagnetic radiation at other wavelengths, for example, a near-infra-red, NIR, camera may be used to capture radiation refracted via refractive element 122 from the surface 110 of boundary structure 106. The imaging apparatus 102 may comprise a depth camera and / or an event camera in some embodiments of the optical system according to the disclosed technology.[000130] In FIG. 3, the imaging apparatus 102 comprises a camera with an external pupil 120 provided by a lens arrangement 300. Lens arrangement 300 is located in a lens mount 302 (which may also be referred to herein as a lens housing 302). The lens arrangement 300 comprises a plurality of lenses configured to provide an external pupil 120, for example, a plurality of spherical lenses may be provided as is illustrated schematically in FIG. 3.[000131] An example of a suitable lens arrangement 300 providing an external pupil 120 comprises spherical lens 304,306, 308 which are shown in FIG. 3 in glass-to-glass contact along a common optical axis 316 and with free space 312 off the optical axis. Another spherical lens 314 is shown in the lens arrangement 300 in FIG. 3 which is separated from spherical lens 308 by a spacer 310. Each lens shares a common optical axis 316 and collectively they are configured to capture light as it emerges from refractive element 122 to focus the light forming the FoV 112 at imaging plane 318, for example, at a film or image sensor 318. To ensure theimaging apparatus 102 is held in position relative to the refractive element 122 and the boundary structure 106, one or more suitable fixations or mounting points may be provided. For example, if the camera system is screwed into position, recesses 320a, 320b may be provided to receive bolts or screws.[000132] An advantage of using an imaging apparatus 102 having an external pupil 120 is that more light may be captured after its confined passage along a refractive element 122 whilst keeping the refractive elements 122, 402, 502, 504 and the optical system 100, 400, 500 as compact as possible. The use of an external pupil 120 also allows the imaging apparatus to be placed in close proximity or adjacent to the refractive element 122, 402, 502, for example, as FIGS. 2A -2F, 4, 5, 6, 7, and 9 show schematically.[000133] FIG. 4 retains the numbering scheme of the elements illustrated in the previous FIGS, for like elements. In FIG. 4, an example embodiment of an optical system 400 is illustrated in which refractive element 122 comprises a monolithic refractive element 402. The monolithic refractive element 402 is configured to both refract and reflect incident light 118. As in the example embodiments disclosed herein, light 118 comprise rays of 118a, b,c which form the FoV which includes light raysl 18c which have followed a path along a grazing near grazing angle e along the surface 110 of boundary structure 106.[000134] The refractive window 116 of the monolithic refractive element 402 is disposed at an angle A relative to surface 110 in order to guide light 118a, c towards a suitable imaging apparatus 102 having an external pupil 120 in order to capture a FoV 112 which includes light 118c which has followed a path at a grazing angle of incidence e along surface 110. The angle A may be dependent on the refractive properties of the composition of the refractive element as well the geometry and size of the optical system and the desired extent of the FoV 112 along the surface 110.[000135] In the example embodiment of FIG. 4, the refractive element 402 is configured to reflect light 118 internally via reflective surface 404a which then exits via refractive element surface 404b. The refractive element 402 accordingly causes light, 118 incident on the window 116 of the refractive element to follow a return path along which it is first refracted at the window 116, and then reflected at surface 404a, and then refracted again at surface 404b back towards the imaging apparatus 102, passing through the external pupil 120 of the imaging apparatus’s optics before entering the imaging apparatus. This returned or folded optical path accordingly allows for optical system 400 to have an even more laterally compact form than the optical system 100 shown schematically in FIG. 2B for example.[000136] In some embodiments, the refractive element 122, 402 has uniform refractive material properties, in other words, it is homogeneous. However, it is also possible in some embodiments for the refractive element to have non-uniform refractive properties, in other words, it may have inhomogeneous or heterogenous refractive properties. In other words, in some embodiments, the refractive element shown in FIGS. 2Ato 2F and 4 may be composedinstead of two or more materials having different refractive properties, thus forming two refractive regions within the refractive element or elements such as FIG. 5 shows schematically.[000137] In FIG. 5 an example embodiment of an optical system 500 is illustrated schematically in side view in which the refractive element 122, 402 is not uniform and instead takes the form of a composite refractive element formed by two adjacent refractive regions or elements 502 and 504 which share a common surface 510. In FIG. 5, like elements share the numbering scheme of the previous FIGURES.[000138] In some embodiments of the disclosed embodiment, alternatively, the refractive element may have a non-homogenous refractive index may be provided, for example, a refractive element may have a gradient refractive index, GRIN, where the refractive index varies in a linear or non-linear way, or radially from its optical centre.[000139] The refractive element shown in FIG. 5 comprises at least two distinct refractive regions or elements 502, 504 which share at least one partially transparent interface at surface 510. The transparent (or partially transparent) surface 510 allows light 118 to pass through from a first refractive region or element 502 to the adjacent refractive region or element 504. It will be apparent that additional refractive regions or elements may be provided in other embodiments of the disclosed technology. These other optical components and refractive regions or elements may be used in addition / instead in some embodiments of the disclosed technology in order to obtain a desired compactness of the optical design based on the geometry and size of the refractive element or refractive component and imaging apparatus 102 shown in FIG. 5, taking into account the depth of the aperture 104 formed in the boundary structure 106 and the desired FoV 112 along surface 110. The first refractive region or element 502 is configured to pass through an aperture such as the aperture 104 in the boundary structure 106 shown in FIGS. 2A and 2Bin order to present a refractive window 116 on the surface 110.[000140] The refractive window 116 may be formed from one or more surfaces, at least one of which is a planar surface in some embodiments. For example, a convex or concave refractive surface may also be used to form refractive window 116 in some embodiments. If a plurality of refractive surfaces may form the refractive window 116, at least one refractive surface is a planar refractive surface. In some embodiments, the refractive element comprises at least one of: a prism; a prism with a mirrored internal surface; and a prism with a partially reflective mirrored internal surface. The refractive element may be configured to refract light via at least one airgap towards at least one mirror in some embodiments with the refractive element and / or the at least one mirror is configured to reflect light towards the imaging apparatus.[000141] The planar surface forming the refractive window 116 which is disposed at an angle, A is shown in both FIGS. 4 and 5 by the angle between the dot-dashed line representing the planar surface 110 of the boundary structure 106 and the planar surface of the refractive window 116.[000142] The size of angle A, in other words the disposition of window 116, at surface 110 is configured to allow the imaging apparatus 102 to capture a FoV 112 (such as the FOV 112 shown in FIG. 1D) which includes light 118c which is captured by the refractive element 502 at a grazing or near-grazing angle of incidence along the plane of the surface 110. Angle A is accordingly greater than zero but will usually be less than approximately 20 degrees. The refractive element 502 extends through and spans the boundary structure 106 in order to guide light along its body of the first refractive element 502 to enter a second refractive element 504. The second refractive element 504 is configured with an inner reflective surface 506 configured to reflect received light to towards a refractive surface 508 via which light 118 escapes and passes in via the external pupil of the imaging apparatus 102 to the imaging plane 318 (see FIG. 3) where the FoV 112 is brought into focus .[000143] In the example shown in FIG. 5, light 118 passes from the first refractive element 502 via surface 510 into the second refractive element 504 and is reflected off inner reflective surface 506 and then refracted as it emerges via refractive surface 508 towards an external pupil 120 (see FIG. 2B, not labelled in FIG. 5) of the imaging apparatus 102. This second refractive element 504 thus causes light, 118 to follow a folded or return optical path in some embodiments towards the imaging apparatus 102 which allows the optical system 500 to have a more compact lateral footprint.[000144] In the disclosed example embodiments, references herein to light refer equivalently to any suitable form of electromagnetic radiation for imaging the desired FoV along surface 110 using a suitable imaging apparatus 102. References to light accordingly may refer to visible light and / or any other suitable part of the electromagnet spectrum which can be imaged by the image capture device either as a still image, a series of images, or as a feed or video of images, for example, visible light, infra-red or ultra-violet -ray part of the electromagnetic spectrum.[000145] In some examples of the embodiments of the disclosed technology illustrated schematically in FIGS. 2B-2F, 6 and 7 , the refractive window 116 and the solid body of the refractive element 122 have a streamlined configuration and form a seal with the boundary surface, for example, with the boundary structure 106. Alternatively, in some embodiments, a hood provides the streamlined surface 124, such as is illustrated schematically in FIG. 2A, 4, 5, and 8 to 11 where the refractive window 116 of the refractive element 122, 402, 502 in combination with a protective hood adjacent to and / or partially over the refractive element 122 provide a streamlined configuration which forms a seal with the boundary structure 106.[000146] In some embodiments of the disclosed technology, for example, as illustrated in FIGS. 2A to 11 , a protective film or other form of additional transparent protective window or filter is provided over the refractive window 116. For example, a dirt repelling coating may be provided in some embodiments, such as a hydrophobic or oleophobic coating depending on the environment to which the refractive window is exposed. In addition, a coating may be providedto improve the transmissibility of light at the grazing angle of incidence in some embodiments.[000147] A protective transparent window over the refractive window 116 may also be beneficial in embodiments where, for example, there is a risk the material composition of the refractive element 122, 402, 502 could degrade over time if not protected.[000148] The height HF normal to the plane of the surface 110 shown in FIG. 5 may be increased and a bigger FoV 112 captured in some embodiments where the image captures system comprises a camera system with a tilt-mechanism.[000149] FIG. 6 shows schematically an embodiment of the disclosed technology where an optical system 600 such as optical system 100, 400, 500 disclosed herein comprises an imaging apparatus 102 configured with an external pupil 120. The optical system 100, 400, 500 may be otherwise the same as that described earlier herein, but now in addition comprises an optical geometry arrangement which uses the Scheimpflug principle to align the optical object plane more closely to the target object, through the use, for example, of a tilt-lens (not visible in the imaging apparatus 102 shown in FIGS. 6 or 7).[000150] The imaging apparatus 702 in FIG. 7 differs from that shown in FIG. 6 as it has an internal pupil 202 , whereas the imaging apparatus 102 as shown in FIG. 6, and in Figures 2A and 2B, has an external pupil 120. Both imaging apparatus 702,102 may be adapted in some embodiments of the disclosed technology to have a better aligned focal plane by conforming to the Scheimpflug principle.[000151] For example, in some embodiments of the optical geometry shown in FIG. 6, for example, the lens arrangement of the imaging apparatus 102, that is to say the orientation of the plane of focus, the lens plane, and the image plane of the optical lens arrangement of the imaging apparatus 102 are all configured to maintain a particular geometric relationship complying with the Scheimpflug principle when the lens plane is not parallel to the image plane. The Scheimpflug principle may be used in some embodiments of the optical system 100, 400, 500, 600, and 700 where the imaging apparatus 102 has similar functionality to, or comprises, a type of view camera which may invert the FoV 112 in its images.[000152] Accordingly, another example embodiment of imaging apparatus 102 for any of the embodiments of the optical systems disclosed herein comprises a view camera. For example, imaging apparatus may comprise a camera lens arrangement which forms an inverted image at the plane where the image is focused which allows both the distance of the plane of focus from the camera lens and the orientation of the plane of focus be adjusted and perspective controlled by tilting the lens arrangement standard backwards or forwards. Tilting the lens arrangement in this way changes, the angle between the lens axis and the perpendicular to the imaging plane changes, in other words the lens tilt changes whilst based on the Scheimpflug principle, the plane of sharp focus is also adjusted to allow any plane to be brought into sharp focus.[000153] In other words, by tilting the lens plane, or standards, relative to the focusedimage plane, it is possible for any plane to be brought into sharp focus at the focussed imaging plane. The term focused imaging plane here refers to a plane at which the focused image is captured or recorded for example, by using an image sensor or sensor array(s) or by using photographic film. By tilting the lens plane (or the plane of the lens arrangement), the plane of sharp focus is also titled according to the geometrical and optical properties of the imaging apparatus 102, and the three planes, namely the lens plane, the plane of sharp focus, and the focussed imaging plane, will intersect along a line below the view camera for a downward lens tilt, and above for an upwards tilt.[000154] This tilted plane of sharp focus may be useful in various embodiments of the disclosed technology as the plane of sharp focus may coincide with both a near and a far object, for example, a near and far object along the surface 110 or a point on the surface 110 and above the surface 110 depending on the orientation of tilt. This allows both near and far objects or locations on surface 110 to be in focus.[000155] Scheimpflug differs from increasing the Depth of Field to encompass more of the physical object by instead aligning the optical object plane more closely to the physical object, resulting in more of the physical object being in focus. This allows for the use of a shallower depth of field, DoF, the benefits of which are understood by those knowledgeable in photography.[000156] The disclosed embodiments of the optical systems 100, 400, 500, 600, 700 share the feature of having a much lower protrusion height H as a result of the low protrusion of the streamlined portion adjacent to or covering the inclined streamlined surfaces provided by the refractive element 122 protruding from the surface 110 of the boundary structure 106. This height H is the extent to which the curved surface provided by the convex portion of the optical system protrudes above the boundary surface 110. H may be the extent which either the hood of the refractive element 122, 402, 522 extends from the boundary structure surface 106 or the extent to which a convex portion of the refractive element itself extends.[000157] By lowering the protrusion height H it helps reduce turbulence and ideally results in a non-turbulent fluid flow over the refractive element 122, 402, 502, as was shown in FIG. 3 described above which shows schematically the flow lines for an example embodiment of the disclosed technology which indicates a non-turbulent flow can be achieved using the disclosed embodiments. Reducing or eliminating turbulence over the refractive window 116 may provide various technical advantages.[000158] Another technical advantage may be provided by keeping the aperture 104 as narrow as possible which is achievable by using an embodiment of the optical systems 100, 400, 500, 600, 700 such as those shown schematically in the accompanying FIGS, as this may result in a greater degree of protection being provided in the interior of the object 108 from the exterior environment. For example, if the window 116 looks out on an environment with hazardous or adverse external environmental effects such as high levels of radiation etc, orexposure to projectiles, a wider aperture 104 through the boundary structure this could increase the likelihood of damage such as the radiation or projectile penetrating into the interior and damaging the interior of the object. In other words, a benefit of having a narrower aperture 104 via which the refractive element 122 spans a boundary structure such as an boundary structure is that it may reduce potential damage to the imaging apparatus 102 of the optical system, any optics and / or other internal components compared to when a wider aperture (e.g. W) is used to bridge the boundary structure 106 (see FIGS. 2A and 2B for example).[000159] Returning now to FIG. 2F, this shows schematically an overhead view of an embodiment of the optical system 100 in a perspective view, in other words, looking down at an angle on the surface 110 of a boundary structure 106, similar to that shown in FIG. 1 D. As shown in FIG. 2F, a planar refractive window 116 of the refractive element 122, 402, 502 of optical system 100, 400, 500, 600, and 700 protrudes from the surface 110 at a non-zero angle A, where A is preferably less than say 20 degrees but greater than zero, allowing it to capture light 118c which has following a grazing angle of incidence along the surface 110 along which fluid is flowing. The curved surface 124 of the convex portion of the refractive element 122, 402, 502 which protrudes out of the surface 110 is streamlined to support laminar flow over the convex portion and over the planar surfaces (of which two are shown in FIG. 3). Other modifications may be made to the surfaces of the refractive element to accommodate a shape which permits a stable fluid flow over its surface in some embodiments.[000160] The refractive window 116 is configured in the embodiment shown in FIG. 2F as a circular or oval planar surface which is angled at an angle A relative to surface 110 so as to capture a FoV 112 (not shown in FIG. 2F) which includes light at a grazing or near grazing angle of incidence, e, to the surface 110 according to some embodiments of the disclosed technology. Also shown in FIG. 2F are the fluid flow lines 200 which illustrate how the projection H of the hood or the curved convex surface of the optical system which may be provided by a hood or by part of refractive element itself is low enough measured from surface 110 to avoid a large increase in drag or causing turbulence in the fluid flow.[000161] FIG. 2F illustrates how a lower profile height H of an optical system 100, 400, 500, 600, 700, resulting from using a refractive window 122, 402, 502 which allows less projection height H from the boundary surface in combination with a convex portion protruding from the boundary surface 110 results in less turbulent fluid flow over the curved surface 124 of convex portion. It is also possible for the convex portion to be shield by a convex hood or coating which conforms to the curved surface 124 of the convex portion so as to provide a curved streamlined surface which results in less turbulent, in other words, more laminar, fluid flow over refractive window 122, 402, 502 of the optical system. In embodiments where the object is a propelled object such as a vehicle, aircraft or spacecraft, train, or accessory vehicle, such as a trailer, tug, or launch vehicle for example, less turbulence means less drag which correspondingly reduces the amount of fuel which may be used.[000162] In some embodiments where the optical system 100, 400, 500, 600, 700 s deployed, the external environment is a fluid such as a gas or liquid, for example, air or water. In other embodiments, however, the optical system may be used to obtain a FoV along a surface of an object which is subject to low-pressure or high-pressures, or low or high temperatures. For example, in some embodiments, the environment to which the surface of the boundary structure is exposed may be a high-pressure environment such as is found at a great depth in water. For example, in some embodiments, the environment to which the surface is exposed may be a low-pressure environment or vacuum. For example, the exterior environment may be space if the optical system is deployed on a space-craft or launch vehicle or a satellite.[000163] Advantageously, embodiments of the optical system according to the disclosed technology, of which example optical systems 100, 400, 500, 600, 700, 800, 900, 1000, 1100, have been described herein, provide a field of view, FoV which permits greater visibility along surface 110 closer to the edge of aperture 104 via the refractive element 122 even when a lip is provided along the boundary of the aperture with the perimeter 126 of the refractive element 122 at surface 110.[000164] As shown in FIG. 2F, where the aperture is an oval or circular aperture 104 in the surface 110 of the boundary structure 106, and the refractive element 122, 402, 502 has a curved convex protruding portion into which one or more planar surfaces from a refractive window 116, a compact imaging apparatus 102 (not visible in FIG. 2F) can be used to capture light 118 guided via the refractive element 122, 402, 502 to form a FoV 112 of surface 110 by capturing in the FoV 112 rays of light 118c which have followed a path at a grazing or neargrazing angle of incidence along the surface 110 of the boundary structure 106.[000165] In some embodiments, accordingly, of the optical system 100, 400, 500, 600, 700 shown in FIG. 2F, the optical system 100, 400, 500, 600, 700comprises: a refractive element 122, 402502, configured to pass through the aperture 104 of a boundary structure 106 in order to present a convex portion providing a streamlined curved surface 124 which protrudes from a surface 110 of the boundary structure, the convex portion being truncated to include at least one planar surface which forms a refractive window 116. At least one of the planar surfaces is disposed at an angle A to the plane of the surface 110 of the boundary structure wall which is to be observed using the optical system, wherein the disposition of at least one surface forming the refractive window 116 at the boundary surface 110 is configured to capture a FoV 112 including incident light 118c at a grazing or near-grazing angle of incidence along the plane of the surface 110 of the boundary structure 106 and guide light 118c along the body of the refractive element 122, 402, 502; and an imaging apparatus 102 configured to focus received light refracted via the refractive element 122 comprising the FoV 112 including the incident light 118c at the grazing or near grazing angle of incidence.[000166] As shown schematically in the particular embodiment of FIG. 3, the refractive element 122, 402, 502 projects in the form of a convex portion including one or more planarsurfaces, and may take the form of a truncated sphere or hemisphere or a truncating cone from the surface 110 of the boundary structure, where the truncation forms a window 116 having a circular or oval shape on the refractive element 122. The remainder of the refractive element 122, 402, 502 is concealed within a hood. In other words, the refractive element may have a frustoconical configuration in some embodiments in addition to one or more inclined planar sections allowing light which has followed a grazing or near grazing angle along the boundary surface to be captured, where the frustroconcial section may be formed using curved surfaces or be provided by straight, e.g., frusto-pyramidal, surfaces in some embodiments.[000167] The disclosed optical systems 100, 400, 500, 600, and 700 may be of use for a variety of different purposes ranging from, but not limited to, covert operations where the refractive window may misdirect a casual observer of the object regarding the direction of the FoV being monitored to monitoring the structural integrity of one or more boundary surface(s) of objects such as buildings, machines, vehicles, space as well as submersible, sub-sea, and surface sea-craft such as ships and submarines, including remotely operated objects.[000168] In some embodiment, the boundary structure comprises the object or a part of the object or comprises a shell or a housing for the object.[000169] In some embodiments, the optical system is mounted on or located on the object.[000170] In some embodiments, if the optical system is located or mounted on the object for use in one or more environments where the surface of the boundary structure surface experiences at least from time-to-time fluid flow, the optical system is configured with a low projection height, for example, less than 7mm, above the surface of the boundary structure surface of the object. For example, the refractive window may project less than 7mm in some embodiments of the disclosed technology above a surface of the boundary structure.[000171] Some embodiments of optical system 100, 400, 500, 600, 700, 800, 900 1000, 1100, as shown in the accompanying FIGS, may be provided pre-assembled on a mount or in a housing for attachment to side A of the boundary structure 106 or another internal feature of object 108. Alternatively, the optical system 100, 400, 500, 600, 700, 800900, 1000, 1100 may be provided for assembly or partially assembled, for example as a kit of parts.[000172] As mentioned above, references herein to light include references to non-visible light, for example, to ultra-violet, UV, NIR, Far IR, or any other type of retractable electromagnetic radiation. It will be apparent that some embodiments of the disclosed technology that the imaging apparatus 102 comprises a camera as shown in FIGS. 2A-2F, 3, 4, 5, 6, and 9 which may have a lens arrangement which has an external pupil 120, for example, see the lens arrangement 300 shown in FIG. 3. Alternatively, a camera 1 may be used which has an internal pupil 202 in some embodiments such as FIG. 1C, 2B and 6B show schematically.[000173] Advantageously, by using an imaging apparatus 102 having an external pupil 120 it allows the optical system to be provided in a more compact form by placing the imagingapparatus 102 closer to the refractive element and / or by truncating the refractive element 122, 402, 504. This closer proximity is also benefit if it is desirable to reduce the width of the aperture 104 through the boundary structure 106.[000174] In some embodiments, the boundary structure 106 is configured to separate two different environments in use. Examples include one of the following different types of environment: a vacuum environment; a fluid environment, where the fluid environment comprises one of a liquid environment or a gas environment. Alternatively, in some embodiments the environments are the same type of environment but have different physical characteristics. For example, the environments may have different temperatures, pressures, or fluid flow. In some embodiments, the imaging apparatus is fixed in use to one or more of: the boundary structure, the refractive element, a part of the object so as to stabilize the optical system. Vibrational supports may be used to fix the optical system in place in order to reduce vibration in some embodiments.FIG. 8 shows schematically an embodiment of an optical system comprising for providing a window through a boundary structure of an object, wherein the optical system comprises a refractive element spanning both sides of the boundary structure, the refractive element being configured to form a refractive window through the boundary structure via which light passes, the light following a path at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure, where the optical system is configured as an illumination system.[000175] In FIG. 8, the illumination system 800 comprises a light source 804 configured to inject light into a refractive element 802. This refractive element may comprise the same type ora similar type of refractive element 122, 402, 502 shown in FIGS. 1D, 2A, 4, 5, 6 and 7, spanning both sides of a boundary structure 106. Examples of a boundary structure 106 which a refractive element 822 may span, include, but are not limited to an object 108 or part of an object such as a shell or boundary structure of an object 108. The object may, for example, be part of an aircraft, vessel, automobile, truck, truck or vehicle accessory, building infrastructure or any other suitable object.[000176] In some embodiments, the example illumination system 800 shown in FIG. 8 comprises the same elements shown in FIG. 1D with the imaging apparatus 102 replaced with a suitably configured light source 804. The light source 804 may comprise a point light source in some embodiments. The light source may be collimated or uncollimated. The light source may be polarized or unpolarized. An example of a suitable light source for some embodiments comprises a light emitting diode.[000177] An embodiment of the illumination system 800 shown in FIG. 8 comprises an optical system similar to the optical systems 100, 400, 500, 600, or 700 which is configured to provide a refractive window 116 through a boundary structure, for example, an boundary structure 106 of an object 108 for illuminating a surface of the boundary structure. The opticalsystem comprises: a light source 804 located on a first side of the boundary structure, for example, a boundary structure 106 of the object 108 and a refractive element 122 spanning both sides of the boundary structure. The refractive element 802 may be of the type of refractive element 122, 402, 502, 1004a, b 1102, described in any of the other embodiments disclosed herein in that it is configured to form a refractive window through the boundary structure from which light 806 shown as light rays 118a, 118c illuminates the other, opposite, surface 110 of the boundary structure 106 from which the illumination source 804 is located. It will be appreciated that the light rays 118a, 118c shown in the FIGS, for optical systems acting as sources of illumination of the boundary surface 110 travel in the opposition direction through the refractive elements to light which forms an observable FoV 112 of the boundary surface 110.[000178] In some embodiments, the refractive element 802 comprises a plurality of refractive elements and / or reflective, for example mirrored, elements which may be separated by air gaps from each other and / or the illuminating light source. As illustrated in FIG. 8, a refractive element 802 comprises a refractive window 116 having at least one refractive surface via which light 806c emerges at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure so as to illuminate the surface of the second side of the boundary structure. The surface 110 of the second side of the boundary structure may be a surface opposite the surface on the first side of the boundary structure, for example, a surface opposite the surface on the first side of a boundary structure 106.[000179] The illumination system shown in FIG. 8 may comprise any of the disclosed optical system aspects or their embodiments for imaging a FoV along a surface of a boundary structure but with the imaging apparatus replaced with a suitably configured light source so that the FoV is instead illuminated by light from the light source.[000180] FIG. 9 shows an illumination system 900 comprising the illumination system of FIG. 8 configured so that light from light source 904 passes through a suitable lens arrangement, for example, the compound lens arrangement 300 of an optical system 100, 400, 500, 600, 700 according to any of the other disclosed embodiments such as those shown by way of example in FIGS. 2A to 2F, 4, 5, 6, and 7, before entering refractive element 802.[000181] The refractive element 802 may comprise any of the disclosed refractive elements 122, 402, 502, 602, 1004a,b, 1102 or another type of refractive element. Thus the term refractive element may refer to one or to a plurality of refractive and / or reflective components and / or airgaps which collectively form the refractive element passing through the boundary structure to provide refraction of light from the light source 904 in FIG. 9.[000182] The lens arrangement 300 shown in FIG. 9 provides a light source 904 with an external exit pupil 120 to the light source 904 as illustrated but may be provided with a lens system configured to provide an internal pupil 202 in alternative embodiments. In either case, the lens arrangement 300 in combination with light source 904 guides light 806a, c towards an external point so that the light 806 appears to come from a point light source 902 located beforethe location of the refractive element 802. In some embodiments of the illumination system 900, light source 904 and a lens arrangement 300 configured to provide a point light source 902 located within the adjacent refractive element 802.[000183] Some embodiments of the disclosed optical systems 100, 400, 500, 600, and 900, 1000, 1100 may comprise an imaging apparatus 102 having an external exit pupil configured at a location within a refractive element 122, 402, 502, 802, 1004a, b, 1102 to reduce the lateral footprint of the imaging apparatus 102 and enable a more laterally compact optical system to be provided.[000184] Advantageously, the embodiment shown in FIG. 9 provides a laterally compact illumination system which is capable of illuminating a larger surface area on the other side of any boundary structure which the refractive element bridges than if the point light source was located outside the refractive element.[000185] In some embodiments, the optical light source of the illumination system may comprise a collimated or uncollimated light source.[000186] In some embodiments, the light may be polarized light.[000187] In some embodiments, the light may be unpolarized light.[000188] In some embodiments, one or more surfaces of the refractive element is provided with a suitable coating to improve the transmission of light from the light source through the one or more surfaces.[000189] FIG. 10 shows schematically an example of an illumination optical system 1000 according to the disclosed technology comprising a similar optical system to that shown schematically in FIG. 5 but replacing with a light source 1002 the imaging apparatus 102. The light 118 now travels in the other direction through the refractive element and FIG. emerges as light 118a,c. Light 118c follows a path having a grazing or near grazing angle of incidence relative to the boundary surface 110 allowing illumination of the boundary surface 110. The light source 1108 of FIG. 10 may comprise a light source such as that described hereinabove with reference to FIG. 8 or FIG. 9.[000190] In some embodiments of the example illumination system illustrated schematically in FIG. 10, the refractive element 1004 comprises a composite or inhomogeneous refractive element having two different refractive elements or regions 1004a, 1004b separated by a boundary 1010. It will be appreciated however that the refractive element may have other components and form factors and still achieve a reversal of the direction of light from light source 1002 so as to allow for a more laterally compact size of illumination system. For example, light 118 is folded back from the light source 1002 by the refractive element 1102 having a coating in some embodiments providing an internally reflective surface 1006 so that when it emerges out of the another side of the boundary structure via refractive window 116, it is travelling in the reverse direction to the direction it emerges from the light source 1002. In some embodiments, the refractive regions 1004a, 1004b may be separated by an airgap insome embodiments. In addition, or instead, in some embodiments, refractive regions or elements 1004a, 1004b may have a mirrored or reflective element disposed between them. Additional reflective elements may be provided by external mirrored surfaces separated by air gaps from the refractive regions (also referred to herein as refractive elements) 1004a,b in some embodiments.[000191] The refractive element 1004 is shown with an optional truncated corner 1012 form factor in the example embodiment of FIG. 10 which further reduces its lateral size.[000192] FIG. 11 shows schematically an example of an illumination system 1100 according to the disclosed technology where the illumination system of FIG. 10 has a simpler single body refractive element 1102 having a reflective surface 1106 and a light source 1104. The illumination system 1100 may comprise similar components to those described herein above for FIG. 10 and / or FIG. 4, as the illumination system 1100 comprises a similar optical system to that shown as optical system 400 in FIG. 4 but with the light source replacing the imaging system 102.[000193] In FIG. 11, light travelling in a first direction from light source 1104 passes through the refractive element 1102 and is reflected from mirrored surface 1106 before emerging as light rays 1108 a, c in a reverse direction to illuminate the opposite surface 110 of a boundary structure, for example, of an boundary structure 106, of object 108 from the side of the boundary surface the light source 1104 is located on. The light source 1108 of FIG. 11 may comprise a light source such as that described hereinabove with reference to FIGS. 8, 9 or FIG.10. Accordingly, even the simpler arrangement of FIG. 11 allows a highly laterally compact light source to be used to illuminate along the surface 110 of the other side of a boundary structure 106.[000194] The disclosed example embodiments of optical systems provide a window through a boundary structure of an object, where the optical system comprises a refractive element spanning both sides of the boundary structure, the refractive element being configured to form a refractive window through the boundary structure via which light passes, the light following a path at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure.[000195] In some embodiments, the optical system is configured as an imaging system, and further comprises an imaging apparatus located on a first side of the boundary structure of the object, where the refractive element spanning both sides of the boundary structure is configured to form a refractive window through the boundary structure via which light incident at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure, opposite the first side, is refracted towards the imaging apparatus to form a FoV at the imaging apparatus.[000196] In some embodiments, the optical system is configured to provide a window through a boundary structure of an object to illuminate a surface of the boundary structure,wherein the optical system further comprises a light source located on a first side of the boundary structure of the object and wherein the refractive element spanning both sides of the boundary structure is configured to form a refractive window through the boundary structure via which light emerges at a grazing or near grazing angle of incidence relative to illuminate the surface of the second side of the boundary structure, opposite the first side.[000197] In some embodiments, the optical system may be configured to both illuminate and image a surface area of a boundary structure by comprising at least one optical system configured to provide a window through a boundary structure of an object to illuminate the surface of the boundary structure and at least one optical imaging system as disclosed herein which is configured to image the boundary structure, for example, an optical system as shown in the FIGS, and described herein and / or according to the first aspect of the optical system set out above.[000198] The optical system disclosed herein may, in other words, replace the imaging apparatus of the above disclosed embodiments with an illumination source in some embodiments and illuminate the surface of the boundary structure. Such an illumination system may be configured in some embodiments to provide a window through a boundary structure of an object to illuminate a surface of the boundary structure, wherein the optical system further comprises a light source located on a first side of the boundary structure of the object, and wherein the refractive element spanning both sides of the boundary structure is configured to form a refractive window through the boundary structure via which light emerges at a grazing or near grazing angle of incidence relative to illuminate the surface of a second side of the boundary structure, opposite the first side.[000199] Where the above disclosures refer to a streamlined portion of an optical system, this may be convex, that is to say, have a curved surface in some embodiment, however, it is also possible to for the streamlined portion of the optical system to comprise one or more planar surfaces which are linear in profile when viewed sideways to the direction of the fluid flow, such as Figure 2B shows schematically. Such streamlined portions may comprise one or more protective components such as hoods which partially cover the protrusion of the refractive element from the boundary surface whilst exposing a planar surface of the refractive element via which light which has followed a path at a grazing angle of incidence along a boundary may form a FoV of the boundary surface.[000200] What has been described and illustrated herein is an example along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims -- and their equivalents -- in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Claims

1. CLAIMS1. A streamlined optical system for providing a window through an opaque boundary structure of an object, wherein the optical system comprises:a refractive element spanning both sides of the boundary structure, the refractive element being configured to form a refractive window through the opaque boundary structure via which light passes through at least one refractive surface of the refractive window, the light following a path at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure,wherein the refractive element is configured to project from the surface of the second side of the boundary structure, the projection of the refractive element forming a streamlined portion including one or more planar surfaces inclined at a non-zero angle A from the surface of the second side of the boundary structure, wherein the light following a path at a grazing or near grazing angle of incidence along the surface of a second side of the boundary structure passes via at least one of the one or more planar surfaces of the streamlined portion to form at least one non-distorted field of view, FoV which includes a portion of the surface of the second side of the boundary structure.

2. The optical system of claim 1, wherein the configuration of the streamlined portion and one or more planar surfaces form two non-distorted FoVs which include different portions of the surface of the second side of the boundary structure.

3. The optical system of claim 1 or claim 2, wherein the optical system is housed in a streamlined structure.

4. The streamlined optical system of any one of claims 1 to 3, wherein the streamlined portion projects into a fluid flow along the surface of the second side of the boundary structure, wherein the configuration of the stream-lined portion and one or more planar surfaces forming the refractive window are configured to allow non-turbulent, laminar, fluid flow across the refractive window.

5. The streamlined optical system of claim 4, wherein the optical system comprises a selfcleaning optical system, wherein the configuration of the stream-lined portion and the one or more planar surfaces forming the refractive window allow the non-turbulent, laminar, fluid flow across the refractive window to wash the refractive window.

6. The streamlined optical system of claim 4 or 5, wherein the optical system is an embedded optical system extending through the opaque boundary structure, wherein the optical system is configured to have a portion having a streamlined flat or streamlined curved surface which protrudes into the fluid flow along the surface of the opaque boundary structure, wherein and the refractive window comprises two or more planar surface, at least one planar surface being inclined at a non-zero angle, A, to the surface of the boundary structure along which fluid is flowing to capture a field of view, FoV, of the surface which includes light which has following37a grazing or near grazing angle of incidence along that surface, and wherein the streamlined flat or curved surfaces are configured to allow non-turbulent, laminar, fluid flow across the refractive window.

7. The optical system of claim 6, wherein the optical system is a self-cleaning optical system comprising a refractive element configured to be covered in part by a streamlined convex portion protruding into the fluid flow, wherein the streamlined convex portion and the refractive window are configured so that the laminar fluid flow across the refractive window cleans the refractive window.

8. The optical system of any one of the previous claims, wherein the non-zero angle of inclination, A, of at least one of the planar surfaces forming the refractive window is less than 20 degrees from the plane of the observed boundary surface and the optical system is configured to provide a non-distorted field-of-view, FoV, of the boundary surface to an imaging system.

9. An optical system as claimed in any one of the previous claims, wherein the optical system is configured as an imaging system, and further comprises:an imaging apparatus located on a first side of the opaque boundary structure; wherein the refractive element spanning both first and second sides of the boundary structure is configured to form a refractive window through the boundary structure via which light incident at a grazing or near grazing angle of incidence relative to a surface of a second side of the boundary structure, opposite the first side, is refracted towards the imaging apparatus to form at least one FoV, each FoV including an area of the surface of the second side of the boundary surface, at the imaging apparatus.

10. The optical system of claim 8, wherein the boundary structure comprises the object or a part of the object or comprises a housing or shell or a boundary structure for the object.

11. The optical system of claim 9 or claim 10 when dependent on claim 9, wherein the refractive element is configured to form the refractive window by passing through an aperture in the boundary structure in order to present the at least one refractive surface disposed at an angle, A greater than zero and less than 20 degrees to the plane of the surface on the second side of the boundary structure, wherein the disposition of the at least one refractive surface at the surface on the second side of the boundary structure is configured to capture at least one FoV, at least one FoV including incident light at a grazing or near-grazing angle of incidence along the plane of the surface of the second side of the boundary structure and guide light along the body of the refractive element towards the imaging apparatus.

12. The optical system of any one of the previous claims 9 to 11, wherein the object comprises one of: a vehicle, an aircraft, a vessel, or a pipeline or conduit for conveying fluid.

13. The optical system of any one of the previous claims, wherein the refractive element comprises a prism having a streamlined configuration above the boundary surface.

14. The optical system of any previous claim, wherein a hood is provided over or adjacent to the prism, wherein the hood has a streamlined surface.

15. The optical system of any one of claims 13 and 14, wherein the prism further includes38one or both of a mirrored internal surface and a partially reflective mirrored internal surface.

16. The optical system of any one of the previous claims, wherein the refractive element is configured to refract light via at least one airgap towards at least one mirror, wherein the refractive element and / or the at least one mirror is configured to reflect light towards the imaging apparatus.

17. The optical system of any one of the previous claims dependent on claim 10, wherein the first side of the boundary structure is inside the object, and the second side of the boundary structure is outside the object, and wherein the imaging apparatus is located inside the object.

18. The optical system of any one of the previous claims dependent on claim 9, wherein the optical system is mounted on or in an object and the boundary structure of the object and the refractive element of the optical system form a seal on the boundary structure to separate an external environment from an internal environment of the object.

20. The optical system of claim 18, wherein the external environment is a non-turbulent flowing fluid environment.

21. The streamlined optical system of any one of the previous claims, wherein a streamlined hood is attached over the refractive element and / or adjacent to the streamlined portion of the refractive element.

22. The optical system of any one of the previous claims, wherein the refractive element is configured to reflect refracted light towards the imaging apparatus in the opposite direction to the direction light is incident on the refractive window.

23. The optical system of any one of the previous claims, wherein the refractive element is a composite or non-homogenous refractive element comprising at least two refractive regions having different refractive properties, wherein the refractive regions share at least a partially transparent interface allowing light to pass through from the first refractive region to at least a second refractive region.

24. The optical system of claim 23, wherein the refractive element comprises at least two refractive elements separated by an air gap.

25. The optical system of claim 23 or 24, wherein a first refractive region of the at least two refractive regions is configured to pass through the aperture in order to present the refractive window at an angle to the plane of the second surface of the boundary structure which allows the refractive element to capture incident light at a grazing or near-grazing angle of incidence along the second surface and guide the captured light along the body of the first refractive region to a second refractive region of the at least two refractive regions, wherein the second refractive region is configured with an inner reflective surface configured to reflect received light towards the imaging apparatus.

26. The optical system of any one of the previous claims 9 to 25, wherein the imaging apparatus is configured to capture an image or a series of images or a video feed of images, each image providing at least one FoV or a series or video of one or more FoVs along the first surface of theboundary structure.

27. The optical system of claim 26, wherein the imaging apparatus comprises one or more of:a camera;an event camera;a stereoscopic camera;a depth camera system;a charge-coupled device, CCD, optical system.

28. The optical system of any one of the previous claims, wherein the optical system further comprises an imaging apparatus configured to capture the FoV, the imaging apparatus including a compound lens arrangement configured to provide an external pupil for forming the FOV.

29. The optical system of claim 28, wherein the external pupil is located within or adjacent to the refractive element configured to refract light towards the imaging apparatus.

30. The optical system of claim 28, wherein the external pupil is located adjacent to a mirror configured to reflect light from the refractive element towards the imaging apparatus.

31. An optical system for observing a surface of an opaque boundary structure along which fluid flows, the optical boundary system comprising:a refractive element configured to have a convex portion having a streamlined curved surface which protrudes into the fluid flow along the surface of the boundary structure and a refractive window formed in or adjacent to the convex portion, the refractive window comprising at least one planar surface, wherein at least one planar surface is inclined at a nonzero angle, A, to the surface along which fluid is flowing to capture a field of view, FoV, of the surface which includes light which has following a grazing or near grazing angle of incidence along the surface, wherein the streamlined convex portion and the refractive window are configured to allow non-turbulent, laminar, fluid flow across the refractive window; andan imaging apparatus configured to capture at least one FoV, including the FoV of the surface which includes light which has follows a grazing or near grazing angle of incidence along the surface.

32. The optical system of claim 31 , wherein the optical system is a self-cleaning optical system comprising a streamlined portion protruding into the fluid flow, wherein the streamlined portion and the refractive window are configured so that the laminar fluid flow across the refractive window cleans the refractive window.

33. The optical system of claim 31 or 32, wherein the non-zero angle of inclination, A, of each planar surface forming the refractive window which is less than 20 degrees from the plane of the observed boundary surface configures the optical system to provide a non-distorted field-of-view, FoV, of the boundary surface to the imaging system.

34. An optical system as claimed in any one of the previous claims, wherein the opticalsystem is further configured to provide a window through a boundary structure of an object to illuminate a surface of the boundary structure, wherein the optical system further comprises:a light source located on a first side of the boundary structure of the object, and wherein the refractive element spanning both sides of the boundary structure is configured to form a refractive window through the boundary structure via which light emerges at a grazing or near grazing angle of incidence relative to a surface on the second side of the boundary structure so as to illuminate the surface of the second side of the boundary structure, opposite the first side.

34. A kit of parts for assembling an optical system according to any one of the previous claims 1 to 33.

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