Collimating systems having a plurality of collimating apparatuses forming an array and a homogenising optical arrangement configured to receive light from each collimating apparatus

The present invention describes a collimating system comprising a plurality of collimating apparatuses forming an array. Each collimating apparatus comprises a light source 12, and a collimating optic 18a configured to collimate light received from the light source 12. The collimating system further comprises a homogenising optical arrangement 50 spaced apart from the array and configured to extend substantially perpendicular to collimated light exiting each collimating apparatus. The homogenising optical arrangement 50 is configured to receive light from each collimating apparatus, the received light having a non-uniform relative radiance profile. The homogenising optical arrangement 50 is configured such that light received from adjacent collimating apparatuses superimposes on exiting the homogenising optical arrangement 50, and forms a flattened relative radiance profile 48b.

FIELD OF INVENTION

The present invention relates to a collimating system and a method of flattening the relative radiance profile of collimated light. The present invention also relates to an artificial skylight comprising said collimating system and method. Such artificial skylights are used to generate a realistic appearance of a sky comprising a virtual sun set at infinity.

BACKGROUND OF INVENTION

Partial collimation of a bright point light source, for example a Light Emitting Diode (LED) or High Intensity Discharge (HID) lighting, is generally achieved using in-line optics usually consisting of a lens or a reflector. Hand torch lights and car headlights often use parabolic reflectors, convergent lenses or a combination of the two. Spot lights such as those found in ceilings tend to use total internal reflection optics using a solid parabolic lens made of polymethyl methacrylate or glass.

In the case of a car headlight, limitations exist as to how large the partially collimated beam can be made. A large headlight requires commensurately sized collimating optics and as its aperture size increases it becomes deeper, thereby intruding into the under bonnet space which is generally quite limited.

A virtual or artificial skylight is similarly constrained in that its depth dictates the spaces into which it can fit. A skylight of shallow depth can conceivably fit into more ceiling spaces than a deeper one. The Applicant's International Patent application number WO2017048569 describes a compact artificial skylight which can be installed within a typical ceiling cavity such that they can be installed in office spaces or other rooms without natural ambient light. The artificial skylight described in WO2017048569 generally includes at least one light source, at least one first collimator, a prism sheet, and at least one transmissive material. The at least one first collimator is configured to collimate light from the at least one light source. The prism sheet is disposed adjacent to the at least one first collimator and is configured to reflect and refract collimated light received from the at least one first collimator. The at least one transmissive material is disposed adjacent to the prism sheet and is configured to radiate light diffusely.

Embodiments of the present invention seek to provide a new and alternative artificial skylight to the arrangement proposed in WO2017048569 which may, for example, provide an even more compact artificial skylight, and also improve the appearance of the artificial sky.

SUMMARY OF INVENTION

According to the first aspect of the present invention there is provided a collimating system comprising a plurality of collimating apparatuses forming an array, each collimating apparatus comprising:

a light source; and

a collimating optic configured to collimate light received from the light source;

the collimating system further comprising a homogenising optical arrangement spaced apart from the array and configured to extend substantially perpendicular to collimated light exiting each collimating apparatus;

wherein the homogenising optical arrangement is configured to receive light from each collimating apparatus, the received light having a non-uniform (and/or discrete) relative radiance profile; and,

wherein the homogenising optical arrangement is configured such that light received from adjacent collimating apparatuses superimposes on exiting the homogenising optical arrangement, and forms a flattened (and/or substantially continuous) relative radiance profile.

Relative radiance may be referred to herein as intensity. The homogenising optical arrangement may be a homogenising prism set. The homogenising prism set may comprise a plurality of prism sheets. Preferably, the homogenising prism set may comprise a first prism sheet and an opposing second prism sheet. As viewed along the longitudinal axis of a collimating apparatus, the first prism sheet may be located closest, or proximally, to the array, and the second prism sheet may be located furthest away, or distally, from the array. The first and second prism sheets may be adjacent, that is next to each other, but physically separated by a distance in a direction substantially parallel to the longitudinal axis of the collimating apparatus.

In an exemplary embodiment, the first and second prism sheet each may comprise a planar surface and a prismatic surface. Each prismatic surface may comprise a plurality of facets. Each pair of facets may be symmetrical.

In some embodiments, the prismatic surface of the first prism sheet and the prismatic surface of the second prism sheet may be oriented away from each other. In alternative embodiments, the prismatic surface of the first prism sheet and the prismatic surface of the second prism sheet may be oriented towards each other.

In both such embodiments, the homogenising optical arrangement may be configured to split the light received from each individual collimating apparatus into two distinct light beams each travelling in a different direction. After splitting the light beam, the homogenising optical arrangement may then realign the two distinct beams in substantially the same direction as the received collimated light beam. The realigned split light beams may be spatially separated, but overlapping. After realigning, any overlapping split beams may be superimposed to form the flattened relative radiance profile. The light may be split and/or realigned by refraction and/or reflection.

In some embodiments, an intermediate material may be provided adjacent to the first prism sheet. The intermediate material may be provided in embodiments where the prismatic surfaces are oriented away from each other, and also embodiments where the prismatic surfaces are oriented towards each other. In embodiments, the intermediate material may be provided between the first prism sheet and the second prism sheet. The prismatic surface of the first prism sheet and the prismatic surface of the second prism sheet may be located on either side of a single transparent substrate.

The intermediate material may be a solid and/or a liquid. The intermediate material may be a third prism sheet.

The third prism sheet may comprise a plurality of facets. The facets of the third prism sheet and the facets of the first prism sheet may be interlockable. The collimating system may further comprise a gap between the first and third prism sheets when the facets of the first and third prism sheets are interlocked. The gap may be an air gap. The gap may be sufficiently small, such that scattering of the light rays may be reduced. In embodiments, the facets of the third prism sheet and the facets of the first prism sheet may be in direct contact.

Preferably, the intermediate material has a higher refractive index than air. The intermediate material may have a lower refractive index than the first prism sheet.

In embodiments, the homogenising optical arrangement may comprise a single substrate. Preferably the first and second prism sheets may be integrally formed on a first and second side of the substrate respectively. The first and second prismatic surfaces may face away from each other.

In use, the direction of light exiting the second prism sheet may be substantially parallel to the direction of light entering the first prism sheet.

The non-uniform relative radiance profile receivable by the homogenising optical arrangement may comprise a series of high amplitude peaks and troughs. The flattened relative radiance profile exiting the homogenising optical arrangement may typically comprise a series of relatively lower amplitude peaks and troughs. The non-uniform relative radiance profile receivable by the homogenising optical arrangement, and the flattened relative radiance profile, typically comprise half sinusoidal wave patterns.

In embodiments, the light source may be a point source. The light source may be a hemispherically emitting light source (where light radiates in a hemisphere from the light source). The light source may radiate light normal to the emissive surface of the light source. The light source may be a Light Emitting Diode (LED).

The collimating system may further comprise a waveguide adjacent to the light source. The waveguide may be configured to receive light from the light source. Preferably the waveguide is a light pipe.

The waveguide may be tapered in shape. The width of the waveguide proximal to the light source may be smaller than the width of the waveguide distal to the light source. The waveguide may be configured to reduce the divergence of the light beam received from the light source.

The collimating system may further comprise a light steering optic. The light steering optic may be configured to steer the light received from the waveguide towards the collimating optic. The light steering optic may be configured to reflect and/or refract the light. Preferably the light steering optic may comprise a Porro prism. The Porro prism may be configured to receive light from the waveguide and redirect said light towards the collimating optic. The light steering optic may alternatively comprise a prism that, like a Porro prism, has at least two surfaces that use total internal reflection to bend the light towards the collimating optic.

In embodiments, the light steering optic may comprise only a mirror, for example when it is not necessary to steer the path of the received light through an angle approaching 180 degrees (such that it is traveling in substantially the opposite direction). In some embodiments, it may be required to steer the path of the received light through an angle approaching 180 degrees (such that the light is travelling in substantially the opposite direction). In said embodiments, the light steering optic may be comprised of several such mirrors. The mirrors may be arranged separately, or alternatively they may be joined together.

Alternatively, the light steering optic may comprise a specular reflector and a prism. The specular reflector may be a mirror. In use, the light may exit the light pipe and be refracted by the prism, such that the light then travels towards the specular reflector. The specular reflector may then redirect the light back towards the collimating optic.

The waveguide and the collimating optic may both be physically located on a first side of the light steering optic. In embodiments, the light steering optic may be part of or incorporated into the waveguide.

According to a second aspect of the present invention there is provided a method of flattening the relative radiance profile of collimated light, the method comprising:

collimating light emitted from a light source to provide one or more collimated light beams with associated non-uniform relative radiance profiles;

refracting and/or reflecting the or each collimated light beam as it passes through a homogenising optical arrangement;

refracting and/or reflecting the or each light beam again as it passes through the homogenising optical arrangement, causing the or each light beam to resume travel in substantially the same direction as the or each collimated light beam;

superimposing any overlapping light beams exiting the homogenising optical arrangement to form homogenised light having a flattened relative radiance profile.

The homogenising optical arrangement may comprise a first prism sheet and a second prism sheet.

In embodiments, refracting and/or reflecting the or each collimated light beam as it passes through the homogenising optical arrangement may cause the or each collimated light beam to split into two distinct light beams. For example, refracting and/or reflecting the or each collimated light beam as it passes through the first prism sheet may cause the or each collimated light beam to split into two distinct light beams. Refracting and/or reflecting each split beam as it passes through the homogenising optical arrangement may cause each split beam to resume travel in substantially the same direction as the collimated light beam. For example, refracting and/or reflecting each split beam as it passes through the second prism sheet may cause each split beam to resume travel in substantially the same direction as the collimated light beam. Superimposing each overlapping split beam may then form a homogenised light beam having a flattened relative radiance profile.

Prior to the step of collimating light emitted from the light source to provide the collimated light beam with an associated non-uniform relative radiance profile, the method may further comprise:

receiving light emitted from the light source in a first aperture of a waveguide; and

emitting the light through a second aperture of the waveguide.

In some embodiments, the method may further comprise:

directing the light from the second aperture of the waveguide towards a corresponding light steering optic; and

steering the light received from the waveguide towards a corresponding collimating optic to provide the collimated light beam.

In alternative embodiments, the method may further comprise directing the light from the second aperture of the waveguide towards a corresponding collimating optic to provide the collimated light beam.

According to a third aspect of the present invention there is provided an artificial skylight comprising the collimating system as described herein.

Whilst the invention has been described above, it extends to any inventive combination set out above, or in the following description or drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1band 1cshow a collimating apparatus10having a light source12and a waveguide in the form of a tapered light pipe14adjacent to the light source12and configured to receive light from the light source12. The collimating apparatus10also comprises a light steering optic16in the form of a Porro prism which is configured to receive light30from the tapered light pipe14, and a collimating optic18which is configured to collimate light received from the Porro prism16. The collimating optic18inFIG. 1ais depicted as two alternative embodiments18aand18b. In particular, the collimating optic could be a Fresnel lens18a(shown inFIG. 1b) or alternatively a parabolic surface mirror18b(shown inFIG. 1c).

The tapered light pipe14and the collimating optic18are both physically located on a first side A of the Porro prism16. In use, the Porro prism16is configured to steer the light30received from the tapered light pipe14towards the collimating optic18. In the embodiments shown inFIGS. 1a-c, the Porro prism16bends, that is primarily reflects, the light beam through an angle greater than 90 degrees, such as 180 degrees. An advantage of embodiments of the present invention may be that the act of folding/steering the light emitted from the light source allows the collimating apparatus to be contained within a small profile enclosure.

The light source12is a hemispherically emitting light source, in particular a Light Emitting Diode (LED). The LED is a small diameter light source such that it approximates a point source.

The tapered light pipe14extends longitudinally away from the light source12along an axis between the light source12and the Porro prism16. The tapered light pipe14comprises a first aperture13which is proximal to the light source12, and configured to receive light from the light source12. The tapered light pipe14also comprises a second aperture15distal to the light source12, and configured such that, in use, the light exits through the second aperture15and travels towards the Porro prism16. The diameter of the first aperture13is smaller than the diameter of the second aperture15. The Porro prism16is positioned proximal to, but spaced apart from, the second aperture15of the tapered light pipe14.

The tapered light pipe14is smoothly tapered from the first to the second apertures13,15. Advantageously, such a design reduces the angular divergence of a beam exiting the tapered light pipe via the second aperture.

The tapered light pipe14may have a circular cross-section, although other shaped cross-sections can be used instead. The tapered light pipe14is constructed of any suitable transparent material, such as glass. In some embodiments, the first and second apertures13,15may have an anti-reflective coating for improved light transmission.

The Porro prism16is made of any clear material generally considered suitable for optics, for example borosilicate glass (BK7 glass).

The tapered light pipe14is configured to reduce the divergence of the light received from the LED12. In particular,FIG. 3shows the mathematical relationship between the first and second apertures13,15and the degree to which light emitted from an LED positioned proximal to the first aperture13is converged (and also expanded) upon exiting the light pipe14. An inevitable consequence of using the tapered light pipe14to reduce beam divergence, is that the beam diameter will increase due to the law of conservation of etendue.

The etendue of a light source is calculated by the area of the light source times the angle of the light beam it emits. InFIG. 3(A), the etendue at the first aperture13can be calculated using A1which is the area of the first aperture13, and θinwhich is the angle of the light source perceived at the first aperture13. The etendue at the second aperture15can also be calculated using A2which is the area of the second aperture15, and θoutwhich is the angle of the light source perceived at the second aperture15. In an ideal optical system, etendue will be conserved. To conserve etendue in this example, a larger aperture must correspond to a smaller angle, and as such the light beam exiting the tapered light pipe is more converged than the light beam entering the tapered light pipe.

FIG. 3(B)shows the relationship between deviations α1in the input angle θin, and the resulting deviations α2in the output angle θout.

As shown inFIG. 1a, the collimating optic18can be a Fresnel lens18a(seeFIG. 1bandFIG. 2) or a parabolic surface mirror18b(seeFIG. 1c). The eventual diameter of the expanded light beam is controlled using the collimating optic18.

Generally it is preferred to use lenses or mirrors with long focal lengths (and larger f-numbers) such that the resulting image, such as the virtual sun, is set close to or at infinity. Typically as the f-number of the lens/mirror increases, the ability of the collimating optic to set images at infinity is improved. In use, and as can be seen in each ofFIGS. 1a-cand2, the tapered light pipe14is located within the focal length of the Fresnel lens18aor mirror18b. Advantageously, this further improves the compactness of the apparatus. In embodiments, the tapered light pipe14is substantially the same length as the focal length of the collimating optic18, whereby the shortest acceptable focal length is f/1. In general, the longer the tapered light pipe, the more ordered the exiting light beam.

FIG. 2shows an alternative embodiment of the collimating apparatus20comprising a light steering optic in the form of a refractive prism21aand mirror21b. As before, the tapered light pipe14and collimating optic18are physically located on a first side A of the refractive prism21a, and the mirror21bis located on a second side B of the refractive prism21a. The second side B of the refractive prism21ais directly opposite the first side of the refractive prism21a. The refractive prism21ais positioned proximal to, but spaced apart from, the second aperture15of the tapered light pipe14, and the mirror21bis positioned further away, or distal, from the second aperture15of the tapered light pipe14.

Similar to the Porro prism16, the refractive prism21aand mirror21bare together configured to receive light30from the tapered light pipe14and steer the light towards the collimating optic18a. In use, the light30exits the tapered light pipe14and passes through the prism21a. The prism21athen refracts the light such that the light is directed towards the mirror21b, which then redirects the light back towards the collimating optic18a. It will be apparent to the skilled person in the art that redirection of a light beam can be accomplished in a variety of ways, such as conventional mirrors inclined at an angle.

It may be an advantage of embodiments of the present invention that having a light steering optic, whether it be a Porro prism, a refractive prism and mirror, or any other suitable light steering apparatus, can help to conserve space, therefore allowing the collimating apparatus to be installed in small roof spaces.

The embodiment shown inFIG. 2further comprises a straightening prism24and a diffusive reflector25. In use, the straightening prism24receives light from the lens18a, and has the function of steering the light towards the viewer, whilst expanding the visible area of the beam. A further function of the straightening prism24is to partially hide the optics and other structures located behind it that would otherwise be visible to a viewer. The straightening prism24achieves this by steering light that is incident at an angle other than the virtual sun beam angle, away from the viewer.

The diffusive reflector25provides additional space in which to house the tapered light pipe14. Any stray light in the apparatus20(because of inevitable inefficiencies) will be partially recycled by the diffusive reflector25. The proportion of light that would otherwise be wasted is partially returned and redirected back through the straightening prism24and out of the apparatus20, thus contributing to the “sky” portion of the radiated light in the virtual skylight.

In use, because the light beam30reflects off the mirror21bwith an angle of reflection equal to the angle of incidence, the collimating lens18bcan be in some embodiments located at an angle (such as, approximately parallel or at a small angle) to the central axis of the tapered light pipe14, as shown inFIG. 2(instead of perpendicular as inFIGS. 1a-c), in order to intercept the full width of the reflected light beam to provide collimated light. Although in embodiments, the light beam can be reflected off the mirror at an angle of 180 degrees, or even slightly more, such that the collimating lens can be perpendicular to the central axis of the tapered light pipe to intercept the full width of the reflected light.

The collimated light is ultimately directed toward the straightening prism24which then steers the light towards the viewer, as described above.

Advantageously, embodiments having a collimating optic positioned at an angle to the central axis of the tapered light pipe, can help to make the collimating apparatus smaller in depth, and thus even more compact.

The position of the Porro prism16of theFIG. 1a-cembodiment, and the refractive prism21aof theFIG. 2embodiment, is such that not only does their width substantially capture the width of the light beam30exiting the tapered light pipe14, but they are also positioned such that the captured light beam can be substantially steered towards the collimating optic18.

The light source12, tapered light pipe14, light steering optic16, or21aand21b, and collimating optic18together form the collimating apparatus10, or20. Any number of collimating apparatuses10or20can be arranged in an array41a, such as a contiguous one-dimensional array, as shown inFIG. 4a. Such a one-dimensional, or linear, array41aof collimating apparatuses10or20is typically used in an artificial skylight. As mentioned previously, the artificial skylight only requires a relatively shallow enclosure42agiven the folding of the light path. For example, the enclosure42amay be around 350 mm deep.

Advantageously, as shown inFIG. 4a, the one-dimensional array41aof collimating apparatuses10can be used to create shallow, large aperture beam collimators which can be used to enhance the performance of an artificial skylight by capturing almost all the light from an array of LEDs12, and through beam expansion (using the tapered light pipe14) and collimation (using the collimating optic18aor18b), provide a collimated output that can be used as a virtual sun in an artificial skylight. A cross-sectional area of the collimated beam44ahas a length41awhich is equal to the length of the array, and a height d equal to the aperture diameter of a single collimating lens18a.

In embodiments of other aspects of the invention, the act of arraying light emitting units can in itself accomplish compactness. The light steering optic16, or21aand21b, may therefore be deemed unnecessary, and thus omitted from collimating apparatus40, as shown inFIG. 4b. An array41bof light emitting units40comprising collimating lenses18aor reflectors18bacts similar to a “fly's eye lens” with a shallow focal length and a wide field-of-view. Such an array41bcan produce a full aperture beam44bvia collimation of multiple sources. However, the presence of a light steering optic16, or21aand21b, is preferred as this adds an additional level of compactness, as can be seen by comparing the required enclosure sizes42aand42b.

In use, collimating apparatuses10are firstly arranged as shown inFIG. 4a. Each collimating apparatus10in the array41ais as described above with respect to theFIG. 1bembodiment.

Each LED12is then switched on to create a small source of hemispherical light. The light rays are transmitted into the first aperture13of the tapered light pipe14. The length of the tapered light pipe14is sufficient to allow the light beam to be adequately expanded as the light travels along its length in accordance with the law of conservation of etendue. The length of the tapered light pipe14is also long enough to reduce the divergence angle of the light rays every time they undergo reflection.

When the light beam exits the tapered light pipe14through the second aperture15, the beam is wider in diameter but more focussed than the light rays entering the tapered light pipe. In the case of a tapered light pipe with a first aperture13having a diameter of 3 mm and a second aperture having a diameter of 12 mm (and assuming a hemispherically radiating light source), light will exit the tapered light pipe in the form of a light cone and will also be constrained to a divergence angle of approximately 29 degrees (subtended), that is 0.214 steradians. Reflection occurs by way of total internal reflection, but in some embodiments it can be produced by application of a specularly reflective coating to the inner surface of the tapered light pipe.

After the expanded light rays exit the tapered light pipe14, they travel towards the Porro prism16. A lower portion of the Porro prism16is wide enough to encompass the width of the expanded light beam exiting the tapered light pipe14. Advantageously this helps to conserve light efficiency.

The Porro prism16refracts the light received from the tapered light pipe14, such that the light leaving the Porro prism16is travelling in the opposite direction to the received light. Ideally, as shown inFIG. 1a, the light exits through an upper portion of the Porro prism16. As the light beam travels away from the Porro prism16, it expands even further.

The Fresnel lens18ais located at the point where the diagonal distance of the lens is substantially equal to the diameter of the light beam. As the light rays pass through the Fresnel lens, they are again refracted, but this time the light rays converge and are collimated, such that the beam stops expanding and wide-beam parallel light is produced. Ultimately, the light beam exits the collimating apparatus as a substantially collimated beam of light.

FIG. 5shows the resulting intensity profile of the light beam (i.e., the relative radiance profile of the light beam) before46and after47passing through the collimating lens18aof unit40, but the same intensity profile is found when collimating apparatus10(with the light steering optic) is used instead. That is, regardless of whether or not the light beam passes through a light steering optic (such as the Porro prism), the intensity profile (that is, the relative radiance profile) will be the same. One problem that will arise from arranging the collimating apparatuses10or40into a contiguous one-dimensional array41aor41b, is that the resulting beam of light44aor44bwill appear to have an uneven intensity profile46due to bright spots associated with each tapered light pipe14.

The relationship between the first aperture13having area A1, the second aperture15having area A2, and beam intensity I1(and assuming that the LED12is of equal cross-sectional size to first aperture13) is:

wherein the intensity I1is defined as the relative radiance, α1is the deviation in the input angle θin, and α2is the resulting deviation in the output angle θout(as shown inFIGS. 3(A) and 3(B)).

The equation for I1shows that the intensity (relative radiance) profile46of the beam exiting the tapered light pipe14, will be parabolic in shape and have a “hot spot”, or a particularly bright region, in and around the central area of the beam. Likewise, the intensity (relative radiance) profile47of the beam exiting the collimating lens18bwill also be parabolic in shape and have a “hot spot” near the centre of the exiting beam. As shown inFIG. 6c, the reason for the parabolic shape is due to the elliptical profile of the light exiting the light pipe being redirected and consequently transformed by the collimating lens. The elliptical profile of the light exiting the light pipe is best described by Lambert's cosine rule.

FIGS. 4aand 4bshow that the resulting intensity profile47of the collimated light beam will be non-uniform, and will comprise a series of high amplitude peaks and troughs along the array41aor41b. The intensity profile47has the appearance of a half sinusoidal wave pattern. The intensity of the collimated light beam I2must therefore be evened out, or flattened, to prevent these “hot spots”. One way around this problem is to homogenise the exiting beam using a homogenising optical arrangement, such as a homogenising prism set50as shown inFIGS. 6aand6b.

The homogenising prism set50is spaced apart from the collimating lens18aand extends across the width of the array41a, that is it extends across the width of each collimating apparatus10. The homogenising prism set50comprises a first prism sheet51and a second prism sheet52. The first prism sheet51is located closest to the collimating lenses18a, and the second prism sheet52is located furthest away from the collimating lenses18a.

AlthoughFIG. 6ashows an embodiment comprising collimating apparatuses10, the homogenising prism set can also be combined with embodiments comprising collimating apparatuses40, that is without the light steering optic (without any folding of the light path), as is shown inFIG. 6b. Furthermore, the homogenising prism set can also be combined with embodiments comprising collimating apparatuses20, that is with a collimating optic in the form of a refractive prism21aand mirror21b.

Prism sheet51comprises a planar surface53aand a prismatic surface54a, and prism sheet52comprises a planar surface53band a prismatic surface54b.

As shown inFIG. 7, prismatic surface54acomprises a plurality of symmetrical facets55aand55b. Prismatic surface54bcomprises a plurality of symmetrical facets56aand56b. Each pair of facets55a,55band56a,56bis equilateral, such that the inclusive angle between each pair of facets55a,55band56a,56bis around 60 degrees.

In the embodiment shown inFIGS. 6 and 7, prismatic surfaces54aand54bare oriented such that they are facing towards each other. In such embodiments, in use, a light beam normally incident on the planar surface53atotally internally reflects off facet55aof the first prismatic sheet51at an angle of around 60 degrees to the normal. The light beam then exits facet55bof the first prismatic sheet51undeviated, that is at the same 60 degree angle. The light beam then incidents facet56aof the second prism sheet52, whereupon the process is reversed and the light beam exits planar surface53bat an angle normal to its planar side, thus realigning the light beam in the same direction as the light beam initially entering the homogenising prism set50.

In use, the light exiting the collimating lens18ahaving a non-uniform and parabolic intensity profile subsequently passes through the homogenising prism set50. This light is first received normally incident to the first prism sheet51which refracts the light and causes the collimated light beam to split into two equal but separate light beams49aand49b. The two distinct light beams49aand49bexit the first prism sheet51travelling in different directions, and then enter the second prism sheet52which refracts each of the split light beams49aand49b, and causes them to assume their original direction prior to splitting. The realigned light beams49aand49bexit normally to planar surface53b, but the light beams are now separated in distance.

As a result, the parabolic intensity distribution48aof each separated light beam will overlap. Each overlapping light beam is superimposed to form a homogenised light beam48bhaving a flattened or averaged intensity profile, and the intensity of any “hot spots” is reduced or nullified.

The flattened intensity profile48bexiting the homogenising prism set50comprises a half-sinusoidal wave pattern of relatively lower amplitude peaks and troughs, compared to the intensity profile47of the collimated light beam, as is shown inFIGS. 6aand 6b. As can further be seen inFIGS. 6aand 6b, intensity profile of the homogenised light beam48bhas double the frequency of the intensity profile47of the collimated light beam.

Any light beams45not normally incident on the planar surface53amay exit facet55b, and then re-enter the first prism sheet51at facet55acausing the light to scatter, leading to chromatic artefacts. Similarly, light beams that miss facet56awill instead totally internally reflect off the planar face53bcausing them to incident off prism facets56aand56bleading to chromatic separation, an unwelcome artefact. To reduce or avoid this problem, a third prism sheet60is provided adjacent to the first prism sheet51, as shown inFIG. 8, between the first prism sheet51and the second prism sheet52, and is made of the same material as the first prism sheet51. Due to the inclusive angle between prism facets65and66being less than 60 degrees, rays49aand49bwill miss facets65and66(a geometric axiom) by a small distance157thereby eliminating the scattering45apresent inFIG. 7within the first prism sheet51. Similarly, rays49aand49bwill always strike prism facets within prism sheet52provided their inclusive angle is greater than 60 degrees thereby preventing rays from striking the planar face of53bof prism sheet52and in so doing, eliminating rays45bpresent inFIG. 7that would otherwise totally internally reflect off planar face53band subsequently undergo chromatic separation.

Similar to the first and second prism sheets51,52, the third prism sheet60comprises a plurality of facets65,66. The facets65,66of the third prism sheet60, and the facets55a,55bof the first prism sheet51interlock in use. However, a small air gap61is maintained between the first and third prism sheets51,60when the facets of the first and third prism sheets55a,55band65,66are interlocked. The air gap61helps to prevent the uncoupling of incident light via evanescent wave interaction. In embodiments, there is no air gap, and the first and third prism sheets51,60are in direct contact.

The third prism sheet60has a higher refractive index than air, but a lower refractive index than the first prism sheet51. Thus, the third prism sheet60influences (refracts) the direction of the light beam during use.

Due to the inclusive angle of the facets55a,55bof the first prism sheet being less than 60 degrees, and the third prism sheet60influencing the direction of light wave propagation, it is necessary for facets56a,56bof the second prism sheet52to have an inclusive angle greater than 60 degrees to reinstate the direction of the light beam such that the light beam exits normal to the planar surface53bof said second prism sheet52.

In some other embodiments, prism facets55a,55bof the first prism sheet51can be in direct contact with a material of lower refractive index (but higher than air) such that the two are optically coupled. However, in this situation the refractive index of the material must be carefully selected so as not to allow uncoupling of the light from facet55aof the first prism sheet51.

In use, after the split light beams49aand49bexit the first prism sheet51they pass through the air gap61and into the third prism sheet60. The third prism sheet60refracts the light as it enters the third prism sheet60and again as it exits. As before, the light beam then enters the second prism sheet52which refracts each of the split light beams49aand49b, and causes them to assume their original direction prior to splitting. The split light beams then superimpose to form a homogenised light beam with minimal or no chromatic artefacts.

It has been found that the three prism arrangement described above works best for monochromatic sources due to the high degree of refraction experienced by the beam49b.

FIGS. 9aand 9bshow a further embodiment of the homogenising prism set150, where the prismatic surface154aof the first prism sheet151and the prismatic surface154bof the second prism sheet152are oriented away from each other. The third prism sheet described above inFIG. 8, can also be provided in embodiments where the prismatic surfaces face away from each other.

In use, this embodiment receives light through facet155bof the first prism sheet151at an angle Ω from the normal to facet155b(as seen in the lower right hand prism ofFIG. 9a, and alsoFIG. 9b). Angle Ω is the angle which causes the received light to be refracted such that it propagates through the first prism sheet151parallel to facet155a. In this case, the light beam is refracted such that it propagates parallel to adjoining facet155auntil it exits the planar surface153aof the first prism sheet151and enters the planar surface153bof the second prism sheet152. The light beam is then refracted upon entry to the second prism sheet152, and the original direction of the light beam, which is approximately normal to the planar surface153aand153bof both prisms151and152, is reinstated as it passes through facet156aof the second prism sheet152.

In this embodiment, the light beam is split in a similar way as described above inFIGS. 7 and 8. After splitting the light beam, the collimated light is first refracted from separate areas on prism sheet151, and is then merged by the second prism sheet152.

As demonstrated inFIG. 9a, a collimated light beam passing through the first prism sheet151is split into two equal but separate light beams149aand149bon exiting the first prism sheet151. The two discrete light beams149aand149btravel in different directions, and then enter the second prism sheet152which refracts each of the split light beams149aand149b, and causes them to assume their original direction. The realigned light beams149aand149bthen exit the second prism sheet152but are now spatially separated. The split light beams149aand149bcan then be superimposed, in the same way as described previously, to form a homogenised light beam having a flattened or averaged intensity profile, such that the intensity of any “hot spots” is reduced or nullified.

As shown inFIG. 9a, R1and R2represent the two outermost light rays contained within a ray bundle, having a divergence angle δ. Any light entering the first prism sheet151that deviates slightly from Ω and incidences facets155aand/or155bin the process, will totally internally reflect off said facets at an angle R1(as seen in the lower left hand prism ofFIG. 9a). Assuming deviation from Ω to be symmetrical about a central ray Ÿ which is precisely normal to planar surface153aof the first prism sheet151(seeFIG. 9b), then, for small angles R1is approximately equal to R2, thus beam divergence is still δ and there are no stray light rays Therefore, the divergence angle δ of the incident light (shown in the lower left hand prism ofFIG. 9a) does not increase and etendue is conserved.

Similarly, when light incidences the prismatic surface154bof the second prism sheet152, the opposite occurs. Light beams striking facet156bof the second prism sheet152will still remain within the divergence angle δ of the incident light.

To ensure that any light rays which totally internally reflect off facets155aand/or155bof the first prism sheet151remain within the divergence angle δ of the incident light, the central ray Ÿ shown inFIG. 9bmust, upon refraction, propagate parallel to at least one of the facets155aor155b.FIG. 9bshows the central ray Ÿ incident to facet155b, refracting, and then propagating parallel to facet155a. For this to occur, the following relationship between incident light and prismatic surface154aexists:
θ=Ω−β,

where Ω is the angle between the incident central ray Ÿ and the normal to facet155b, β is half the inclusive angle between facets155aand155b, and θ is the angle between the refracted central ray Ÿ and the normal to facet155b. Using Snell's Law, we obtain the equation:

n1n2=cos⁢⁢2⁢βcos⁢⁢β
where n1and n2are the refractive index of air and the first prismatic sheet respectively.

Taking this one step further, in some examples, the first prism sheet is made of acrylic, which has a refractive index n2of 1.492. Solving for β we then obtain, β=Arccos 0.89=27°.

Since the inclusive angle of the facets is 2β, the inclusive angle will therefore be 54 degrees. This means that for a refractive index of 1.492, the inclusive angle must be 54 degrees for any totally internally reflecting light to remain within the divergence angle δ.

FIG. 9cshows a further embodiment of the homogenising optical arrangement250comprising a substrate258. The first and second prismatic surfaces254aand254bare located on a first and second side of the substrate258respectively, and face away from each other. Such a prism may be referred to as a dual faceted homogenising prism. In use, only one such substrate is required to homogenise a collimated light beam.

The light beam is once again split in a similar way as described above forFIGS. 7, 8, and 9a. However, in this embodiment, the single substrate258performs the functions of splitting the incident light beam into two distinct light beams which travel in different directions, and then realigning the two light beams such that they assume their original direction upon exiting the substrate258. The realigned light beams are now spatially separated, and can be superimposed, in the same way as described previously, to form a homogenised light beam having a flattened or averaged intensity profile, such that the intensity of any “hot spots” is reduced or nullified.

Overall, an advantage of the invention may be that it not only provides a means of producing substantially collimated light from a large aperture device housed within a shallow enclosure, for example an enclosure that is no deeper than the focal length of the collimating optic; it also provides a means of efficiently collimating light harnessed from a small diameter, hemispherically emitting light source. In some embodiments, collimating apparatuses can be nearly seamlessly combined in a contiguous one-dimensional array, without the presence of “hot spots” and shadows.

Although the invention has been described above with reference to an exemplary embodiment, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

For example, embodiments of the homogenising optical arrangement as described herein can also be used in non-optimised collimating systems, that is systems that rely on inefficient light harnessing techniques to constrain the light cone from an LED (such as where a suboptimal optic is used instead of a light pipe). Inefficient light harnessing techniques can result in a significant portion of the light from a hemispherically emitting LED to expand beyond a given lens aperture, thus increasing etendue. Although inefficient, such non-optimised collimating systems may be used where it is necessary to reduce component costs, and so simpler and less expensive light harnessing optics may be favoured.

Furthermore, the homogenising optical arrangement can also be used in applications where the light from an LED is collimated without any intermediate light harnessing optics whatsoever (as shown inFIG. 6b). However, as described above, the homogenising optical arrangement will average out the series of high amplitude peaks and troughs defining the relative radiance profile of the non-uniform collimated light beam (particularly evident where Fresnel lenses are used as the collimating optic) by superimposing adjacent beams.