FIELD OF INVENTION

The present invention relates to optical conduits for transmitting light from luminescent concentrators or collectors and, in particular, to an optical conduit in which trapped light generated from a luminescent solar concentrator may be transmitted and released.

Such optical conduits have particular application as means for substantially increasing the useful light provided by a luminescent concentrator whose output is transmitted to the interior of a building by a clear, smooth, optical conduit. In particular, the invention seeks to provide a means by which light that is ordinarily trapped in the concentrator can travel down the conduit, and a means by which this light can be released from the conduit at a location where illumination is required. BACKGROUND ART

Luminescent solar concentrators (also called light receiving stacks) are of increasing interest because of their ability to contribute to the transmission of sunlight to the interior of buildings, owing in large part to their lower installation, running and maintenance costs over both conventional lighting systems and solar lighting systems that use tracking mirrors.

Luminescent solar concentrators contain at least one luminescent species capable of emitting luminescent radiation upon excitation by incident solar radiation. A large proportion of the emitted luminescent radiation is totally internally reflected by the surfaces of the medium from which the concentrator is fabricated and propagates inside the medium to the concentrator's end surfaces.

For example, in a luminescent solar concentrator comprising a flat rectangular sheet, light emitted by luminescent species at small angles to the planar axis of the sheet is totally internally reflected by the sheet's upper, lower and side surfaces and propagates to one end surface where it can escape. It is also apparent that light emitted by luminescent species nearly perpendicular to the sheet's planar axis quickly escapes through an upper, lower or side surface without undergoing total internal reflection.

However, some of the luminescent light emitted at intermediate angles to the sheet's planar axis is totally internally reflected by the sheet's upper, lower and side surfaces and propagates to one end surface where total internal reflection from the end surface causes it to reverse its path and be reflected back down the sheet. This light is completely trapped within the sheet and is unable to escape through any smooth surface of the sheet. For example, in a flat rectangular sheet of refractive index 1 .5 surrounded on all sides by air, each of the sheet's six surfaces release approximately 1 2.7% of the luminescent radiation, and 23.6% of the luminescent radiation is trapped within the sheet. (Most of the trapped light is eventually removed by absorption by the luminescent molecular species or by scattering from defects.) The prior art has not successfully provided a means by which this trapped light may be released from the conduit at a location where illumination is required.

Luminescent concentrator/conduit systems known in the prior art consist of a luminescent concentrator which is connected to a smooth, transparent optical conduit which is, in turn, connected to a luminaire

(which may be no more than the end of the optical conduit). Luminescent radiation from the concentrator enters the conduit where it is channelled by

means of total internal reflection to the luminaire which allows the light to escape from the system in the required directions(s). For efficient light transfer to occur from the concentrator to the conduit, and along the conduit, the cross sectional area of the conduit (which may change along its length) must never be smaller than the exit area of the concentrator.

It has been found by the present inventors that if the joint between the concentrator and the conduit has a mismatch in refractive indices (as will always occur with an air gap and may occur with some glued joints), then a substantial fraction of the luminescent radiation striking the joint is reflected away from the conduit, back into the concentrator. For many concentrator geometries, this light is unable to escape through any surface. It is therefore an object of the present invention to provide an optical conduit that includes luminaire means through which such 'trapped light' can exit the system in a useful manner. It is another object of the present invention to ensure that the luminescent concentrator and conduit are sufficiently closely coupled to enable the concentrator's 'trapped light' to enter the optical conduit, where it will substantially increase the amount of light that reaches the luminaire means at the end of the conduit. SUMMARY OF INVENTION According to the present invention there is provided an optical conduit for transmitting and releasing luminescent radiation emitted from a luminescent concentrator, the luminescent concentrator and optical conduit comprising an optical system, a portion of the luminescent radiation being otherwise trapped in the optical system by total internal reflection, wherein the optical conduit includes a luminaire means for enabling the otherwise trapped luminescent radiation to acquire an angle of incidence to a surface

of the optical conduit so as to enable release of the trapped luminescent radiation from the optical conduit.

Preferably, the luminaire means comprises a plurality of scattering regions for scattering trapped luminescent radiation so that the scattered radiation acquires the said angle of incidence.

In a preferred form of the invention, the optical conduit is prepared by firstly extruding or casting a sheet of polymer material, then cutting to size and suitably polishing the edges.

Preferably, the plurality of scattering regions comprise shape variations or irregularities on the surface of the optical conduit at specific locations and of a predetermined spatial extent. The surface shape variations may comprise non-flat surfaces made by external abrasion, texturing, moulding or chemical etching.

For instance, the surface shape variations of the optical conduit may comprise a surface roughened by sand paper.

The plurality of scattering regions may also comprise a surface coating on the optical conduit, wherein the surface coating includes particulate matter capable of scattering otherwise trapped luminescent radiation. In another embodiment of the invention, the plurality of scattering regions comprise particulate matter embedded within the optical conduit or inhomogeneities within the optical conduit.

The purpose of the scattering centres is to scatter the trapped light out of the conduit which would otherwise remain trapped in the complete concentrator/conduit system.

The plurality of scattering regions function by creating a change in the angle of incidence of the trapped light with respect to a surface of the optical conduit so that the light is emitted or released through that surface. In yet another embodiment of the invention, the luminaire means comprises a portion of the conduit which is expanded greatly in cross sectional area so that the otherwise trapped light strikes a surface of the expanded portion at an angle that permits transmission through that surface. In this embodiment, the luminaire means may be joined to the remainder of the conduit by an optical joint or the luminaire means may be cast simultaneously with the remainder of the conduit.

For most efficient results, the luminescent concentrator is coupled to the optical conduit, and the conduit is coupled to the luminaire means thereof, by optical joints at which there is no mismatch in refractive index between the concentrator, joint material, conduit and luminaire means. Preferably, the optical joint is provided by a transparent coupling agent with a refractive index as close as possible to the square root of the product of the refractive indices of (a) the concentrator and the conduit and (b) the conduit and luminaire means thereof (ie the geometric mean of their refractive indices) . UV cured optical cements or optical grade epoxy glues are suitable coupling agents. Another suitable coupling agent is optical gel although, if this is used, the optical conduit must be held in mechanical alignment by other means.

It may also be possible to couple the conduit to the concentrator and the luminaire means to the remainder of the conduit by other techniques known in the art, such as by solvent welding, ultrasound welding, and the like.

It is possible to eliminate the need for a coupling agent between the concentrator and conduit, by coating the luminescent material onto part of a continuous optical conduit or by casting the concentrator as a continuation of a preformed optical conduit, or by casting the optical conduit as a continuation of a preformed concentrator. Alternatively, the concentrator and optical conduit may be cast simultaneously.

However the joint from the luminescent concentrator to the optical conduit is made, it should ideally be defect free with no bubbles or voids and there should be no surplus coupling agent on the surfaces near the joint so that the concentrator/conduit system is as optically continuous as possible and so that light may freely pass from the concentrator to the optical conduit without reflection or scattering. This optical continuity enables the 'trapped light' (ie the light that would be trapped in the absence of optical continuity) to enter the optical conduit, whereas a simple alignment or butt joint, even with very smooth surfaces, would not.

Preferably, the luminaire means of the optical conduit comprises a light scattering portion at a first of two opposed ends of a light fitting adapted to be located in an area to be illuminated, the light fitting forming a terminal part of the conduit and being optically coupled to a main body part of the conduit at the second of its opposed ends, the light scattering portion having been treated in such a way so as to enable otherwise trapped luminescent radiation to be released therefrom.

In a still further embodiment of the invention, there is provided an optical system comprising a luminescent concentrator optically coupled to any one of the aforementioned optical conduits.

Preferably, the luminescent concentrator is illuminated with sunlight.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be readily understood and put into practical effect, reference will be made to the accompanying drawings, in which:- Fig. 1 is a schematic representation of the path of light emitted by luminescent species at small and large angles to the planar axis of a flat rectangular sheet used as a luminescent solar concentrator, Fig. 2 is a schematic representation of the path of light emitted by luminescent species at an intermediate angle to the planar axis of the sheet shown in Fig. 1 , Fig. 3 is a schematic representation of a luminescent solar concentrator/conduit system known in the prior art, Fig. 4 is a schematic representation of a luminescent solar concentrator/conduit system which includes an optical conduit according to a first preferred embodiment of the invention, Fig. 5 is a schematic cross-sectional representation of an end portion of an optical conduit according to a second preferred embodiment of the invention, Fig. 6 is a schematic representation of a luminescent solar concentrator/conduit system which includes the optical conduit to Fig. 4, and Fig. 7 is an isometric view of a luminescent solar concentrator/conduit system according to another preferred embodiment of the invention.

MODES FOR CARRYING OUT THE INVENTION

In the luminescent solar concentrator sheet 1 1 shown in Fig. 1 , light 1 2 which is emitted by luminescent species (shown as the origin of the arrows) at small angles to the planar axis of the sheet 1 1 is totally internally reflected by the sheet's smooth lower surface 1 3 and smooth upper surface 1 4 and propagates to the end surface 1 5 where its angle of incidence and the refractive index at the interface enables it to be released or to escape from the sheet 1 1 . Light 1 6 which is emitted by luminescent species nearly perpendicular to the planar axis of the sheet 1 1 immediately escapes through the upper surface 1 1 without undergoing total internal reflection.

In the luminescent solar concentrator sheet 1 8 shown in Fig. 2, light 1 9 which is emitted by luminescent species at intermediate angles to the planar axis of the sheet 1 8 is totally internally reflected by the sheet's smooth lower surface 20 and smooth upper surface 21 and propagates to the smooth end surface 22 where its angle of incidence and the refractive index at the interface causes it to be totally internally reflected. The general direction of the path of the light 1 9 is now reversed by the total internal reflection and the light 1 9 is reflected back down the sheet 1 8. This light 1 9 is completely trapped within the sheet's smooth surfaces.

The traditional luminescent concentrator conduit system 23 shown in Fig. 3 comprises a luminescent solar concentrator 24 connected to a smoothly surfaced transparent optical conduit 25 which is, in turn, connected to a cupped luminaire 26. The system 23 is surrounded by air. The cross-sectional area of the conduit 25 is, in this embodiment, the same as the cross-sectional exit area 27 of the concentrator 24, and there is an optical joint 28 between the concentrator 24 and conduit 25 (whereby the

refractive indices (Rl's) of the concentrator 24, joint 28 and conduit 25 are identical), thereby enabling efficient light transfer from the concentrator 24 to the conduit 25 and along the conduit 25. However, not all of this light is able to escape through the end surface 29 so that the cupped luminaire 26 may direct the released light as shown in Fig. 3.

If the luminescent concentrator/conduit system 23 has an Rl of 1 .5, each of the six surfaces of the system 23 will release about 1 2.7% of the luminescent radiation, whereas about 23.6% of the luminescent radiation will be trapped within the system 23, most of this trapped light being eventually absorbed by the luminescent species in the concentrator 24 or being scattered by defects.

The effect of the cupped luminaire 26 as a means for directing light concentrated by the system 23 is, therefore, not significant, as it is only able to direct light that has been released through the end surface 29, and much useful light is either lost through the other surfaces or trapped within the system 23.

The luminescent concentrator/conduit system 30 shown in Fig. 4 comprises a luminescent solar concentrator 31 connected by an optically continuous joint 32 to a smooth optical conduit 33. A luminaire 34 for the conduit 33 is produced by introducing light scattering centres at the appropriate portion of the conduit 33 where illumination is required. In the present embodiment, the scattering centres are on the surface of the conduit 33, but they may be in the bulk material from which the conduit 33 is fabricated. The scattering centres scatter the trapped light out of the conduit 33 by creating a change in the angle of incidence of the trapped light with respect to the surface portions. Such scattering centres serve as the luminaire 34.

Fig. 5 shows an enlarged end portion 35 of an optical conduit 36. The enlarged end portion 35 is optically continuous with the conduit 36 and has a greatly enlarged cross-sectional area so as to enable the totally internally reflected light 37 to strike a surface of the end portion 35 at an angle that permits the light to be released through that surface. The optical continuity is provided by an optical joint between the separately cast conduit 36 and the enlarged end portion 35, or by casting the optical conduit simultaneously with the enlarged head portion 35. The shapes which may be suitable for the enlarged head portion 35 will be described later in the specification.

As shown in Fig. 6, a concentrator sheet 40 dyed with about 70 ppm Lumogen 083 (tm) "yellow" dye (which emits green light at the concentrations used) was exposed to a fluorescent lamp 41 only at one end, as shown, with the middle portion 44 of the sheet 40 serving as an optical conduit 42 as it did not have any light exposure thereon. The total light output from the optical conduit 42 at the opposite end was measured with an integrating sphere 43. No optical joint was considered necessary for this example of a concentrator/conduit system.

The sheet dimensions were 270 mm x 20 mm x 2 mm. The final 50 mm of the optical conduit 42 was treated in various ways so that the total internally reflected light 45 could be scattered and released out the side.

As will be described later, it was found that light was released from the conduit 42 both through the end surface 46, hereinafter defined as end light, and through the side surfaces at the end of the conduit 42 (top and bottom side surfaces 47 and 48 shown, but near side and far side surfaces not shown), hereinafter defined as side light.

Various treatments were found suitable for the final 50 mm of the optical conduit 42 including (a) roughening one or more surfaces with 1 200 grade, 600 grade, 400 grade, 240 grade and 1 20 grade "wet and dry" sand papers (b) attaching "diffuse" sticky tape to the top and bottom side surfaces (c) gluing diffuser sheets to the surfaces and (d) dipping the final

50 mm into acetone for various intervals (which roughens the surfaces) . The grooves made with sand papers were mostly perpendicular to the long axis of the conduit 42 and their direction seemed not to be important. These various treatments have different efficiencies. All of the above treatments gave more light output than no treatment, where the only light output was through the end surface 46 as end light. The best results of 63% more light were obtained when 1 200 grade paper was used to roughen the top side surface only. However, even 1 20 grade paper on all side surfaces gave 43% more light than no treatment. In the conduit treated with 1 200 grade paper, the side light leaked out over the first 3 or 4 centimetres of the 50 mm treated length. In the more roughly treated conduits, the side light leaked out within the first centimetre of the 50 mm treated length of conduit. However, in all of the conduits treated with sand paper, about one quarter of the light came out the end surface 46. This suggests that the surface roughness was imperfect, as ideally almost all the light should have been released as side light if the surface was sufficiently rough.

In experiments with various conduits, the level of trapped light gains has also been sensitive to the quality of the joint and of the luminescent concentrator. Gains in excess of 50% are practical.

The extent to which the side surfaces of the conduit should be roughened with sand paper or other forms of surface abrasion must not be

such that it will cause a reversal in the direction of the internally scattered light, and nor should the length of the roughened region be so short that any reversely scattered light cannot undergo a second or more subsequent forward scattering. Some treatments have been found to backscatter both trapped and end light, and these treatments must be avoided.

It is envisaged that an even more improved light output may be achieved when bulk scattering regions are present on or within the optical conduit. For instance, during the manufacture of the optical conduit, bulk scattering centre forming materials, such as calcium carbonate, zeolites and titanium dioxide, may be included in the medium that is shaped and solidified into the conduit. Alternatively, these materials may be included in a paint or other surface applicable material that is coated on to the conduit. A coating of a polymer that includes fine scattering particles may also be used.

Small bubbles or other inhomogeneities may also be incorporated in the conduit during its manufacture to generate scattering regions. Such inhomogeneities may be produced by adding particles of a polymer or other scattering material which has a slightly different Rl to the other material from which the conduit is made. The region of conduit at which such inhomogeneities or small bubbles occur serves as the luminaire means.

The above treatments may result in an increase in the frequency of scattering interactions within the conduit, so that otherwise trapped light can be scattered such that it acquires an angle of incidence to the side surfaces for the scattered light to escape the conduit. Preferably, the increased scattering is in a forward direction along the conduit.

Fig. 7 shows a luminescent concentrator conduit system 50 comprising a three layered stack solar collector or concentrator 51 coupled by an optical joint 52 to a flexible optical conduit 53 which comprises three overlaid light guides 65a, 65b and 65c and a luminaire fitting 55. The solar collector 51 (consisting of three overlaid fluorescent sheets 51 a, 51 b and

51 c) is located externally of a building or the like so that it is exposed to sunlight (shown impacting the solar collector 51 by arrow 57 and being absorbed by luminescent species at locations 58, 59 and 60, so that these species fluorescently emit light (luminescent radiation) shown by arrows 61 , 62 and 63 that is trapped within the solar collector 51 by total internal reflection). The optical cable 53 passes through a wall 54 of the said building to the area to be illuminated.

The aforementioned system 50 is similar to a sunlight collecting and transmitting system disclosed in Australian Patent No. 661 ,71 6 to the same inventors. The teachings of Australian Patent No. 661 ,71 6 are included herein by reference.

The specially fabricated luminaire fitting 55, adapted to be located in the area to be illuminated, is coupled to the end surface of the three overlaid light guides 65a, 65b and 65c by an optical joint 56. At the free end of the fitting 55 is a terminal scattering portion 57 which is treated in any of the aforementioned ways so that the portion 57 can serve as a luminaire for the trapped light to exit the system (as shown by the arrows which radiate outwardly from all side surfaces of the portion 57).

Alternatively, the terminal scattering portion may comprise the entire luminaire fitting 55 so that the flexible overlaid light guides are optically coupled directly to a terminal scattering member. Such a scattering member may be made of diffuse material, such as opalescent plastic, or

include an outer layer of diffuse material. The diffuse material is envisaged to cause a gradual scattering of light in the forward direction.

As previously mentioned with reference to Fig. 5, the conduit may also terminate in an enlarged end portion 35, such as a cone shaped member with a curved end surface (where the conduit is cylindrical) or as an angular sector of a cylinder member (where the conduit is a rectangular prism) of the same refractive index as the conduit but, say, about five times the thickness of the conduit. The angular sector of a cylinder member 66 is shown in Fig. 8 coupled by an optical joint 67 to a flexible rectangular prism conduit 68 consisting of overlaid light guides similar to that shown in Fig. 7. As a result of the enlarged configuration of the end portion, which serves as the luminaire means, previously trapped light will pass from the conduit into the enlarged end portion and escape out of the end surface of the enlarged end portion to illuminate the adjacent area. The side surfaces 70 and 71 of the enlarged end portion 35 or 66 may include a plurality of scattering regions such as surface coatings, or shape variations formed as a result of the casting process or abrasion to assist in release of trapped light. The surface coating includes particulate matter capable of scattering trapped light. Various other modifications may be made in details of design and construction without departing from the scope and ambit of the invention.