AU2003201296A1 - Daylight collecting and projecting system - Google Patents

Description

C

1 DAYLIGHT COLLECTING AND PROJECTING SYSTEM FIELD OF INVENTION This invention relates to a sunlight collection and projection system for the illumination of the interior buildings. The invention may be adapted for illumination through windows or skylights.

BACKGROUND ART The natural lighting or daylighting of large office buildings is difficult due to the fact that, of the daylight entering through windows, the amount useable falls off very rapidly with distance from the windows. An additional factor which limits the utilisation of daylight in buildings is the necessity to shade windows with external shades or reflectivelabsorbing glazing to reduce radiant heat gain through windows which would otherwise lead to excessive cooling loads. Studies, for example Li and Lam, Solar Energy, Vol 65, (1999), have shown that the two main components of peak cooling electricity load in high rise office buildings are the cooling load to remove the heat of artificial lighting and the cooling load to remove radiant heat gain through windows. Natural light has a higher efficacy (more lumens per Watt) than artificial light. Therefore using natural light rather than artificial light, should, in principle, reduce the cooling load associated with electrical light as well as reducing the cost of electrical lighting, natural light being free. However, if windows are left unshaded to increase natural light the result is radiant heat gain and a cooling load increase.

Conversely, if the windows are shadedto reduce radiant heat gain the energy load due to electrical lighting increases.

To address this problem Edmonds AU 657 749 provides a device that shades windows while collecting and directing some natural light deep into the building. This device relies on reflectors to direct radiant energy into the building within a specified angular range. However, this device is constrained by the second law of thermodynamics such that as the specified angular range of the light input to the building is made narrower the amount of incident sunlight which can be utilised by the device is reduced.

It is therefore a first objective of this invention to provide a sunlight collecting and light projecting device which is not constrained by the second law of thermodynamics in respect to providing light output in a narrow angular range. It is a second objective of this invention to provide a sunlight collecting and light directing device which can also provide effective shading of windows from radiant heat gain.

Bornstein et al US 4,539,625 provide a lighting system comprising a stack of luminescent solar concentrators connected by an optical conduit and collimating lens to a fixture for piping and distributing light into a building. (As the actual process of light emission is usually termed fluorescence rather than luminescence the term fluorescent solar concentrator (FSC) will be used in this application). The device of Bornstein et al relies on a collimating lens and a light pipe to guide light from the output of the FSC into the interior of the building. The device is unsatisfactory as a collimating lens is inefficient as a means of collecting light from the output of a FSC. Light refracted through the FSC to air interface occupies an angular range +90' to -90 whereas the numerical aperture of collimating lens (conventional or Fresnel) limits the angular range of efficient light collection to about 300. Thus, much of the light emitted by the FSC in the system of Bornstein et al is not collected by the collimating lens. Secondly the device is unsatisfactory as guiding and distributing the light by means of a light piping fixture extending into the building implies that much of the ceiling space or ceiling plenum space of a building is occupied by a substantial and expensive rigid light piping fixtures and associated light distributing systems.

Smith et al, US 6,059,438 provide a device which transmits the light from a FSC via a flexible solid transparent light guide into the interior of a building. The flexibility of the solid light guide is achieved by making the light guide only as thick as the FSC itself. While being less bulky than the hollow light pipe of Bornstein et al transmission by means of transparent solid light guide (Smith et al) or by hollow light guide (Bornstein et al) implies that much of the ceiling area or ceiling plenum area of an office building be occupied by expensive light guiding materials.

It is therefore a third objective of this invention to provide an optical angle to angle light transformer which when combined with a FSC provides for light in the wide angular output range of a FSC to be efficiently transformed into a narrow angular range and to be projected within this narrow angular range from a FSC at the external facade of a building deep into the interior and over the ceiling of the building to be illuminated.

The systems of Bornstein et al US 4,539,625 and of Smith et al US 6,059,438 are unsatisfactory in requiring the use of complex light distribution systems to be coupled to the end of, or integral with the light transmission systems. Thus, in a preferred embodiment, Bornstein et al provide that their light transmitting system be comprised of a rectangular box of prism light guiding material with the input from the FSC being collimated and directed at an angle close to 27.60 into the light guide such that light is progressively extracted from the light guide to provide illumination in the building.

Smith et al US 6,059,438 indicate that light distribution from their solid transparent light guide is most effectively achieved by fixing a suitable light fitting to the end of the transparent light guide although Smith et al do not disclose the form of a suitable light fitting.

Therefore, it is a fourth objective of this invention to provide an inexpensive and simple interior light distribution means whereby light collected from a FSC external to a building, is transformed into a narrow beam for projection deep into a building, where it is reflected diffusely from the ceiling so as to provide a glare free illumination to workplaces deep within a building.

SUMMARY OF THE INVENTION The invention is a lighting system for the interior of a building comprising a stacked or tandem panel of fluorescent sheets for collecting and converting sunlight into concentrated light, the arrangement of fluorescent sheets forming a substantially rectangular panel with the upper surface of the panel being the sunlight collecting surface and the lower surface being contiguous with a reflective panel of the same or similar rectangular dimensions as the fluorescent sheets; the fluorescent panel and the reflective panel together being suitable for external shading of the windows of the building; a wide angle to narrow angle optical transformer abutting the edge of the fluorescent panel closest to the building window such that the wide angle light output from the edge of the fluorescent panel is transformed into a narrow angle light beam that is projected from the output end of the optical transformer through the window and over the ceiling of the building; tiles or covers, fixed to the area of ceiling illuminated, having a raised surface patterning such that the projected light is diffusely reflected to work surfaces below the illuminated ceiling with minimal glare.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 shows a tandem FSC panel and angle to angle optical transformer.

Fig 2 shows a stacked FSC and angle to angle optical transformer.

Fig 3 shows a combined lighting system supported to project narrow angle light through awindow..

Fig 4 shows the diffuse reflection of light from patterned ceiling tiles.

Fig 5 shows an air filled ideal optical transformer.

Fig 6 shows a dielectric filled ideal optical transformer.

Fig 7 shows a dielectric filled len-Vtrough optical transformer.

Fig 8 shows the deeper projection of light into a room with an ancillary reflector.

Fig 9 shows means of tilting the plane of the optical transformer with respect to the plane of the FSC.

Fig 10 shows the proj ection of li ght from a combined system mounted paral lel to the building facade with an ancillary reflector.

Fig I I shows a stacked FSc combined wit two reflective plates forming a triangular angle to angle transformer.

Fig 13 shows the "pitch circle" construction determining the output angular range from a triangular optical transformer.

Fig 14 shows light rays traced through a triangular optical transformer.

Fig 15 shows a 90' angular range transformed into a 12* angular range by a triangular optical transformer.

Fig 16 shows the essentially unchanged output range when the input is tilted with respect to the optical transformer.

Fig 17 shows a stacked FSC panel the plane of which is tilted with respect to a triangular optical transformer.

Fig 18 shows the projection of light into a building from a combined lighting system parallel to the facade of a building by means of an ancilliary reflector.

Fig 19 shows the distribution of light over the ceiling of a room. Fig 20 shows the deeper distribution of light obtained with an ancillary reflector.

Fig 21 shows the deeper penetration of light with combined lighting system and ancillary reflector external to a building.

Fig 22 shows the projection of light throgh a window above a venetian blind.

Fig 23 shows a combined lighting system delivering light to a vgrtical light pipe for distribution in a building.

Fig 24 shows a three dimensional view of a combined lighting system and vertical light pipe system.

Fig 25 shows a combined lighting system and vertical light pipe at approxianitely the same scale.

Fig 26 shows a near horizontal combined lighting system coupled to a vertical light pipe via a 450 reflector.

DETALED DESCRWPTION OF PREFERRED EMBODIMENTS The sunlight collector and fluorescent concentrator 1 of Fig I a comprises a tandem arrangement of a red fluorescent sheet 2 separated from a blue fluorescent sheet 4 by an intermediate green fluorescent sheet 3 with the width of the sheets and fluorescent dye concentrations in the sheets such that the combined fluorescent light emitted from the output edge 5 of the blue fluorescent sheet 4 approximates white light. All other external edges of the fluorescent sheets, that is, all edges other than the edges between sheets 2 and 3 and the edges between sheets 3 and 4 and edge 5 are provided with a reflective coating to improve the efficiency of light output at edge 5. A reflective panel 6 is fitted below the tandem arrangement of fluorescent sheets 2, 3 and 4 to improve the collection efficiency and to shade the window below the reflective panel 6 if the collector 1 is delivering light through a window. A wide angle to narrow angle optical transformer 7 is joined optically to the output edge of the blue fluorescent sheet 5 by either abutting the input edge 8 of the optical transformer 7 to the output edge 5 of the blue fluorescent sheet or, in the case where the angle to angle transformer 7 is made from acrylic plastic, by optically joining output edge 5 to the input edge 8 of the angle to angle transformer by transparent acrylic cement. The angle to angle transformer 7 is designed, as outlined below, according to the theory of non imaging optics (Welford and Winston (1978)) to transform the angular range of the output light from the edge of the blue fluorescent sheet 90* when the angle transformer is air filled, or 480 when the angle transformer is solid acrylic) into a suitably narrow angular output range as measured in the plane perpendicular to the line of the output edge of the blue fluorescent sheet 4 and perpendicular to the upper surface of the blue fluorescent sheet 4. Output angular ranges suitable for deep projection of light into building interiors range from about 150 down to about 5' In a preferred embodiment where a stacked panel of fluorescent sheets is used, as illustrated in Fig 2, a green fluorescent sheet 3 is stacked between an upper blue fluorescent sheet 4 and a lower red fluorescent sheet 2. The output edges of fluorescent sheets 2,3 and 4 abut or are optically joined to the input face 8 of angle to angle transformer 7. All edges of the fluorescent sheets other than the output edge abutting the angle to angle transformer 7 have a reflective coating to improve light output efficiency. A reflective panel 6 is fitted below the stacked arrangement of fluorescent sheets 2,3 and 4 to improve the collection efficiency and to shade the window below the reflective panel 6 if the combination of collector I and angle to angle transformer 7 is delivering light through a window.

In subsequent embodiments either the tandem fluorescent panel or the stacked fluorescent panel are used to illustrate different embodiments. However it is understood that either the stacked panel or the tandem panel could equally well be used in any embodiment.

The combination of the tandem of fluorescent panels 2,3 and 4 with the reflective panel 6, or the combination of a stack of fluorescent panels 2,3 and 4 with reflective panel 6, can form, in both cases, an external window shading panel supported to extend from the upper edge of building windows at a small downward tilt angle as illustrated in Fig 3. The support may be provided by means such as a metal frame 8 fixed to the building by fixings 9 and struts Light projected in a narrow beam from the output face 11 of the angle to angle transformer 7 is projected through the window 12 and projected over an area of the ceiling deep in the interior of the building as illustrated in Fig 4. The area of the ceiling which is illuminated by the projected beam may be fitted with ceiling tiles or other ceiling covering 13 that has a raised surface pattern such that the incident projected light is reflected substantially downward from the ceiling to work surfaces below without giving rise to significant forward or side directed glare. The preferred surface patterning of the ceiling cover 13 is of triangular indentation or pyramidal indentation as illustrated in Fig 4 in cross section. The triangular pattern of indentation, where the triangular ridge lies parallel to the plane of the building windows, reduces the amount of light, and therefore glare, being reflected from the ceiling more deeply into the building. As both triangular and pyramidal patterning are readily available as forms of expanded polystyrene and plaster ceiling tiles this arrangement provides a convenient and inexpensive means of distributing the projected input light while reducing glare reflected from the ceiling. However, with ceilings of conventional white diffuse painted surface the downward spread of diffusely reflected light may be sufficient to reduce any glare effect to an acceptable level. Where it is desired that the penetration of reflected light deep into the room be substantially assisted by forward reflection off the ceiling the use of smooth or moderately diffuse ceiling covering would be preferred.

A FSC provides a means of light concentration which is not constrained by the second law of thermodynamics or equivalently by the conservation of optical etendue. Thus a high flux of light can be concentrated at the thin output edge of the FSC without the necessity of tracking the FSC to follow the sun. The angle to angle transforner 7 provides an efficient means of transforming the wide angular range of output light from the FSC into a narrow angular range suitable for projection deep into the building. An angle to angle transformer is constrained by the second law of thermodynamics or conservation of optical etendue. However, as the fluorescent light is concentrated at the narrow output edge of the FSC it is possible, by the invention disclosed here, to collect, concentrate and optically transform much of the light energy incident on the collecting surface of the FSC panel into a narrow output beam without tracking mechanisms or requiring the optical transformer to have an output aperture much larger than the collecting aperture. In fact, as evident from the illustrations, the output light is projected, into a narrow beam, through an aperture which can be much smaller than the collecting aperture of the FSC panel. This means that only a small percentage the window space is occupied by the light collection and projection system of this invention, allowing almost the full extent of the window to be used for other functions such as view and admission of upwardly reflected light from the ground and surroundinos.

Embodiments of the angle to angle transformer are now described. These include ideal, or near ideal, forms of the optical transformer which provide high efficiency light throughput and, forms based entirely on planar reflectors which, while being slightly less efficient in light throughput are much simpler to fabricate. Ideal forms are described first.

The air filled compound parabolic transformer, Fig 5, takes light emitted from the output edge 5 of the FSC panel in a 900 angular range and transforms the light by reflection off two parabolic reflectors 14 and 15 into a narrow angular range output beam. The parabolic shape illustrated would provide an output beam of 110 half angle.

The dielectric filled compound parabolic transformer, Fig 6, takes light emitted from the output edge of the FSC panel in the angular range and transforms it by total internal reflection off planar surfaces 16 and 17 close to the input aperture and parabolic surfaces 18 and 19 into narrow angular range light incident on the dielectric to air interface 20 at the output of the transformer 7 whence it is refracted into a wider angular range in air. The dielectric filled V trough/lens transformer, Fig 7, takes light in the angular range 48' emitted from the output edge of the FSC panel and transforms it by total internal reflection off planar faces 21 and 22 and refraction at cylindrical output surface 23 into a narrow angular output range. The air filled V trough/lens transformer works in a similar manner except that the planar reflecting faces are metal reflectors and the lens is a cylindrical optical lens of conventional design fixed at the output aperture, (not illustrated here).

The angle to angle transformers 7 illustrated in Figures 5,6 and 7 are shown in the same relative scale as the FSC panel. Fluorescent sheets commonly available are 3 mm in thickness and three stacked fluorescent sheets give a FSC panel 9 mm thick. Thus the width of the input face of the optical transformers shown is 9 mm, the width of the output face about 50 mm and the length of the transformers ranging from 70 mm to 120

MM.

The dielectric filled V trough/lens of Fig 7 is considerably shorter than the other configaurations and can be produced fairly readily by extrusion in transparent acrylic. As acrylic is the same material as the material of the fluorescent sheets the input surface of the angle to angle transformer 7 may be readily fixed to the output edge of the FSC panel 5 with acrylic cement thereby minimising transmission loss at the interface and providing a simple means of fixing the angle to angle transformer 7 to the fluorescent sheet 5. However, a much simpler and less expensive form of optical transformer may be provided by planar reflective sheets in triangular configuration as discussed in a following section.

In the embodiments shown previously the plane of the FSC panel I is in line with the axis of the angle to angle transformer 7. In order to provide deep projection of light into a building the combination of FSC panel 1 and angle to angle transformer 7 should be tilted upward at a relatively small angle from the horizontal as illustrated in Fig 3.

The tilt of the FSC panel may be insufficient to provide self cleaning of accumulated dust on the upper face of the panel thereby reducing the efficiency. In this case the combined panel may be covered by a transparent sheet of glass or other material 24 at sufficient tilt to the horizontal to provide a self cleaning surface as illustrated in Fig 3.

An alternative embodiment is shown in FigureS8 where the in line combination of the FSC panel 1 and the angle to angle transformer 7 is tilted at an angle sufficient to provide self cleaning. The light beam from the angle to angle transformer 7 is projected onto a planar reflector 25 fixed just inside the window so that the light beam, on reflection from the planar reflector 25, is projected at the desired angle deep into the building interior. As is evident from this diagram, if the planar reflector 25 is placed closer to the output face of the angle to angle transformer the planar reflector may be smaller in size. Thus a further embodiment (not illustrated) is where the planar reflector is placed against the face of the angle to angle ti ,ansformer 7 and all components of the combination are placed outside the Window.

Fig 9 shows a further embodiment where the angle to angle transformer 7 is coupled to the output edge of the FSC panel 5 by a cylindrical section of clear acrylic 26 such that light from the output face 5 of the fluorescent sheet is substantially guided by total internal reflection into the input face 8 of the angle to angle transformer 7, here shown as an air filled compound parabolic transformer.

Fig 10 shows a further embodiment where angle to angle transformer 7 is of the asymmetric compound parabolic form according to the theory of Rabl, Solar Energy, (1989). In this case input face 8 of the angle to angle transformer 7 may be coupled directly to the output faceS5 of the FSC panel thereby accommodating a suitable angle between the plane of the tilted FSC panel and the output projection direction of the angle to angle transformer 7.

While, in the embodiments of the light collection and light projection system described above, a window shading function is provided by the placement of the system over the window, there are cases where the daylighting system may be required without the requirement of also providing window shading.

Thus, the invention includes embodiments as illustrated in Figure 11 where the FSC panel 1 and angle to angle transformer 7 are fixed substantially parallel to the wall of the building and the narrow beam of light projected from the output of the optical transformer 7 is reflected by pianar reflector 27 through the window and over the ceiling deep inside the building. Shading of the window, if required, may then be provided by other means such as venetian blinds 28 as illustrated in Figure 11, external shades fixed below the planar reflector 27 or reflective glazing installed below planar reflector 27.

The previous embodiments of the optical transformer are all based on various forms of ideal concentrator derived from the theory of non-imaging optics. These provide an efficient optical form to transform light from a wide input angular range to a narrow output angular range with a minimal number of reflections. However where these optical transformers comprise curved reflective or refractive optical surfaces they may be relatively difficult to manufacture. It is known (Welford and Winston 'The optics of non-imaging concentrators" P 67), that the performance of very narrow triangular concentrators approaches closely the optical performance of very narrow ideal concentrators. As the optical transformer included in this invention is required to transform to a very narrow output angular range it is possible to use an optical transformer 7 comprised of two flat reflective sheets or plates 28 and 29 arranged in a narrow triangular form as illustrated in Figure 12 to provide the required angular transformation, hereafter referred to as triangular optical transformer. Provided the angle of the triangular optical transformer is narrow the performance should not be significantly less effective than that of a similarly narrow "ideal" optical transformer.

The angular width of the output beam is determined by the diameter of the "pitch-circle" relative to the width of the output aperture as illustrated in Figure 13.

To illustrate the performance Figure 14 shows light rays traced through a triangular optical transformer comprising two flat reflective plates 28 and 29 arranged between an input aperture 30 and a wider output aperture 3 1. The configuration of input aperture and output aperture illustrated in Figure 14 is the same as would be required for an ideal optical transformer to transform from input light in the angular range from +900 to -900 into an output angular range from 100 to 10' degrees. Input light rays may be traced through the system as illustrated. When the full angular range from +90' to -90* passes through the input aperture the output rays appear as in Figure 15. The output range obtained is from +12' to -12o. This range is nearly as narrow as the angular range achievable by an ideal reflective system of similar size- However the flat plate triangular system is much simpler to manufacture and therefore is a preferred embodiment in this invention.

A further advantage of the triangular optical transformer is that the output light is the combination from many reflected images of the FSC panel edge as indicated by the "pitch- circle" diagram in Fig 13. Thus the different wavelengths of fluorescent light are well mixed within the angle to angle transformer.

In Fig 9 and 10 an embodiments were described where the plane of the FSC panel was tilted with respect to the axis of the optical transformer. This was seen to be useful in cases where the FSC panel also acts as an external window shade. It can be demonstrated that the performance of the triangular optical transformer is substantially independent of the tilt between the direction of the input light and the axis of the optical transformner provided the tilt is not too great, 200. This is illustrated by using ray tracing in Figure 16 where the principal direction of the input angular range is tilted about 200 with respect to the axis of the triangular optical transformer. It is evident that the output distribution is essentially the same as the distribution obtained in Figure where the tilt is zero.

Thus an additional embodiment to the invention is illustrated in Figure 17 where the FSC panel I and bottom reflector 6 ?orms an external sloping shade exterior to a building and couples to a triangular optical transformer compising reflective plates 28 and 29 with the tilt angle between the plane of the FSC panel and the principal plane of the angle to ange transformer being such that the output beam from the optical transformer would project output light deeply into the interior of the building.

A further additional embodiment to the invention is illustrated in Figure 18 where a FSC panel I is fixed in vertical orientation parallel to the building facade and couples to a triangular optical transformer 7 and a plane reflector 27 such that the output beam from the optical transformer is projected through the top of a window deeply into the interior of the building. This is seen as useful where the windows of the building are not externally shaded and have internal shading means such as venetian blinds 28 and it is desirable to collect and project additional light collected from the facade deep into the interior of the building.

To illustrate the light projection performance of the daylighting system of this invention refer to Fig 19 where a FSC panel I couples to triangular optical transformer 7. The triangular optical transformer 7 is formed with an input aperture to match the width of the FSC panel. This is typically a stack of three fluorescent sheets each of 3 mmn thickness so the input aperture to the optical transformer is 9 Trm wide. The corresponding length of the triangular optical transformer is 170 mm with output aperture 50 mm wide so that the wide angle input light from the edge of the FSC panel is transformed into an output beam of 120 half angle width, ie 120. The assembly of FSC panel and triangular optical transformer is tilted, typically, by 150 from the horizontal so that the output light is projected between 30 above the horizontal, ray 30, and 270 above horizontal, ray 3 1, through the window and into the room of typical dimensions 3 m high and 9 mn deep. The room is shown at about 1/10 scale relative to the FSC panel and optical transformer for clarity. However, as the output aperture of the optical transformer is still small relative to the scaled down room the geometry of projection is essentially the same as if the scales were equal. It is evident that the light between ray 30 and ray 32 will fall on the ceiling of the room near the window (within 3 m from the window) while light between ray 31 and ray 32 willI penetrate deeper into the room. It is possible to deflect most of the light between ray 31 and ray 32 deeper into the mom by adding a planar reflector 27 to the upper end of the optical transformer as illustrated in Fig The overall system, shown closer to actual relative scale between the daylighting system and the window, is illustrated in Fig 21. The combination of fluorescent stack 1, optical transformer 7 and ancillary reflector 27 is supported in the form of a sloping shade at the upper edge of a window 36 to a room by supports 33 and 39 which fix to the facade of the building 35. Wide angle light from FSC panel 1 is transformed into a narrow angular range by optical transformer 7 and reflector 27 and projected as a narrow angular beam of light through window 36 and into the room.

The same combination of FSC panel 1, optical transformer 7 and ancillary mirror 27 can be placed parallel to the facade in a similar manner as illustrated in Fig 22 if it is not desired to use the assembly as a shade to the window.

The light collection and light projection system of this invention is designed to provide natural illumination and window shading for deep plan office buildings where the plan size of the building (typically 40 m x 40 mn) and the depth into the central interior (typically 15 in) is much greater than the floor to ceiling height (typically 3 in). In this case the system as described above will provide illumination over an area of the ceiling extending from about 5 in from the window to about 15 mn from the window.

Assuming the efficiency of conversion of sunlight to fluorescent light in the FSC panel is 1001, the optical efficiency of projection through the optical transformer and window is 80% and the efficiency of reflection of projected light from the ceiling to work surfaces is 60% the resulting overall efficiency is about If the required illumination over the 10 m deep, 1 m wide area illuminated by the projected light is 500 lux, (500 x 10)10.05 100,000 lumens must be incident on each metre width of FSC panel. If the FSC panel extends 1 m in length out from the building the required incident illuminance is 100,000 lux. Direct sunlight provides about 100,000 lux while overcast skies provide about 20,000 lux. This suggests that rectangular FSC panels extending about I m outwards as window shades and coupled with optical transformers for narrow beam light projection through windows as described in this invention should provide adequate natural illumination to deep plan office buildings under direct sunlight conditions. Electrical cost savings can then be made by reducing installed electrical lighting or by providing electrical light control.

With FSC panels extending from the wall by 1 m and a floor to floor spacing of 4 m, shading of one FSC panel by the one above does not occur (in the simple case of light incident in the vertical plane nonnal to the building facade) until the elevation of the sun exceeds 75'. This sun elevation does not occur on equatorial facing walls in temperate latitudes and occurs only at the height of summer in sub tropical latitudes. When it does occur there will be significant variation in the spectral content of the output from a tandem FSC. However when a stacked FSC panel as in Fig 2 is used the variation from in the spectral content of the output as shading progresses across the panel will be much less noticeable. When the FSC panel is fixed parallel to the building facade as in the embodiment of Fig 22,'no self shading occurs and both the tandem and stacked fluorescent panel embodiments should show insignificant variation in the spectral content of light output with time of day or sun elevation.

Natural light arrives at a collector from a wide angular range due to the fact that the sun moves across the sky both diurnally and seasonally and that the diffuse skylight arrives from all parts of the sky hemisphere visible from the collector. Light pipes are not effective in transmitting light incident at a large angle to the light pipe axis due to losses from multiple reflections that occur when wide angle light is transmitted through a light pipe. Thus there are significant advantages in using the fluorescent panel concentrator and optical transformer of this invention with light pipes to provide efficient transfer of natural radiant energy deep into buildings. An embodiment illustrating this concept is shown in Fig 23.

Fig 23 shows a FSC panel 1 erected vertically on top of a building. A triangular optical transforner 7 abuts the lower and light emitting edge of the FSC panel transforming the wide angle light output from the FSC panel into a nan-ow angle light output from the lower end of the optical transformer 7. The lower end of the optical transformer feeds into a reflective rectangular light pipe comprised of flat reflective sheets 37 and 38 which penetrate through one or several floors of the building. Within the reflective pipe are partially reflective panels 39 and 40 which serve to deflect some of the light out through windows 41 and 42 in the reflective pipe and over the ceilings of the building.

The embodiment of Fig 23 is shown as viewed obliquely in Fig 24. Here it is shown that the reflective panels within the reflective pipe are made up of reflective areas 43 and 44which do not fill the pipe but rather, increase in reflecting area progressively from one floor to the next so that approximately equal amounts of light are extracted at each floor. Further, Fig 24 shows that the distance across the reflective pipe in the direction in which the optical transformer narrows the emergent light, 42, is much less than the dimension of the rectangular reflective pipe in the direction in which the optical transformer has no narrowing effect, W. As this ratio is important in examining the advantages of this embodiment of the invention it is desirable to provide typical values for both dI, d2and W.

Fluorescent panels are generally available in standard sheet size of 2.4 mnx 1.2 m. x 3 mm. The important dimension is the 3 mmn thickness. A stack of one blue, one green and one red sheet each of 3 mm. thickness will typically convert about 10% of the incident sunlight into visible output light. In this embodiment the 9 mm. thickness of the fluorescent sheet will be the width of the input aperture, dl, of the optical transformer. Referring to the equation dl .Sin(thetal) d2.sin(theta2) which relates to a two dimensional "ideal" optical trasformer we note that the input aperture dlI is 9 m in width and theta 1, the half angle spread of the fluorescent light emitted from the bottom of the FSC panel, is 9Q0*. To optically transform the output light to a half angle width theta2 100 the output aperture of the optical transformer is d2 =9 10)) 51 mm. (As discussed earlier a triangular optical transformer, as illustrated here, closely approximates the performance of an "ideal" optical transformer for small output angular width). Thus the output aperture of the required optical transformer is only 51 mm and this is the minimum width of the reflective light pipe. It may be wider than this, but cannot be less than this, if all light from the optical transformer is to enter at angles less than or equal to 10 degrees.

The horizontal length of the system, W, might typically be 2.4 m, about 50 times the minimum thickness, d2. The height of the FSC panel might typically be im.

The number of reflections, N, in traversing a one dimensional light pipe of thickness d2 is given by N L.Tan (theta)/d2 where L is the length of the pipe, theta is the angle the light makes with the side of the pipe and d2 is the thickness of the pipe. If we consider a building 5 stories high and light being piped to the bottom floor, the length of the light pipe, L, would be about m. With theta 1O* and d2 51 nun, the number of reflections for light input at 100, would be N 52. The transmission after multiple reflections is given by T RN where R is the reflectivity of the reflective material. For a silvered reflector R 0.95 and for N 52, T 0.9552 0.07. That is, the transmission of light at I0' is only 7%.

However the pipe can be wider than 17 mm. Making the pipe 17. cm wide N 15.5 and T 0.9515-5 0.45. That is about 1/2 the light at the largest angle is transmitted through five floors. It is worth noting that within a triangular optical transformer, emitted fluorescent light suffers a maximum number of reflections within the optical trahsformer given approximately by 90/(2.theta) where theta is the maximum emitted angle in degrees. Thus the maimum number of reflections suffered by light in passing through the optical transformer in the present case is 90/(2 x.1O) 4.5. Thus the total number of reflections to be considered is 15.5 +4.5 =20 and the transmittance of light at 100 is 0.9520 0.36.

In the other dimension, the horizontal width of the pipe is typically W 2.4 m. The number of reflecti ons is given by N L.Tan (theta)/W. Thus for a ray emitted into the pipe at 450 N 6 and T =-0.956 0.74. Thus reflective loss for light travelling in this direction is much less significant.

The advantage of this invention can now be assessed. The fluorescent panel has a collecting area of 1 x 2.4 2.4 square metres. However the cross sectional area of the light pipe is just 2.4 x 0. 17 =0.4 square metres. This is 1/6 th of the area of a light pipe, or skylight or atrium which collects light from the same area directly, that is, without the assistance of the FSC panel and optical transformer. Thus the floor space that the light pipe occupies within the building is very significantly reduced. For example a light pipe of width 17 cm might be readily included within an internal wall.

The relative scale of the fluorescent panel, optical transformer and light pipe relative to the building dimension is illustrated more accurately in Fig 20 (4Q2) A further advantage of the present invention is that a vertical panel placed on the roof of a building is very effective in collecting low elevation light whereas conventional roof lighting systems are ineffective in collecting low elevation light. The collection efficiency of a vertical fluorescent panel falls off approximately as cosine of the elevation angle of the incident light. However as the sun elevation seldom exceeds degrees in temperate latitudes this is not a significant problem in temperate latitudes when the FSC panel/optical transformer and light pipe system is oriented approximately East West.

A further advantage of the present invention is that the sunlight collection surface is vertical. This greatly reduces the accumulation of dust and significantly reduces the possibility of damage from hal.

The present embodiment was described in terms of a vertical FSC panel coupled via an optical transformer to a vertical hollow light pipe. However, the FSC panel 1 and optical transformer 7 may be mounted substantially horizontally on the roof of a building 45 with the output coupled to a vertical hollow rectangular light pipe 9 by means of a 450 planar reflector 48 as illustrated in Fig 26.

The foregoing preferred embodiments are considered illustrative only and all other modifications and variations apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as defined in the claims appended hereto.