Background to the Invention Lighting panels having a two-dimensional array of light sources and a diffusing screen are well known and are operable to give moderately uniform illumination of the diffusing screen by virtue of the location of light sources at both the centre and periphery of the panel.

Summary of the Invention According to the invention there is provided a lighting panel comprising light source means and a reflector, the light source means being located at or near to the periphery of the panel and the reflector being operable to reflect light from the light source means so as to give the appearance of substantially uniform illumination across the panel.

The invention therefore provides a lighting panel that achieves the appearance of substantially uniform illumination of the panel whilst avoiding the need to have an array of light sources extending across the whole panel. This enables a lighting panel to be constructed more simply and cheaply than has previously been possible.

The panel may advantageously be square.

Preferably the panel is adapted to fit into a suspended grid structure for supporting ceiling tiles.

Preferably the panel is adapted to fit into a 600 mm by 600 mm or 2ft by 2ft suspended grid structure for supporting ceiling tiles or similar.

Alternatively, or in addition, the panel may advantageously be adapted to be laid in a floor or mounted on or in a wall.

The reflector may advantageously be generally pyramidal, having a plurality of triangular reflective faces.

These faces may be suitably curved or stepped to aid even illumination of the panel.

The panel may advantageously have a housing with a back plate.

Preferably if the faces are stepped each reflective face includes two non parallel, non perpendicular facets, an apical facet that includes an apex of the reflector and an abapical facet.

Preferably the apical facet is more steeply inclined relative to the light source means than is the abapical facet. For example, where the panel has a housing having a back plate the angle between the back plate of the housing and apical facet of each face is preferably greater the angle formed between the back plate and the respective abapical facet.

In the case of a square panel, the reflector may advantageously have four triangular reflective faces.

Preferably the apex of the reflector is situated at or near to the centre of the panel, with an edge of each reflective face parallel and adjacent to a respective side of the panel. The reflector may advantageously further be operable to scatter the light from the light source means. This enables the reflector also to diffuse or help to diffuse light from the light source means.

Preferably the reflector is formed from a white plastics material.

Preferably the reflector is vacuum formed from said material.

The light source means may advantageously comprise a plurality of light-emitting diodes (LEDs).

Preferably the plurality of LEDs comprises a first set of LEDs of a first colour, a second set of LEDs of a second colour and a third set of LEDs of a third colour.

The first, second and third colours may conveniently be red, green and blue respectively.

Preferably the lighting panel further comprises illumination control means responsive to control signals to modulate a supply of electrical energy to the sets of LEDs so as to vary the apparent colour of light produced by the lighting panel.

In the case of a square panel, the plurality of LEDs may advantageously be distributed along each of the four sides of the panel.

Preferably the LEDs are so distributed along the sides of the panel that, for each side of the panel, the distances between neighbouring LEDs decrease with distance from the ends of the side in the direction of the midpoint of the side. In this way, the effect of the illumination of each region of an intersection of two faces of the reflector by both the LEDs at the end portion of a first side of the panel and by the LEDs at the contiguous end portion of a second side is negated by the lower intensity of light produced per unit length of the side at the end portions of each side, thus ensuring the appearance of uniform illumination of the panel.

Preferably the LEDs are distributed along each side of the panel in a repeating sequence according to their colours, typically red, green, blue, red, green, blue, et c.

The lighting panel may advantageously further comprise a translucent element operable to diffuse the light reflected from the light source means by the reflector so as to improve the appearance of substantially uniform illumination across the panel.

Preferably the element comprises a sheet of plastics material.

The translucent material may advantageously be opalescent polymethyl methacrylate, polycarbonate or PET.

The lighting panel may advantageously include further diffusion means interposed between the light source means and the translucent element and operable to diffuse light emitted from the light source means in the direction of the translucent element.

Preferably the diffusion means comprises a strip of translucent plastics material adapted to be located adjacent to the light source.

Brief Description of the Drawings The invention will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a lighting panel in accordance with the invention; Figure 2 is a cutaway plan view of the lighting panel of Figure 1; Figure 3 is a sectional side view of the lighting panel of Figure 1 in a 600 mm by 600 mm suspended grid structure for supporting ceiling tiles.

Figure 4 and 5 are views, respectively corresponding to figures 2 and 3, showing second embodiment of lighting panel in accordance with the invention; Figure 6 is a view, corresponding to figure 5, showing a modification to the second embodiment; and Figure 7 and 8 are views, respectively corresponding to figures 4 and 5, showing third embodiment of lighting panel in accordance with the invention.

Detailed Description of an Embodiment Referring to Figure 1, a lighting panel 10 comprises an aluminium housing 12 having a back plate 13 and containing four elongate printed circuit boards, sections of two of which, 14 and 16, are shown in Figure 1, and a reflector 18. The housing, when viewed in plan, is square, measuring 590 mm x 590 mm x 75 mm. Each circuit board is attached to the internal surface of the back plate 13 of the housing adjacent to and parallel with a respective side of the housing.

The reflector 18 is vacuum formed from white flocked styrene and is generally pyramidal, having four reflective triangular faces. The reflector is located against the internal surface of the back plate 13 and at the centre of the panel. The reflector has an apex 19 lying on a axis which is perpendicular to the plate 13 and runs through the centre of the plate 13, and hence the panel. The side of each face that is located against the back plate 13, i. e. the base of each triangular face, of the housing is approximately 400 mm in length and runs parallel to a respective adjacent side of the panel. This, for example, reference numeral 23 in figure 2 denotes the base of one of the facets 17, which base is parallel to the adjacent side, denoted by reference numeral 25, of the panel. Each face has a respective apical facet 15 and abapical facet 17. A notional line produced from (and parallel with) each apical facet 15 forms an angle of 14° with the back plate 13, and each abapical facet 17 forms an angle of 4° with the back plate, so that the facet 15 is more steeply inclined relative to the plate 13 than is the facet 17. This change helps to compensate for the grater distance of the central region of the reflector from the LEDs.

The front of the housing 12 is closed by a translucent element in the form of a 3 mm thick Perspex'diffusing lens 20, which is attached to the housing by reusable fasteners (not shown).

The lighting panel further comprises a socket with 24 V dc, ground, and 0-10 V dc red, green and blue control signal terminals. The socket is attached to the lighting panel by flying leads. The 24 V dc, ground, and 0-10 V dc red, green and blue control signal terminals are electrically connected to the four printed circuit boards. The socket, which is not shown in Figure 1, enables the lighting panel to be connected to a lighting controller and power supply such as a ChromaZonew made by Pulsar Light of Cambridge Limited.

Such a controller enables the LEDs to be controlled so that the effective intensity of light emitted by LEDs of any of the three colours may be continuously varied. The controller feeds a respective continuously variable drive current to each set of LEDs to power the LEDs. As well as being continuously variable, the drive currents are continuous, so that the LEDs, and hence the panel emit light which is flicker free.

Each printed circuit board has mounted on it a respective row of 33 LEDs, made up of 11 red, 11 green and 11 blue LEDs, the LEDs being ordered in the row in a repeating red, green, blue, red, green, blue sequence. Each row extends parallel to its board, whilst each individual LED in the row projects perpendicularly from the board towards the central axis of the panel. This ordering of the LEDs helps to blend the red, green and blue light produced by the LEDs, so as to maintain the appearance of uniform colour and illumination of the lighting panel. The LEDs are mounted on the circuit boards so that when the circuit boards are attached to the housing, the distance of the LEDs from the back plate 13 is approximately one third of the distance from the back plate 13 to the apex 19 of the reflector 18. A green LED 22 mounted on circuit board 14 and a green LED 24 mounted on circuit board 16 are shown in Figure 1. The magnitude of a 0-10 V dc red control signal applied to the red control signal terminal of the socket is used to control the magnitude of a current that flows through the red LEDs. The green and blue LEDs are controlled in the same manner by 0-10 V dc green and blue control signals, respectively.

The lighting panel further comprises four strips of translucent polystyrene, each strip being attached to a respective circuit board adjacent to the row of LEDs so as to be interposed between that row and the lens 20. The strips therefore diffuse light emitted in the direction of the diffusing lens 20 from the sides of the LEDs, which light would otherwise appear as a row of bright red, green and blue spots along the periphery of the lens 20. A first strip 26 attached to circuit board 14 and a second strip 28 attached to circuit board 16 are shown in Figure 1.

Referring to Figure 2, the lens 20 has been omitted to show the inside of the housing. In addition to housing 12, circuit boards 14 and 16,24 and strips 26 and 28, circuit boards 30 and 32 and strips 34 and 36 are visible LEDs. The strips 26,, 28, 34 and 36 have also been partially cut away to reveal LEDs 22 and 24 and the remaining one hundred and thirty LEDs of the panel.

As described above, each circuit board has 33 LEDs mounted on it. The LEDs are mounted on each circuit board such that the distance between neighbouring LEDs decreases linearly with distance from the ends of the circuit board in the direction of the midpoint of the board, so that the distance between LEDs at the middle of the circuit board is half the distance between LEDs at the ends of the board. The longer distances between neighbouring LEDs at the ends of the rows compensate for the contribution of LEDs in the next row to the illumination of the adjacent portion of the board, whilst the greater concentration of LEDs at the centre of the rows (which are closer to the apex than those at the ends) helps to provide adequate illumination of the central region of the reflector 18.

Referring to Figure 3, two sections 38 and 40 of a 600 mm by 600 mm suspended grid structure for supporting ceiling tiles are shown. Although the 75 mm depth of the lighting panel is considerably greater than that of a standard ceiling tile, which typically has a depth of approximately 10 mm, many suspended ceilings have sufficient space between the ceiling tiles and the ceiling proper from which the tiles are suspended to accommodate the lighting panel, so that the lens is flush with the lower surface of the suspended ceiling.

In use, the LEDs of are controlled to emit light which is reflected and diffused by the reflector 18 and further diffused by the lens 20, or which passes through the diffusing strips 26 and 28 to the lens 20. The reflectors 18, lens 20 and diffuser strips 26 and 28 cause that light to give the appearance of constant intensity illumination of the panel across the lens 20.

The perceived colour of that light will depend on the relative intensities of the red blue and green components emitted by the LEDs, so that by controlling the amount of light emitted by each colour group of LEDs it is possible to control the perceived colour of light emitted by the panel. That colour can be continuously varied by correspondingly varying the relative intensities of any of the red, blue and green colour components emitted by the LEDs.

The second embodiment of panel is identical to the first in all aspects other than the reflector. In figures 4 and 5, components corresponding to those of the embodiment are denoted by the same reference numerals used in figures 1 to 3. raised by 100. This, for example, the panel-has a back plate 113, a reflector 118 and a lens 120.

The reflector 118 is in the general shape of a four sided pyramid with an apex 119 at the centre of the panel. However, each of the four faces (reference 150,152, 154 and 156) of the pyramid does not have two planar facets, but instead is slightly concave as can be seen from figure 5 so that each face becomes progressively more steeply sloped, relative to the back plate 113, from its base to the apex 119. The modification to the panel as shown in figure 6 is to the shape of the reflector which in this case is a pyramid having four triangular, planar faces, two of which are denoted by references 158 and 160.

The third embodiment is also identical to the other two embodiments in all aspects other than the shape of its reflector. In figures 7 and 8, the components corresponding to those shown in figures 5 and 6 are denoted by the same reference numerals as are used in figures 5 and 6, raised by 100. The reflector 218 is, as with the second embodiment, a concaved pyramid, but in this case the curvature of each face is more pronounced, and the reflector has a larger lateral extent so that the base of each face is situated closer to the respective adjacent row of LEDs.

It will be apparent that the above description relates only to three embodiments of the invention, and that the invention encompasses other embodiments as defined by the foregoing summary of the invention.