TECHNICAL FIELD

The technical field of this invention is solid-state lighting, and in particular the use of light-emitting diodes (LEDs) to provide a lighting fixture that is energy efficient, readily controllable, and can be used to replace both incandescent and fluorescent luminaires.

BACKGROUND The development of the technology of light-emitting diodes based on the Gallium, Aluminium and Indium Nitrides has now reached the stage where white light sources based on these materials are more efficient that almost any other light sources. In particular, white light, usually but not necessarily based on the use of a blue (450 to 470nm) LED together with a phosphor that is excited by the blue and has a broad emission centred at in the range 550 to 600nm, can now be produced with an efficiency greater than incandescent or fluorescent sources. The efficiency is generally described in terms of the output light energy, expressed in lumens to take account of the physical response of the human eye, relative to the input electrical energy. Typically an incandescent lamp will have an efficiency of 15 lumens/W, a compact fluorescent 601umens /W and a traditional fluorescent tube 100 lumens/W, in comparison to the best commercial white LEDs that are now achieving 1301umens/W, and still improving. For all these light sources the efficiency when incorporated into a luminaire is much less. A typical luminaire incorporating conventional T8 fluorescent tubes will have an efficiency of approximately 70 lumens/W, where the reduced efficiency arises because the light emission from a fluorescent tube is circularly symmetric so that the luminaire absorbs a significant proportion of the light.

This advantage in efficiency of LEDs is supplemented by other major advantages. Unlike all fluorescent tubes the LED lights do not contain mercury, an environmental hazard, and do not require a ballast system to generate the electrical discharge and potentially cause electrical interference. The reliability is much better than that of other sources of light, with expected lifetimes of 50,000 hours or more, in comparison to the lifetimes of incandescent lights, which can be as low as 1000 hours, or compact fluorescent lights with lifetimes of typically 5000 hours. LED lights do not require a glass envelope and so in general are much more robust.

The distribution of light emitted from an LED also has advantages. Light is emitted from most high-brightness LEDs only over a hemisphere so that there is no need to incorporate a reflector to capture half the emitted light. An LED can also be considered as a point source of light, in contrast to a fluorescent tube that acts as an extended source. This allows the designer much more control over the distribution of the emitted light through the use of optical components such as lenses and reflecting surfaces. The technical development of LED lamps has triggered a major effort to use these light sources to produce efficient light sources and luminaires. The main technical problems can be classified into three groups: i) Thermal management. Unlike incandescent and in particular fluorescent tubes the dissipation in an LED is concentrated into a very small volume, and must be removed efficiently if it is not to cause degradation. The problems are magnified because a plastic (silicone or epoxy) encapsulation is usually used around the phosphor and the semiconductor chip for protection and to enhance light extraction. Heat can be removed efficiently from the active region of the semiconductor into the body of the LED lamp, but heat transfer from the luminaire to air by means of free convection is an inefficient process, and is usually the limiting factor in the operation of the light. ii) Optical design. Almost all commercial white LEDs are packaged in such a way that they act as Lambertian emitters, with the intensity of the light varying as the cosine of the angle from the normal. For a focused beam a collimating lens is therefore needed. Moreover, a single LED can provide typically 200 lumens of light, in comparison to 1000 lumens from a typical incandescent bulb or 3500 lumens from luminaire containing T8 fluorescent tubes. A practical light must therefore contain a number of LEDs. Controlling the light, and superposing the light from a number of LEDs involves careful optical design and can often lead to significant reductions of efficiency.

Cost. The major limitation on the widespread adoption of solid state light sources is cost. To some extent this is a reflection of the relative immaturity of the technology, but the high cost also results from the need to satisfy both thermal and optical design constraints. As LED technology develops the cost of LEDs is decreasing, so that this factor should be of decreasing importance.

Most of the LED luminaires that have been developed to date have attempted to provide a light fitting or lamp that can be used as a direct replacement for existing incandescent or fluorescent lights. Although this approach may lead to slightly earlier adoption of solid-state lighting, it does not build on the major strengths of LEDs. In particular, the attempt to provide a "retro-fit" solution does not provide the most cost-efficient solution.

Patent publications relating to the direct replacement of fluorescent tubes by LEDs such as US7,441,922, US7,307,39, US7,438,441, US7,488,086, US7,488,086, US7,507,001 describe how LEDs can be used as direct replacements for fluorescent tubes, either with or without ballasts. Many have been summarised in the lengthy publication US7490957, which also explains at length the problems with the disposal of mercury in standard fluorescent tubes and the environmental implications. However, for the purposes of the present patent publication, US7490957 serves to illustrate the difference between the inventions described in these earlier publications and the present invention. US7490957 explicitly describes an LED lamp in the form of a tube that has electrical connections compatible with existing fluorescent tubes and which can be driven from a conventional fluorescent tube ballast circuit. The invention described here is completely different. It is a novel approach to the design and manufacture of a solid-state luminaire based on LEDs, in which the three design constraints described above are treated within an integrated approach to provide a cost efficient solution to luminaire design, and is intended as a replacement for a complete fluorescent luminaire, and not for a fluorescent tube.

The invention described here makes use of a diffuse light reflecting material (DLR material) such as that produced by the Dupont company and the subject of US patent application 20100014164. This Dupont patent describes the way in which the material is manufactured, and the claims refer to the diffuse reflector itself and also to an application in an optical display. The Dupont patent does not describe any applications to lighting of the kind described in the present document.

Description

The present invention provides a luminaire incorporating light emitting diodes (LEDs) and intended for general illumination characterised by an assembly consisting of:

a support;

an array of LEDs and other circuit elements mounted on the support; and a diffusely reflecting screen;

wherein, when in use, light emitted by the LEDs is directed onto the diffusely reflecting screen which has a shape that gives a desired spatial distribution of the emitted light.

The major innovative feature of the luminaire described in this document and shown in the figures is that light emitted by the LEDs is directed on to and then reflected by a diffusely reflecting surface before leaving the luminaire. Preferably, all of the light emitted by the LEDs is directed on to and then reflected by the diffusely reflecting surface before leaving the luminaire. This has a number of advantages as described in the following sections. The luminaire of the present invention is characterised by an assembly which consists of three essential elements. The first is a support (1). The second is an array of LEDs (2), which is mounted on the support together with any other circuit elements that may be wanted for driving the LEDs and for sensing and control. This support also serves as a heat sink, transferring heat from the LEDs into the ambient air. The third essential element is a diffusely reflective surface (3) that serves to reflect the light from the LEDs that is directed onto it. It is arranged so that most of the light, and in some implementations all the light, emitted by the LEDs falls on the screen, and the shape and surface finish is designed to create the desired pattern of light emission from the luminaire.

The three basic elements are simply attached together to form an assembly. In the invention's most basic form this assembly is a complete luminaire. Some possible additions to, and derivatives of, this basic form are discussed below in relation to the specific embodiments shown in the Figures, as are the factors that determine the shape and surface finish of the screen.

Further features and advantages of the present invention will be apparent from the specific embodiments that are discussed below and that are shown in the Figures.

Drawings

Figure 1 shows the schematic cross-section through a luminaire according to the present invention;

Figure 2 is an expanded schematic cross-section of the support of the luminaire of Figure 1;

Figure 3 is an exploded three-dimensional view showing the linear array of LEDs, the support and the diffusely reflecting screen of the luminaire of Figures 1 and 2;

Figure 4 is an exploded three-dimensional view of the complete luminaire of Figures 1 to 3;

Figure 5 is a schematic cross-section of an alternative embodiment of a luminaire according to the present invention showing the two linear arrays of LEDs together, the diffusely reflecting screen and the supports of that embodiment; and Figure 6 is two exploded views of a further alternative luminaire comprising a circular array of LEDs.

Figure 1 shows a cross section through the luminaire. In this example the LEDs (2) are mounted on a printed circuit board (pcb) (4) which in turn is mounted on the support (1). For ease of assembly and for low cost the LEDs are mounted on the pcb using standard surface mount techniques, although other methods of assembly are possible as is well known in the field. Other circuit components can also be mounted on the pcb for the purposes of supplying regulated current to the LEDs and for sensing and control. Examples of such circuits will be described later. An expanded view of the support is shown in figure 2.

In this embodiment the circuit board is a metal-cored printed circuit board (pcb) of the type that is commonly used for mounting high power LEDs . The high thermal conductivity of the pcb means that the temperature is almost uniform across the pcb, so that the heat generated by the LEDs is spread effectively over the whole area of the pcb, making heat transfer from the LEDs (2) through the pcb (4) to the support (1) as efficient as possible. Other options such as the use of standard FP4circuit board with or without filled metal through-vias to improve the thermal conductivity can also be used. Heat transfer between the pcb (4) and the support (1) is improved by using a thermal interface material such as loaded silicone grease or a graphite mat. Various options are available for this thermal interface material and can be found in the trade literature. If this design is implemented using the most recent generation of high-efficiency high brightness LEDs then cooling by means of free convection from the support (1) is easily sufficient to maintain the temperature of the board at an appropriate level and hence maintain the junction temperature of the LEDs well below the maximum specified in manufacturer's data sheets. As an example, the support may be made from aluminium to ensure a high thermal conductivity which efficiently spreads the heat throughout the support, and as shown in figure 2 can be shaped to increase the effective surface area so as to improve even further heat transfer to the ambient air. In other embodiments the circuit board would carry in addition to the LEDs a range of other circuit components including integrated circuits and passive components as shown in figure (3). These additional components add a negligible additional heat load, but enable much greater functionality to be achieved. For this additional functionality additional supply wires may be needed to control the circuits and provide an additional power supply.

Specific circuit functionality can be identified by way of example, although the selected examples are not intended to provide a complete list. i) A constant current source can be included on the pcb so that the current passing through the LEDs is independent of the DC voltage supplied through the wires (5) for voltages in the range, by way of example, 15 to 30V. In this way a single design of luminaire can be used directly in different applications where supply voltages may be different.

A temperature sensor can be included on the pcb, together with control circuitry that reduces the current supplied to the LEDs if the temperature increases beyond safe limits as specified in the manufacturer's data sheet.

The circuitry mounted on the PCB can have supply voltage monitoring and diagnostic functionality, such that action can be taken to ensure correct and undamaged operation, and can also provide status indicators, giving a visual indication if the temperature is too high or if the supply voltage is outside the allowed ranges.

The circuitry can incorporate a motion sensor or a detector of infrared radiation so as to allow the light to be reduced in intensity or switched off if there are no people in the vicinity. Such circuits can also be linked to other lights or central control systems in order to provide a complete lighting system for improved efficiency and performance. An important aspect of this invention relates to the design that allows the circuit board (4) to be manufactured in a cost effective way. The LEDs and other electrical components can be assembled on the pcb using automated techniques similar to those used, for example, in the mobile phone industry by sub- contractors not skilled in luminaire design.

The light emitted from the LEDs is reflected by the screen (3), which is chosen so that the light is diffusely reflected. In this example the position and shape of the screen are chosen so that all the light leaving the luminaire has been reflected from the screen, and none comes directly from the LEDs. This has the effect of ensuring that the LEDs are not visible directly, so that these extremely bright sources of light do not present a safety hazard or contribute to glare. Materials with a very high reflectivity can be obtained such the Diffuse Light Reflector (DLR) manufactured by Dupont which has a reflectivity in the visible range of at least 98%.

The use of a diffuse surface has a number of important advantages: i) The diffuse surface reduces glare and improves safety by ensuring that people under the light will not see bright reflected images of the LEDs. ii) The diffuse reflection of light by the screen provides mixing of the light from the individual LEDs, thereby ensuring a more uniform colour over the complete luminaire. iii) The diffusely reflecting screen reduces the sensitivity of the distribution of the light emitted by the luminaire to the precise alignment of the LED array and the diffusely reflecting surface. iv) The diffuse nature of the surface reduces the sensitivity of the distribution of light to the precise shape of the reflecting surface.

In the example of a linear array of LEDs shown schematically in figure 4 the shape of the diffusely reflecting screen (3) is shown as a segment of a circle for simplicity. Other shapes are possible, and in general the required distribution of light can be obtained using optical ray-tracing software to determine the required shape of the diffusely reflecting screen and the position of the LEDs as is well known to those in the field.

For linear arrays of LEDs the desired shape of the reflecting screen can be obtained by means of predefined slots or guides produced in the side members (6) of the luminaire. Slots can be produced by direct machining, or as part of the injection moulding process used to produce the side members, or by other standard manufacturing techniques. In the case of long arrays additional support may be necessary along the length of the array. This support can be provided for example by attaching the screen to rigid supporting members, or by laminating the diffusely reflecting screen to a preformed rigid supporting structure of the required shape.

Luminaires housing fluorescent tubes are one-dimensional as a consequence of the need for long tubes for the excitation of the mercury plasmas. This is not true for LEDs and so a wider range of options is possible. Figure (5&6) shows an exploded view of a two-dimensional array of LEDs together with the shaped diffuse reflector. The optimum shape of the reflector can, as for the one- dimensional case, be obtained using optical simulation software. The required shape can be manufactured using moulding or other well-known techniques.