Abstract:
A luminaire for projecting an image comprises a housing ( 1 ) having a light emission window ( 2 ). The housing ( 1 ) accommodates a light source ( 3 ), an optical system ( 4 ) for directing light originating from the light source ( 3 ) along a longitudinal axis ( 5 ), and an image-forming surface ( 6 ) which is positioned orthogonally with respect to the longitudinal axis ( 5 ). According to the invention, the light source is a LED ( 3 ), and the optical system ( 4 ) comprises a collimating element ( 14 ) for collimating the light originating from the LED ( 3 ). Preferably, the luminaire comprises only one LED ( 3 ) which, during operation, has a luminous flux of at least 5 lm. Preferably, the housing ( 1 ) also comprises a focusing lens ( 15 ) for focusing the collimated light originating from the collimating element ( 14 ) on the image-forming surface ( 6 ) and, preferably, a projection lens ( 7, 7′ ) for projecting an image of the image-forming surface ( 6 ). Preferably, parts of the collimating element show total internal reflection. The luminaire according to the invention is a compact, lightweight, low-energy and low-maintenance luminaire.

Description:
FIELD OF THE INVENTION 
     The invention relates to a luminaire for projecting an image by means of a light beam, comprising 
     a housing having a light emission window, which housing accommodates a light source and an optical system for directing the radiation to be generated by the light source along a longitudinal axis, and 
     an image-forming surface which is to be placed in the light beam transversely to the longitudinal axis. 
     BACKGROUND OF THE INVENTION 
     Such luminaires are known per se. They are used in, inter alia, theater lighting, such as stage lighting, in particular for projecting an image or a silhouette on a stage, screen, background or backcloth. Other applications of these luminaires include the projection of a logo, text or otherwise an image onto a facade of a building or on a road surface, for example in front of a shop window, for example, to attract the attention of a potential customer. Dynamic effects are generally brought about by providing the housing with moving mirrors. In addition, said luminaires generally comprise a plurality of image-forming surfaces, also referred to as GOBOs. In an alternative embodiment, said GOBOs are mounted on rotatable, transparent sheets, so that each time a different GOBO is imaged by the luminaire. 
     From EP-A 0 253 081, a so-called light pattern generator is known, which is used as theater lighting, wherein a halogen lamp or a metal halide lamp is used as the light source. Using a reflector, the light originating from the light source is turned into a parallel light beam which is focused on an image-forming surface by means of a lens. Subsequently, the image-forming surface is formed into a corresponding image. 
     Luminaires of the type mentioned above exhibit an important drawback. They often have large dimensions, are comparatively heavy and relatively expensive. In addition, such luminaires require comparatively much maintenance, for example, in that the lamp has to be regularly replaced. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to obviate the above-mentioned drawback. The invention more particularly aims at providing a luminaire of the type mentioned in the opening paragraph, wherein a simpler and cheaper construction is obtained in combination with a comparatively low energy consumption and relatively low maintenance costs. 
     To achieve this, the luminaire is characterized in accordance with the invention in that the light source comprises at least one light-emitting diode, and the optical system includes a collimating element for collimating the light beam. 
     Light-emitting diodes (LEDs) have a service life in excess of 60,000 hours, which is much longer than that of the halogen lamp or metal halide lamp used in the known luminaire. In addition, light-emitting diodes have a relatively low energy consumption (approximately 1 Watt instead of 50-200 Watt). 
     A luminaire having a high optical efficiency is obtained by using opto-electronic elements, also referred to as electro-optic elements, for example electroluminescent elements, such as light-emitting diodes (LEDs) as the light source. As a consequence of this and other factors, the decrease in light output by the use of light-emitting diodes is not proportional to the decrease of the power and the dimensions of the luminaire. 
     If a plurality of light-emitting diodes are used, they are preferably arranged in an array, with one collimating element being associated with each LED. In an alternative embodiment, the luminaire comprises a plurality of light-emitting diodes associated with a plurality of collimating elements, enabling various images of different colors to be alternately or simultaneously projected. One or more LEDs may be associated with one collimating element. As a result, the use of LEDs has the further advantage that dynamic lighting possibilities are created. If different types of LEDs are combined and/or LEDs of different colors are used, colors can be mixed in a desirable manner and also changes in color can be effected without having to use, for example, a so-called color wheel. To achieve this, a favorable embodiment of the luminaire in accordance with the invention comprises control electronics for changing the luminous flux of the LEDs. This measure enables the desired color effects to be achieved. A suitable combination of LEDs also enables white light to be obtained, and control electronics can be used to set a desired color temperature, which color temperature remains constant during operation of the luminaire. The use of this measure further enables the luminous flux to be dimmed. It is to be noted that the control electronics is customarily accommodated in the housing. In principle, however, the control electronics may be provided outside the housing. 
     A further drawback of the known luminaire is that, during operation of the luminaire, the light source used emits comparatively much heat, in practice, in the form of IR radiation. In addition, such a light source generates comparatively much UV-radiation. As a result, it proved impossible to use synthetic resin for the optical parts in the luminaire. It has been found that parts of the optical system which are made of synthetic resin are subject to degradation under the influence of the UV-radiation and/or they melt under the influence of the IR-radiation. Therefore, the optical system in the known luminaire is made of glass. Since LEDs generate much less radiation heat and/or UV light than gas discharge lamps or halogen lamps, it is possible in the luminaire in accordance with the invention to use synthetic resin for the parts of the optical system used to direct the radiation generated by the light source. The temperature of the lighting-emitting diode is much lower than that of the lamp in the known luminaire (approximately 50° C. as compared to approximately 200° C.). Furthermore, it is possible to adapt the optical path in the housing of the luminaire to the light source. In addition, optical parts of synthetic resin having the dimensional accuracy necessary for this application can be manufactured relatively readily and inexpensively as compared to glass parts. In this respect, satisfactory results are achieved using optical systems of poly[methylmethacrylate] (PMMA) or of polycarbonate (PC). 
     By using a collimating element, which surrounds the light-emitting diode, the use of a reflector as in the known luminaire is precluded. The collimating element further causes the light emitted, in operation, by the light-emitting diode to become a parallel light beam. In the collimating element, preferably, both reflection and total internal reflection of light take place, the provision of a reflective coating, as on the reflector in the known luminaire, being precluded. 
     The luminaire in accordance with the invention is compact, lightweight and has a low energy consumption and low maintenance costs. 
     An embodiment of the luminaire is characterized in accordance with the invention in that the light source is composed of a single light-emitting diode, the luminous flux of which is at least 5 lm during operation. 
     An advantage of the use of a single LED is that a very compact luminaire is obtained. At present, so-called high-efficiency, high-output LEDs are commercially available. The luminous flux per LED is at least 10 lm, preferably more than 30 lm per LED. The use of these high-power LEDs enables a luminaire having a very compact housing to be manufactured. 
     A favorable preferred embodiment of the invented luminaire is characterized in that the LED is mounted on a metal-core printed circuit board. If the LEDs are provided on such a metal-core printed circuit board (MC-PCB), heat generated by the LED or the LEDs can be readily dissipated via the PCB through heat conduction. In the case of a luminaire comprising a light source having only a single LED, the use of a MC-PCB is not necessary. 
     An interesting embodiment of the luminaire is characterized in accordance with the invention in that the housing is made of metal and provided with cooling fins, and in that the metal-core printed circuit board is in contact with the metal housing via a heat-conducting connection. Such a heat-conducting connection is preferably formed by mounting the MC-PCB on a metal plate which is connected to the metal housing. In this embodiment, the heat generated in the LED or LEDs can be dissipated, by (thermal) conduction, via the MC-PCB and the metal plate to the housing and the cooling fins, resulting in heat being dissipated to the environment. An advantage hereof is that, unlike the known luminaire, forced air cooling to dissipate heat is not necessary. Unlike the light source in the known lamp, where a halogen lamp or a metal halide lamp is used, an LED used as a light source does not emit heat. The heat originating from an LED is dissipated via conduction. 
     In a preferred embodiment of the luminaire, the optical system further comprises a focusing lens to focus the collimated light beam on the image-forming surface. In a preferred embodiment of the luminaire, the collimating element and the focusing lens form an integrated whole. This results in a greater compactness of the luminaire. In an alternative embodiment of the luminaire, the focusing lens is embodied so as to be a Fresnel lens. The focusing lens preferably consists of a synthetic resin, such as PMMA or PC, the desired optical Fresnel structure being obtained by means of injection molding. 
     In a preferred embodiment, the luminaire also comprises a projection lens for projecting an image of the image-forming surface. Preferably, the projection lens is embodied so as to be a Fresnel lens. This leads to a more compact luminaire. Such a Fresnel lens is preferably made of a synthetic resin, for example PMMA or PC. To avoid lens errors, the projection lens is preferably embodied so as to be an aspherical lens. In an alternative embodiment, the projection lens comprises diffractive structures, which lead to a higher image quality (correction of lens errors). 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a perspective view of an example of a luminaire in accordance with the invention; 
     FIG. 2A is a sectional view of a luminaire in accordance with the invention, as shown in FIG. 1, partly in cross-section and partly in perspective; 
     FIG. 2B is a cross-sectional view of a LED comprising an embodiment of a collimating element integrated with a projection lens; 
     FIG. 3A is a cross-sectional view of a LED comprising an embodiment of a collimating element, and 
     FIG. 3B is a cross-sectional view of a LED comprising an embodiment of a collimating element embodied so as to be a Fresnel lens. 
    
    
     The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts whenever possible. 
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagrammatic, perspective view of an example of a luminaire for projecting an image by means of a light beam in accordance with the invention. The luminaire comprises a housing  1  having a light emission window  2  provided in an adjusting member  71 , which can be moved with respect to the housing  1 . In operation, the luminaire emits the light beam along the longitudinal axis  5 . The housing is provided with cooling fins  11  and supported by a support  20 , which rests on three flexible legs  21 ,  21 ′ in this example. The luminaire may alternatively be suspended or provided with a lamp cap which is known per se, for example of the type E14 or E27. 
     FIG. 2A diagrammatically shows, partly in cross-section and partly in perspective, a sectional view of a luminaire in accordance with the invention as shown in FIG.  1 . The housing  1  is provided with cooling fins  11  and supported by a support  20  which rests on flexible legs  21 ,  21 ′. The housing  1  further comprises an opening  9  for allowing passage of an electrical connection cable. The housing  1  also includes control electronics (not shown in FIG. 2A for the sake of clarity), which is necessary to operate the LED  3 . If necessary, the control electronics may be situated outside the housing  1 . 
     The housing  1  accommodates a light source  3  and an optical system  4  for directing radiation to be generated by the light source  3  along the longitudinal axis  5  (see FIG.  2 B). The housing  1  further accommodates an image-forming surface  6  which is placed in the light beam transversely to the longitudinal axis  5 . The image-forming surface  6  is alternatively referred to as GOBO (an abbreviation used in the theater and movie world: “GO BlackOut”). A GOBO generally consists of a thin metal sheet which serves as a mask for selectively passing light, or it may alternatively be made of (colored) glass provided with an image. The image-forming surface  6  may also comprise a so-called Cookie (originating from the Greek word kukaloris), which causes light to be refracted. In general, a luminaire comprises a plurality of GOBOs mounted on rotatable transparent plates, and the luminaire images a different GOBO each time. 
     In accordance with the measure of the invention, the light source comprises a light-emitting diode  3  which is accommodated in the housing  1  of the luminaire. The optical system  4  comprises a collimating element  14  for collimating the light beam. In the example shown in FIGS. 2A and 2B, the integrated element  4  has a collimating function as well as a focusing function. 
     Since LEDs are used in the invented luminaire instead of the light sources known from the prior-art luminaire, it is favorable to manufacture the image-forming surface from an optically transparent synthetic resin comprising, for example, a diapositive, such as an advertising text and/or an image of a logo and/or a brand name of a company. 
     In the example of FIG. 2A, the housing comprises one LED  3  and one image-forming surface  6  associated therewith. In this manner, a very compact luminaire is obtained. The length of the housing  1  including the light emission window  2 , measured along the longitudinal axis  5 , ranges between 5 and 10 cm. The length of the known luminaire generally ranges from 30 to 100 cm. 
     To obtain a luminaire having a high light output, so-called power packages of LEDs can be used as the light source. For example, high-efficiency, high-output LEDs of the type “Barracuda” (Hewlett-Packard) may be used. The luminous flux per LED is 10 lm for the red LEDs and 13 lm for the green LEDs. The use of these high-efficiency, high-output LEDs has the specific advantage that the number of LEDs may be relatively small for a desired, relatively high light output. This contributes to the compactness and efficiency of the luminaire to be manufactured. In an alternative embodiment, “Prometheus” type LEDs (Hewlett-Packard) are used as the high-efficiency, high-output LEDs, the luminous flux per LED being 35 lm for the red LEDs and 30 lm for the green LEDs. By using one or more high-efficiency, high-output LEDs as the light source, luminaires are obtained which are daylight viewable and whose brightness is comparable to that of the known luminaire. 
     Preferably, the LED  3  is mounted on a metal-core printed circuit board  13 . Preferably, the metal-core printed circuit board  13  is in contact with the metal housing  1  provided with cooling fins  11  via a heat-conducting connection. Such a heat-conducting connection is preferably brought about by connecting the MC-PCB  13  to the metal housing  1 , either directly or via a metal plate. The heat generated in the LED or LEDs  3  can be dissipated by (thermal) conduction via the MC-PCB  13  to the housing  1  and the cooling fins  11 , in which process heat dissipation to the environment takes place. This has the advantage that forced air cooling for dissipating heat, as employed in the known luminaire, is not necessary. 
     The optical system  4  in the luminaire, as shown in FIG. 2A, further comprises a focusing lens  15  for focusing the collimated light beam on the image-forming surface  6 . In FIG. 2B, which shows the optical system  4  of FIG. 2A on an enlarged scale, a LED  3  is shown in combination with a collimating element  14  which is combined with a focusing lens  15  to form an integrated whole. The integrated element  4  has a collimating function as well as a focusing function. The LED  3  is provided on a MC-PCB  13 , has an optical axis  25  and is composed of a body  21  for emitting light during operation. In the example shown in FIG. 2B, the body  21  of the LED  3  is provided with a light-transmitting envelope  21 ′, for example in the form of a lens. The collimating element  14  and the focusing lens  15  jointly form the optical system  4  and have an optical axis  25 ′, which coincides with the optical axis  25  of the LED  3 . In an alternative embodiment of the luminaire, the collimating element is embodied so as to be a Fresnel lens (see FIG.  3 B). 
     The luminaire shown in FIG. 2A further comprises a projection lens  7 ,  7 ′ for projecting an image of the image-forming surface  6 . In the example shown in FIG. 2A, two projection lenses  7 ,  7 ′ are used, namely a first projection lens  7  which is in a fixed position with respect to the housing  1 , and a second projection lens  7 ′ which is mounted in an adjusting member  71  which is displaceably provided on the housing  1  (for example by a helical motion) so that the projection lens  7  can be moved along the longitudinal axis  5 . In this manner, a sharp image of the image-forming surface  6  is obtained. In an alternative embodiment of the luminaire, the focusing lens is embodied so as to be Fresnel lens. Preferably, the focusing lens is made of a synthetic resin. 
     FIG. 3A is a cross-sectional view of a LED  33  with an embodiment of a collimating element  34  associated with said LED  33 . The LED  33  has an optical axis  35  and is built up of a body  31  for emitting light during operation. In the example of FIG. 3A, the body  31  of the LED  33  is provided with a light-transmitting envelope  31 ′, for example in the form of a lens. The collimating element  34  has an optical axis  35 ′, which coincides with the optical axis  35  of the LED  33 . The shape of the collimating element  34  is chosen to be such that light originating from the LED  33  is emitted by the optical system in a direction parallel to the optical axis  35 ′ of the collimating element  34  (see the very diagrammatically shown light rays passing through the collimating element  34 ). Due to this measure, the intensity of the LED  33  is increased in the forward direction (parallel to the optical axis  35 ). To this end, surfaces  38 ,  38 ′ of the collimating element  34  are curved such that light originating from the LED  33  and impinging on the surface  38  forms a parallel light beam after passing the surface  38 ′, which light beam extends parallel to the optical axis  35 ′ of the collimating element  34  (refraction). In addition, surfaces  39 ;  39 ′ and associated surfaces  40 ;  40 ′ of the collimating element  34  are curved such that light originating from the LED  33  and impinging on the surface  39 ;  39 ′ is fully reflected in the direction of the surface  40 ;  40 ′ and, after passing surface  40 ;  40 ′, forms a parallel light beam extending parallel to the optical axis  35 ′ of the collimating element  34  (total internal reflection). A reflective coating can thus be omitted. 
     FIG. 3B is a diagrammatic, cross-sectional view of an alternative embodiment of the luminaire comprising a LED  43  and a collimating element embodied so as to be a Fresnel lens  44 . The LED  43  has an optical axis  45  and is built up of a body  41  for emitting light during operation. In the example of FIG. 3B, the body  41  of the LED  43  is provided with a light-transmitting envelope  41 ′, for example in the form of a lens. The collimating element  44  has an optical axis  45 ′, which coincides with the optical axis  45  of the LED  43 . In this example, the collimating element  44  is embodied so as to be a Fresnel lens, so that light originating from the LED  43  passes through the optical system and is emitted in a direction parallel to the optical axis  45 ′ of the collimating element  44 . 
     It will be obvious that, within the scope of the invention, many variations are possible to those skilled in the art. 
     The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics. Reference numerals in the claims do not limit the scope of protection thereof. The use of the word “comprising” does not exclude the presence of elements other than those mentioned in the claims. The use of the word “a or “an” in front of an element does not exclude the presence of a plurality of such elements.