Abstract:
A lighting device comprises, or consists essentially of, a housing, a solid state light emitter and conductive tracks. The conductive tracks are positioned on the housing and are coupleable with a power supply. The conductive tracks comprise a positive conductive track and a negative conductive track. Each of the solid state light emitters is in electrical contact with a positive conductive track and a negative conductive track. Another lighting device comprises a fixture and a solid state light emitter in which the fixture comprises conductive elements which are coupleable to at least one power supply and the solid state light emitter is mounted on the fixture. There is also provided a lighting device which provides light of an intensity which is at least 50 percent of its initial intensity after 50,000 hours of illumination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/752,753, filed Dec. 21, 2005, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a lighting device, in particular, a device which includes one or more solid state light emitters. The present invention also relates to a lighting device which includes one or more solid state light emitters, and which optionally further includes one or more luminescent materials (e.g., one or more phosphors). In a particular aspect, the present invention relates to a lighting device which includes one or more light emitting diodes, and optionally further includes one or more luminescent materials. 
     BACKGROUND OF THE INVENTION 
     A large proportion (some estimates are as high as one third) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient. It is well-known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 4) but are still quite inefficient as compared to solid state light emitters, such as light emitting diodes. 
     In addition, as compared to the normal lifetimes of solid state light emitters, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, the lifetime of light emitting diodes, for example, can generally be measured in decades. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI) which is a relative measure of the shift in surface color of an object when lit by a particular lamp. Daylight has the highest CRI (of 100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70-85). Certain types of specialized lighting have relatively low CRI&#39;s (e.g., mercury vapor or sodium, both as low as about 40 or even lower). 
     Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hour&#39;s (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs. 
     Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, color rendering index (CRI), efficacy (1 m/W), and/or duration of service. 
     A variety of solid state light emitters are well-known. For example, one type of solid state light emitter is a light emitting diode. Light emitting diodes are well-known semiconductor devices that convert electrical current into light. A wide variety of light emitting diodes are used in increasingly diverse fields for an ever-expanding range of purposes. 
     More specifically, light emitting diodes are semiconducting devices that emit light (ultraviolet, visible, or infrared) when a potential difference is applied across a p n junction structure. There are a number of well-known ways to make light emitting diodes and many associated structures, and the present invention can employ any such devices. By way of example, Chapters 12-14 of Sze, Physics of Semiconductor Devices, (2d Ed. 1981) and Chapter 7 of Sze, Modern Semiconductor Device Physics (1998) describe a variety of photonic devices, including light emitting diodes. 
     The expression “light emitting diode” is used herein to refer to the basic semiconductor diode structure (i.e., the chip). The commonly recognized and commercially available “LED” that is sold (for example) in electronics stores typically represents a “packaged” device made up of a number of parts. These packaged devices typically include a semiconductor based light emitting diode such as (but not limited to) those described in U.S. Pat. Nos. 4,918,487; 5,631,190; and 5,912,477; various wire connections, and a package that encapsulates the light emitting diode. 
     As is well-known, a light emitting diode produces light by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer. The electron transition generates light at a wavelength that depends on the band gap. Thus, the color of the light (wavelength) emitted by a light emitting diode depends on the semiconductor materials of the active layers of the light emitting diode. 
     Although the development of light emitting diodes has in many ways revolutionized the lighting industry, some of the characteristics of light emitting diodes have presented challenges, some of which have not yet been fully met. For example, the emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode&#39;s composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing lighting, such an emission spectrum provides a very low CRI). 
     Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode can produce white light. “White” light emitting diodes have been produced which have a light emitting diode pixel formed of respective red, green and blue light emitting diodes. Other “white” light emitting diodes have been produced which include (1) a light emitting diode which generates blue light and (2) a luminescent material (e.g., a phosphor) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. 
     In addition, the blending of primary colors to produce combinations of non-primary colors is generally well understood in this and other arts. In general, the 1931 CIE Chromaticity Diagram (an international standard for primary colors established in 1931), and the 1976 CIE Chromaticity Diagram (similar to the 1931 Diagram but modified such that similar distances on the Diagram represent similar differences in color) provide useful reference for defining colors as weighted sums of primary colors. 
     Light emitting diodes can thus be used individually or in any combinations, optionally together with one or more luminescent material (e.g., phosphors or scintillators) and/or filters, to generate light of any desired perceived color (including white). Accordingly, the areas in which efforts are being made to replace existing light sources with light emitting diode light sources, e.g., to improve energy efficiency, color rendering index (CRI), efficacy (1 m/W), and/or duration of service, are not limited to any particular color or color blends of light. 
     A wide variety of luminescent materials (also known as lumiphors or liminophoric media, e.g., as disclosed in U.S. Pat. No. 6,600,175, the entirety of which is hereby incorporated by reference) are well-known and available to persons of skill in the art. For example, a phosphor is a luminescent material that emits a responsive radiation (e.g., visible light) when excited by a source of exciting radiation. In many instances, the responsive radiation has a wavelength which is different from the wavelength of the exciting radiation. 
     Other examples of luminescent materials include scintillators, day glow tapes and inks which glow in the visible spectrum upon illumination with ultraviolet light. 
     Luminescent materials can be categorized as being down-converting, i.e., a material which converts photons to a lower energy level (longer wavelength) or up-converting, i.e., a material which converts photons to a higher energy level (shorter wavelength). 
     Inclusion of luminescent materials in LED devices has been accomplished by adding the luminescent materials to a clear encapsulant material (e.g., epoxy-based or silicone-based material) as discussed above, for example by a blending or coating process. 
     For example, U.S. Pat. No. 6,963,166 (Yano &#39;166) discloses that a conventional light emitting diode lamp includes a light emitting diode chip, a bullet-shaped transparent housing to cover the light emitting diode chip, leads to supply current to the light emitting diode chip, and a cup reflector for reflecting the emission of the light emitting diode chip in a uniform direction, in which the light emitting diode chip is encapsulated with a first resin portion, which is further encapsulated with a second resin portion. According to Yano &#39;166, the first resin portion is obtained by filling the cup reflector with a resin material and curing it after the light emitting diode chip has been mounted onto the bottom of the cup reflector and then has had its cathode and anode electrodes electrically connected to the leads by way of wires. According to Yano &#39;166, a phosphor is dispersed in the first resin portion so as to be excited with the light A that has been emitted from the light emitting diode chip, the excited phosphor produces fluorescence (“light B”) that has a longer wavelength than the light A, a portion of the light A is transmitted through the first resin portion including the phosphor, and as a result, light C, as a mixture of the light A and light B, is used as illumination. 
     As noted above, “white LED lights” (i.e., lights which are perceived as being white or near-white) have been investigated as potential replacements for white incandescent lamps. A representative example of a white LED lamp includes a package of a blue light emitting diode chip, made of gallium nitride (GaN), coated with a phosphor such as YAG. In such an LED lamp, the blue light emitting diode chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. For instance, in some designs, white light emitting diodes are fabricated by forming a ceramic phosphor layer on the output surface of a blue light-emitting semiconductor light emitting diode. Part of the blue ray emitted from the light emitting diode chip passes through the phosphor, while part of the blue ray emitted from the light emitting diode chip is absorbed by the phosphor, which becomes excited and emits a yellow ray. The part of the blue light emitted by the light emitting diode which is transmitted through the phosphor is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light. 
     As also noted above, in another type of LED lamp, a light emitting diode chip that emits an ultraviolet ray is combined with phosphor materials that produce red (R), green (G) and blue (B) light rays. In such an LED lamp, the ultraviolet ray that has been radiated from the light emitting diode chip excites the phosphor, causing the phosphor to emit red, green and blue light rays which, when mixed, are perceived by the human eye as white light. Consequently, white light can also be obtained as a mixture of these light rays. 
     Designs have been provided in which existing LED component packages and other electronics are assembled into a fixture. In such designs, a packaged LED is mounted to a circuit board, the circuit board is mounted to a heat sink, and the heat sink is mounted to the fixture housing along with required drive electronics. In many cases, additional optics (secondary to the package parts) are also necessary. 
     In substituting light emitting diodes for other light sources, e.g., incandescent light bulbs, packaged LEDs have been used with conventional light fixtures, for example, fixtures which include a hollow lens and a base plate attached to the lens, the base plate having a conventional socket housing with one or more contacts which is electrically coupled to a power source. For example, LED light bulbs have been constructed which comprise an electrical circuit board, a plurality of packaged LEDs mounted to the circuit board, and a connection post attached to the circuit board and adapted to be connected to the socket housing of the light fixture, whereby the plurality of LEDs can be illuminated by the power source. 
     There is an ongoing need for ways to use solid state light emitters, e.g., light emitting diodes, in a wider variety of applications, with greater energy efficiency, with improved color rendering index (CRI), with improved contrast, with improved efficacy (1 m/W), and/or with longer duration of service, for all possible light colors, including white light (including light perceived as white light). 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention is directed to a lighting device which employs solid state light emitters at the chip/dice level (light emitting diodes, laser diodes, thin film electroluminescent devices, etc) which are attached to the housing of the device, the housing of the device preferably providing both the thermal and optical solution for the device. Such a design eliminates thermal interfaces (to reduce the temperature of the light source (e.g., light emitting diodes)) and reduces cost as the light emitting diode(s) or light source(s) is/are built “bottoms up” within the system to minimize cost and maximize performance. In a preferred aspect, the entire integration involves: a) light emitting diode chips mounted directly to the fixture with the required optics integrated into the fixture and the required drive electronics, in which the fixture provides the function of thermal and optical solutions, thereby reducing the complexity of many subassemblies used in conventional designs. 
     In a specific aspect, the lighting device is one that can produce light that is perceived as “white”. 
     According to a first embodiment, there is provided a lighting device comprising, or consisting essentially of, a housing, at least one solid state light emitter, and conductive tracks. The conductive tracks are coupleable with at least one power supply. The conductive tracks are positioned on at least a first portion of the housing, and the conductive tracks comprise at least a first positive conductive track and at least a first negative conductive track. Each of the solid state light emitters is in electrical contact with at least one positive conductive track and at least one negative conductive track. 
     The expression “on”, e.g., as used in the preceding paragraph in the expression “positioned on”, or in the expressions “mounted on”, “formed on”, “painted on”, “printed on”, or “trace on a circuit board”, means that the first structure which is “on” a second structure can be in contact with the second structure, or can be separated from the second structure by one or more intervening structures. 
     The expression “conductive track”, as used herein, refers to a structure which comprises a conductive portion, and may further include any other structure, e.g., one or more insulating layers. For example, a conductive track mounted on a housing might consist of an insulating layer and a conductive layer, particularly where the housing is capable of conducting electricity (in which case the conductive track is mounted on the housing with the insulating layer of the conductive track in contact with the housing and the conductive layer of the conductive track not in contact with the housing, and one or more light emitting diode chips are electrically connected to the conductive layers of the conductive tracks such that the light emitting diode chips can be powered by electricity and illuminated. 
     In a particular aspect of the invention, the lighting device comprises a plurality of solid state light emitters. In a further particular aspect, the one or more solid state light emitters is/are light emitting diode(s). 
     In a further aspect of the invention, the lighting device further comprises at least a first luminescent material, e.g., a first phosphor. 
     In a second aspect, the present invention provides a lighting device comprising a fixture comprising conductive elements which are coupleable to at least one power supply, and at least one solid state light emitter. The solid state light emitter is mounted on the fixture. The lighting device provides, after 50,000 hours of illumination, light of an intensity which is at least 50 percent of its initial intensity. 
     The invention may be more fully understood with reference to the accompanying drawings and the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a sectional view of a first embodiment of a lighting device according to the present invention. 
         FIG. 2  is a partially schematic sectional view of the embodiment shown in  FIG. 1 , taken along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a sectional view of the embodiment shown in  FIG. 1 , taken along line  3 - 3  in  FIG. 1 . 
         FIG. 4  is a sectional view corresponding to the view depicted in  FIG. 3 , with a modification. 
         FIG. 5  is a sectional view of a second embodiment of a lighting device according to the present invention. 
         FIG. 6  is a sectional view of the embodiment shown in  FIG. 5 , taken along line  6 - 6  in  FIG. 5 . 
         FIGS. 7-12  depict sectional views of a variety of housings of different shapes. 
         FIG. 13  is a schematic electrical diagram showing a plurality of solid state light emitters wired in a mesh pattern. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As described above, in one aspect, the present invention is directed to a lighting device which comprises a housing, at least one solid state light emitter, and conductive tracks for supplying electricity to the solid state light emitter(s). The present invention is also directed to a lighting device which comprises a housing, at least one solid state light emitter, at least one luminescent material and conductive tracks for supplying electricity to the solid state light emitter(s). 
     The conductive tracks can be positioned in any suitable way. For example, the conductive tracks can, if desired, be positioned on at least a first portion of the housing, and comprise at least a first positive conductive track and at least a first negative conductive track. 
     Each solid state light emitter can be mounted in any suitable arrangement. For example, the solid state light emitter(s) can, if desired, be mounted on the housing, in electrical contact with at least one negative conductive track and at least one positive conductive track. 
     Preferably, one or more surfaces of the housing is/are reflective, so that light from some or all of the light emitting diodes is reflected by such reflective surfaces. 
     The housing can be formed of any material which can be molded and/or shaped. Preferably, the housing is formed of a material which is an effective heat sink (i.e., which has high thermal conductivity and/or high heat capacity) and/or which is reflective (or which is coated with a reflective material). 
     The housing can be any desired shape. Representative examples of shapes for the housing include hollow conical (or substantially conical), hollow frustoconical (or substantially frustoconical), hollow cylindrical (or substantially cylindrical) and hollow semi-elliptical (or substantially semi-elliptical), or any shape which includes one or more portions which are individually selected film among hollow conical (or substantially conical), hollow frustoconical (or substantially frustoconical), hollow cylindrical (or substantially cylindrical) and hollow semi-elliptical (or substantially semi-elliptical). In one aspect of the invention, the housing comprises at least a first concave surface, at least one of the solid state light emitters being mounted on the first concave surface. Optionally, the housing can comprise numerous concave surfaces, and one or more light emitting diodes can be mounted on any or all of such concave surfaces. 
     As used herein, the term “substantially,” e.g., in the expressions “substantially conical”, “substantially frustoconical”, “substantially cylindrical” and “substantially semi-elliptical”, means at least about 95% correspondence with the feature recited, e.g., “substantially semi-elliptical” means that a semi-ellipse can be drawn having the formula x 2 /a 2 +y 2 /b 2= 1, where y≧0, and imaginary axes can be drawn at a location where the y coordinate of each point on the structure is within 0.95 to 1.05 times the value obtained by inserting the x coordinate of such point into such formula, etc. 
     Any desired solid state light emitter or emitters can be employed in accordance with the present invention. Persons of skill in the art are aware of, and have ready access to, a wide variety of such emitters. Such solid state light emitters include inorganic and organic light emitters. Examples of types of such light emitters include light emitting diodes (inorganic or organic), laser diodes and thin film electroluminescent devices, a variety of each of which are well-known in the art. 
     In one aspect of the present invention, there is provided a device which comprises at least first and second solid state light emitters, in which the first solid state light emitter emits light of a first wavelength and the second solid state light emitter emits light of a second wavelength, the second wavelength differing from the first wavelength. In such a device, the solid state light emitters can emit light of any desired wavelength or wavelengths (or wavelength range or wavelength ranges) within the ranges of infrared, visible and ultraviolet light, including, e.g., (1) two or more light emitting diodes emitting light within different wavelength ranges within the visible spectrum, (2) two or more light emitting diodes emitting light within different wavelength ranges within the infrared spectrum, (3) two or more light emitting diodes emitting light within different wavelength ranges within the ultraviolet spectrum, (4) one or more light emitting diodes emitting light within the visible spectrum and one or more light emitting diodes emitting light within the infrared spectrum, (5) one or more light emitting diodes emitting light within the visible spectrum and one or more light emitting diodes emitting light within the ultraviolet spectrum, etc. 
     As noted above, persons skilled in the art are familiar with a wide variety of solid state light emitters, including a wide variety of light emitting diodes, a wide variety of laser diodes and a wide variety of thin film electroluminescent devices, and therefore it is not necessary to describe in detail such devices, and/or the materials out of which such devices are made. 
     As indicated above, the lighting devices according to the present invention can comprise any desired number of solid state emitters. For example, a lighting device according to the present invention can include 50 or more light emitting diodes, or can include 100 or more light emitting diodes, etc. In general, with current light emitting diodes, excellent efficiency can be achieved by using a large number of comparatively small light emitting diodes (e.g., 100 light emitting diodes each having a surface area of 0.1 mm 2  vs. 25 light emitting diodes each having a surface area of 0.4 mm 2  but otherwise being identical). 
     Analogously, light emitting diodes which operate at lower current densities provide excellent efficiency. Light emitting diodes which draw any particular current can be used according to the present invention. In some embodiments of the present invention, light emitting diodes which each draw not more than 50 milliamps are employed. 
     On the other hand, current “power chips” can provide excellent performance as well. Accordingly, some embodiments of the present invention are lighting devices which include 30 light emitting diodes or fewer (and in some cases, 20 light emitting diodes or fewer), the light emitting diodes each operating at 300 mA or more. 
     Persons of skill in the art are familiar with various ways of attaching solid state light emitters to housings, and any such ways can be employed in accordance with the present invention. 
     The conductive tracks can be any structure which conducts electricity. Persons of skill in the art are familiar with, and can readily provide, a wide variety of conductive tracks provided in a wide variety of forms. For example, conductive tracks can be metallized traces formed on, painted on or printed on the housing, or can be wires or lead frames placed along a surface or surfaces of the housing. 
     The solid state light emitters can be wired in any suitable pattern. Preferably, the plurality of solid state light emitters are wired in a mesh pattern (see  FIG. 13 , which is a schematic diagram showing a plurality of solid state light emitters  71  arranged in strings with conductive elements  72  connecting the solid state light emitters in a particular string, and with one or more cross-connection conductive elements  73  extending between the strings). Another example of a wiring pattern which can be used is series parallel, such that failure of one of the solid state light emitters would affect only solid state light emitters in series with the solid state light emitter that failed. The expression “series parallel”, as used herein, means electrical paths are arranged in parallel, each electrical path including one or more solid state light emitters. 
     In one aspect of the invention, the conductive tracks (and therefor the solid state light emitters as well) are coupleable, i.e., can be electrically connected (permanently or selectively), to one or more power supply, e.g., to one or more batteries and/or to electrical service. For example, circuitry can be provided in which (1) electricity is normally supplied to the lighting device through electrical service (e.g., connected to the grid) under normal conditions, and in which (2) if electrical service is interrupted (e.g., in the case of a power outage), one or more switches can be closed whereby power can be supplied to some (e.g., at least about 5 percent or at least about 20 percent) or all of the solid state light emitters. Where necessary, there is preferably further provided a device which detects when electrical service has been interrupted, and automatically switches on battery power to at least some of the solid state light emitters. 
     A statement herein that two components in a device are “electrically connected,” means that there are no components electrically between the components, the insertion of which materially affect the function or functions provided by the device. For example, two components can be referred to as being electrically connected, even though they may have a small resistor between them which does not materially affect the function or functions provided by the device (indeed, a wire connecting two components can be thought of as a small resistor); likewise, two components can be referred to as being electrically connected, even though they may have an additional electrical component between them which allows the device to perform an additional function, while not materially affecting the function or functions provided by a device which is identical except for not including the additional component; similarly, two components which are directly connected to each other, or which are directly connected to opposite ends of a wire or a trace on a circuit board or another medium, are electrically connected. 
     In another aspect of the invention, the solid state light emitters can optionally be connected (permanently or selectively) to one or more photovoltaic energy collection device (i.e., a device which includes one or more photovoltaic cells which converts energy film the sun into electrical energy), such that energy can be supplied from the photovoltaic energy collection device to the solid state light emitters. 
     Persons of skill in the art are familiar with various ways of electrically connecting (permanently or selectively) conductive tracks to power supplies, and any such ways can be employed in accordance with the present invention. 
     The one or more luminescent materials, if present, can be any desired luminescent material. As noted above, persons skilled in the art are familiar with, and have ready access to, a wide variety of luminescent materials. The one or more luminescent materials can be down-converting or up-converting, or can include a combination of both types. 
     For example, the one or more luminescent materials can be selected from among phosphors, scintillators, day glow tapes, inks which glow in the visible spectrum upon illumination with ultraviolet light, etc. 
     The one or more luminescent materials, when provided, can be provided in any desired form. For example, in one aspect, a lighting device according to the present invention can comprise at least one luminescent element which comprises a first luminescent material, the luminescent element being attached to the housing, the luminescent element and the housing defining an internal space, at least one of the solid state light emitters being positioned within the internal space. 
     The luminescent element can, if desired, comprise a material in which the first luminescent material is embedded. For example, persons of skill in the art are very familiar with luminescent elements comprising a luminescent material, e.g., a phosphor, embedded in a resin (i.e., a polymeric matrix), such as a silicone material or an epoxy material. 
     In a preferred aspect of the present invention, the lighting device comprises at least one luminescent element which comprises at least a first luminescent element region and a second luminescent element region, the first luminescent element region comprising a first luminescent material, the second luminescent element region comprising a second luminescent material, the first luminescent material, upon being excited, emitting light of a first wavelength (or range of wavelengths), the second luminescent material, upon being excited, emitting light of a second wavelength (or range of wavelengths), the second wavelength (or range of wavelengths) differing from the first wavelength (or range of wavelengths). 
     In accordance with another preferred aspect of the invention, a lighting device can comprise a plurality of luminescent elements, each luminescent element comprising at least one luminescent material, each luminescent element being attached to the housing to define an internal space, at least one solid state light emitter being positioned within each internal space. 
     In embodiments of the present invention in which a plurality of solid state light emitters are mounted on a housing, the heat load produced by the solid state light emitters is distributed over the surface of the housing. The more uniformly the solid state light emitters are distributed over the surface area of the housing, the more uniformly the heat load is distributed. As a result, the housing can provide more efficient heat dissipation, with the result that the housing can, if desired, be made smaller than would otherwise be the case. In addition, by having multiple solid state light emitters (as opposed to a single point source of light), the light source is affected less by shadowing—that is, if an object which is smaller than the light emitting area is placed in front of the light emitting area, only a portion of the light rays would be blocked. Since the light sources follow the Huygens principle (each source acts as a spherical wave front), the viewing of a shadow is not seen, and only a slight dimming of the illuminated source is seen (in contrast to where a single filament is employed, where the light would be substantially dimmed and a shadow would be observed). 
     Persons of skill in the art are familiar with various ways of attaching luminescent elements to housings, and any such ways can be employed in accordance with the present invention. 
     The devices according to the present invention can further comprise one or more long-life cooling device (e.g., a fan with an extremely high lifetime). Such long-life cooling device(s) can comprise piezoelectric or magnetorestrictive materials (e.g., MR, GMR, and/or HMR materials) that move air as a “Chinese fan”. In cooling the devices according to the present invention, typically only enough air to break the boundary layer is required to induce temperature drops of 10 to 15 degrees C. Hence, in such cases, strong ‘breezes’ or a large fluid flow rate (large CFM) are typically not required (thereby avoiding the need for conventional fans). 
     The devices according to the present invention can further comprise secondary optics to further change the projected nature of the emitted light. Such secondary optics are well-known to those skilled in the art, and so they do not need to be described in detail herein—any such secondary optics can, if desired, be employed. 
     The devices according to the present invention can further comprise sensors or charging devices or cameras, etc. For example, persons of skill in the art are familiar with, and have ready access to, devices which detect one or more occurrence (e.g., motion detectors, which detect motion of an object or person), and which, in response to such detection, trigger illumination of a light, activation of a security camera, etc. As a representative example, a device according to the present invention can include a lighting device according to the present invention and a motion sensor, and can be constructed such that (1) while the light is illuminated, if the motion sensor detects movement, a security camera is activated to record visual data at or around the location of the detected motion, or (2) if the motion sensor detects movement, the light is illuminated to light the region near the location of the detected motion and the security camera is activated to record visual data at or around the location of the detected motion, etc. 
       FIG. 1  is a sectional view of a first embodiment of a lighting device  10  according to the present invention. Referring to  FIG. 1 , the first embodiment comprises a housing  11 , a plurality of light emitting diodes  12  mounted on the housing  11 , and a substantially circular luminescent element  13  attached to the housing  11 . The housing  11  and the luminescent element  13  together define an internal space within which each of the light emitting diodes  12  are positioned. The housing  11  has a hollow substantially semi-elliptical shape. The surface of the housing  11  which faces the internal space has a reflective surface coated thereon, as well as conductive tracks  14  printed thereon. The luminescent element  13  comprises a cured polymeric resin with a phosphor powder loaded therein. The lighting device  10  further comprises a power cord including a negative power line  15  electrically connected to negative power tracks and a positive power line  16  electrically connected to positive power tracks, the power cord being connectable to a power supply, such that the conductive tracks are coupleable with a power supply. Each of the light emitting diodes  12  is in electrical contact with at least one positive conductive track and at least one negative conductive track, whereby power can be provided to the light emitting diodes  12  to illuminate them.  FIG. 1  schematically illustrates a power supply  17  attached to the negative and positive power lines  15  and  16 . 
       FIG. 2  is a sectional view of the embodiment shown in  FIG. 1 , taken along line  2 - 2  in  FIG. 1 . 
       FIG. 3  is a sectional view of the embodiment shown in  FIG. 1 , taken along line  3 - 3  in  FIG. 1 .  FIG. 3  shows the luminescent element  13 , in this case containing a single luminescent material. 
       FIG. 4  is a sectional view corresponding to the view depicted in  FIG. 3 , modified in that instead of the luminescent element  13  containing a single luminescent material, the luminescent element  13  has a plurality of regions, each of the regions having a luminescent material selected from among luminescent material which, when illuminated by the light emitting diodes  12 , emit blue light, green light or yellow light. The regions depicted in  FIG. 4  are marked to indicate the type of luminescent material in each region, where a “B” marked in the region indicates that the region contains luminescent material which, when illuminated by the light emitting diodes  12 , will emit blue light, where a “G” marked in the region indicates that the region contains luminescent material which, when illuminated by the light emitting diodes  12 , will emit green light, and where a “Y” marked in the region indicates that the region contains luminescent material which, when illuminated by the light emitting diodes  12 , will emit yellow light. 
       FIG. 5  is a sectional view of a second embodiment of a lighting device  50  according to the present invention. Referring to  FIG. 5 , the second embodiment comprises a housing  51  having a first annular flange portion  57  extending radially inwardly toward the center axis  58  of the housing  51  and a second annular flange portion  59  extending radially outwardly from the center axis  58  of the housing  51 . A plurality of light emitting diodes  52  are mounted on the first annular flange portion  57 . A luminescent element  53  is attached to the housing  51  and to an inner edge  60  of the first annular flange portion  57 . The housing  51 , the first annular flange portion  57  and the luminescent element  53  together define a toroidal internal space within which each of the light emitting diodes  52  are positioned. The housing  51  has a hollow substantially semi-elliptical shape. The surface of the housing  51  which faces the internal space has a reflective surface coated thereon. If desired, any suitable cover, a variety of which are well-known to those skilled in the art, can be positioned over the opening defined by the inner edge  60  of the first annular flange portion  57 . 
       FIG. 6  is a sectional view of the embodiment shown in  FIG. 5 , taken along line  6 - 6  in  FIG. 5 .  FIG. 6  shows the first annular flange portion  57  with light emitting diodes  52  mounted thereon.  FIG. 6  also shows conductive tracks  54  printed on the first annular flange portion  57  to provide electrical power to the light emitting diodes  52 . 
     Referring again to  FIG. 5 , the lighting device  50  is mounted in a circular hole formed in a ceiling  61  (e.g., formed of wallboard or any other suitable construction material), i.e., the second annular flange portion  59  is in contact with the ceiling  61 . The luminescent element  53  comprises a cured polymeric resin with a phosphor powder loaded therein. Referring to  FIG. 6 , the lighting device  50  further comprises a power cord including a negative power line  55  electrically connected to the negative power track and a positive power line  56  electrically connected to the positive power track, the power cord being connectable to a power supply, such that the conductive tracks are coupleable with a power supply. Each of the light emitting diodes  52  is in electrical contact with the positive conductive track and the negative conductive track, whereby power can be provided to the light emitting diodes  52  to illuminate them. 
     As noted above, the housing can generally be of any desired size and shape.  FIGS. 7-12  depict sectional views of a variety of housings of different shapes.  FIG. 7  is a sectional view of a first hollow semi-elliptical housing.  FIG. 8  is a sectional view of a second hollow semi-elliptical housing.  FIG. 9  is a sectional view of a hollow conical housing.  FIG. 10  is a sectional view of a first hollow cylindrical housing.  FIG. 11  is a sectional view of a second hollow cylindrical housing.  FIG. 12  is a sectional view of a housing having a plurality of hollow conical portions. 
     Any two or more structural parts of the lighting devices described herein can be integrated. Any structural part of the lighting devices described herein can be provided in two or more parts (which can be held together, if necessary).