Abstract:
The proposed illuminator relates to white-light lamps based on LEDs with remote phosphor converters. The illuminator comprises a heat removing base with a radiation output opening, and the LEDs secured near the periphery of the opening, with, arranged in series at a distance from the LEDs, a concave phosphor converter layer, wherein the layer&#39;s concavity is oriented towards the LED&#39;s and the opening. White light formed as mix of reflected LED&#39;s radiation and phosphor&#39;s radiation exits via the opening, while white light formed as mix of LED&#39;s radiation passing through the layer and phosphor&#39;s radiation exits through the layer. The layer may have the form of a truncated ellipsoid of revolution, in particular a sphere, or a paraboloid, with a main axis perpendicular to the plane of the opening, or a cylinder truncated by the plane of the opening.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to electrical and electronic equipment, and more particularly to light sources based on semiconductor light-emitting diodes (LEDs), more particularly to a white light source based on LEDs with conversion photoluminescent phosphors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Solid-state lighting technology is starting to conquer the white lighting market, thanks to recent advances in the development of efficient LEDs, especially nitride (InGaN), and the highest achievable lighting efficiency of all white light sources known in the art. LED solutions are widely used in the lighting apparatus, such as linear and street illuminators wherein the illuminator is relatively large, and intensely heated LEDs can be distributed so as to facilitate the efficient removal of heat from them. The development of LED replacements for standard incandescent and halogen lamps having a small form factor and a high luminous flux, in view of significant prospects for solving the energy saving problem, is one of the most critical scientific and technical tasks; however, its solution is significantly hindered by the limited space for installing the control electronics (drivers) and a relatively small surface area for removal of heat emitted by LEDs in such lamps. White LEDs often include a blue LED coated with YAG:Ce phosphor. High-power (one watt or more) blue LEDs have an efficiency of about 30-45%, with approximately 550-700 mW allocated to unit heating from each applied watt. Furthermore, it is believed that when phosphor converts the blue light into the yellow one in white LEDs, approximately 20% of the incident light energy is spent for phosphor heating. Technical specifications indicate that blue LED radiation power loss is about 7% at the temperature of 25-125.degree. C., while power loss of white LEDs is about 20% at the same temperature. Thus, high-power white LEDs have substantial limitations on heat and luminous fluxes. 
       SUMMARY OF THE INVENTION 
       [0003]    The objective of the present invention is to provide a LED lamp with a small form factor, wherein the problems of the known technical solutions have been overcome, to replace standard lamps. 
         [0004]    The structure of any LED lamp designed to replace standard white light lamps is based on LED chips. White light results from mixing of radiation emitted from LED chips and different light colors such as blue, green and red, or blue and orange, etc. 
         [0005]    However, in recent years LED white light sources with photoluminescent phosphor converters, which radiate yellow or orange (red) light while absorbing blue or UV radiation of LED chip, are becoming the most widespread light sources.  FIG. 1  shows a diagram explaining the operating principle of a white light source of this type. 
         [0006]    The apparatus comprises a LED chip that emits primary relatively shortwave radiation and a conversion photoluminescent phosphor medium which is irradiated with the said relatively shortwave radiation and which, upon irradiation by the specified relatively shortwave radiation, is excited and emits in response a second radiation with relatively longer waves. In the particular embodiment, the monochrome blue or UV radiation emitting from the chip is converted to white light by placing the chip in organic and/or inorganic phosphors (photoluminescent phosphors) in a polymer matrix. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows a diagram explaining an operating principle of a white light source generally known in prior art; 
           [0008]      FIG. 2  shows LED white light source with photoluminescent phosphor conversion, as described in prior art; 
           [0009]      FIG. 3  shows a diagram explaining an operating principle of LED white light source with a remote phosphor wavelength converter according to the prior art; 
           [0010]      FIG. 4  shows LED white light source with a remote phosphor wavelength converter, as described in prior art; 
           [0011]      FIG. 5  shows white LED elongated illuminator with a cylindrical remote phosphor wavelength converter, as described in prior art; 
           [0012]      FIG. 6  shows LED white light source with side light radiation of the remote phosphor wavelength converter, as described in prior art; 
           [0013]      FIG. 7  shows LED white light source with a remote phosphor wavelength converter of searchlight type, as described in prior art; 
           [0014]      FIG. 8  shows a cross-section of the proposed illuminator according to a preferred embodiment of the present invention; 
           [0015]      FIG. 9  shows a schematic view of sheets of different thicknesses formed by extrusion from the suspension of particles of an experimental photoluminescent material; 
           [0016]      FIG. 10  shows another embodiment of the proposed illuminator according to the present invention;  FIG. 11  shows a detailed view of the illuminator of  FIG. 10 ; and 
           [0017]      FIG. 12  shows a front view of the illuminator according to the present invention comprising a ribbed heat radiator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 2  shows a known LED white light source with photoluminescent phosphor conversion, as described in U.S. Pat. No. 6,351,069. 
         [0019]    White light source  110  includes a nitride LED chip  112 , which, when excited, emits primary blue radiation. The chip  112  is placed on the conductive frame of the reflector cup  114  and is electrically connected to conductors  116  and  118 . Conductors  116  and  118  supply the chip  112  with electric power. The chip  112  is covered with the transparent resin layer  120  that includes conversion material for converting the wavelength of radiation  122 . Conversion material type used to form layer  120  can be selected depending on the desired spectral distribution of the secondary radiation produced by material  122 . The chip  112  and the fluorescent layer  120  are covered by a lens  124 . The lens  124  is typically made of a transparent epoxy or silicone. When using a white light source the voltage is applied to the chip  112 , wherein the primary radiation is emitted by the upper surface of the chip. The emitted primary radiation is partially absorbed by the conversion material  122  in the layer  120 . Thereafter the conversion material  122 , in response to the absorption of the primary light, emits secondary radiation, i.e. converted light having a peak with a longer wavelength. The remaining unabsorbed portion of the emitted primary radiation is transmitted through the conversion layer together with the secondary radiation. The lens  124  directs the unabsorbed primary radiation and the secondary radiation in a general direction indicated by the arrow  126  as outgoing light. Thus, the outgoing light is a complex light which is composed of the primary radiation emitted by the chip  112  and the secondary radiation emitted by the conversion layer  120 . The conversion material can also be configured so only a small portion or even all primary light remains in the device as in the case of a chip that emits primary UV light combined with one or more conversion materials emitting visible secondary light. 
         [0020]    The aforementioned apparatus known in the art, wherein a photoluminescent phosphor layer is formed on LED surface, has several disadvantages. It is difficult to achieve color uniformity when photoluminescent phosphor is in direct mechanical, optical and thermal contact with the LED surface due to significant changes in the light path length depending on the angle of radiation propagation through the photoluminescent phosphor layer. Furthermore, high temperature of the heated LED can undesirably alter the color coordinates of the photoluminescent phosphor or can lead to its degradation. 
         [0021]    In order to eliminate the specified disadvantages we propose white light sources with a wavelength converter remote from LED, which operating principle is explained in  FIG. 3 . 
         [0022]    The arrangement of the illuminator built according to this principle and described, for instance, in U.S. Pat. No. 6,600,175 (B1) is shown in  FIG. 4 . 
         [0023]    This white light source comprises a shell  207  formed by a transparent medium  211 , with an internal volume. The medium  211  can be formed by any suitable material that transmits light, such as transparent polymer or glass. The internal volume of the medium  211  comprises a light emitting diode (LED)  213  chip placed on the base  214 . First and second electrical contacts  216  and  217  are connected to radiating and back sides  218  and  219  of the LED  213  chip, respectively, and to a radiating side  218  of the LED, which is next to the first electrical contact  216  by the conductor  212 . The light transmitting medium  211  is associated with fluorescent and/or phosphorescent components, or mixtures thereof, in other words, photoluminescent phosphor medium which converts the radiation emitted by the side  218  of the LED  213  into white light. 
         [0024]    Photoluminescent phosphor is scattered in the shell  207  of the medium  211  and/or is arranged in the form of a film coating  209  on the inner wall of the shell  207  surface. Alternatively, photoluminescent phosphor can be a coating on the outer wall of the assembly (not shown) shell if the shell is used exclusively in the environment, wherein such an outer coating can be satisfactorily maintained (e.g., where it is not subject to abrasion or degradation). For example, photoluminescent phosphor can be distributed in polymer, or glass melt, from which the shell is formed to provide a homogeneous composition of the shell and ensure light output from the entire surface of the shell. 
         [0025]    Elongated white LED illuminator with remote cylindrical converter is known in the art, as described in U.S. Pat. No. 7,618,157 B1. Its arrangement is schematically shown in  FIG. 5 . The illuminator  310  comprises a linear heat sink  312 , a plurality of the LEDs  314  mounted on the heat sink  312  along the long side of the heat sink, and the light emitting dome  316  mounted on the heat sink  312  on one line with the LEDs  314 , wherein a portion  318 , half-round in section, of the dome  316  located against the LEDs  314  comprises a photoluminescent phosphor  320  which is excited by the light from the LEDs. The heat sink  312  is made of heat-conductive material, such as aluminum. The dome  316  is made of transparent material, such as glass or plastic. The photoluminescent phosphor  320  can be applied as a coating on the inner side of the dome or introduced into the coating material. The flat portions  326  without photoluminescent phosphor which are attached to the heat sink on both sides of the LEDs have internal reflective surfaces  328 , for example, aluminum coatings reflecting light which comes from the LEDs  314  to the dome portion  318 . 
         [0026]    The conversion layer can comprise photoluminescent phosphor material, quantum dot material, or a combination of such materials, and can further comprise a transparent host material, wherein phosphor material and/or quantum dot material are dispersed. 
         [0027]    It is known that the layers that contain powdered photoluminescent phosphor materials can conduct, absorb, reflect and dissipate the light incident on them. When this layer dissipates the light, it can also conduct, absorb and reflect some of the scattered light. 
         [0028]    Due to this fact a common disadvantage of said known inventions is that the radiation excited in photoluminescent phosphor grains under the influence of LED radiation as well as reflected LED radiation are inevitably partially absorbed in the photoluminescent phosphor layer and on inner elements of the apparatus, which reduces efficiency of the white light source. 
         [0029]    Yamada [1] and Narendran [2] determined the ratio of portions of the radiation propagating back and forth from the conversion layer of the photoluminescent phosphor YAG:Ce excited by blue light radiation with a wavelength of about 470 nm, which is converted into yellow wavelength range radiation. Narendran proved that in this case more than 60% of the light emitted and reflected by the conversion layer extends back to the excitation source, and a large portion of this light is lost within the LED assembly [2]. In [3] it is proved that even in the case of YAG:Ce photoluminescent phosphor with optical refractive index 1.8, which is mixed in epoxy resin with optical refractive index 1.6 at the photoluminescent phosphor density of 8 mg/cm2, which allows creating a balanced white light portions of back-directed and passed forth radiation, including blue and yellow light radiation, are 53% and 47%, respectively, while for the yellow light only such portions are 55% and 45%, respectively. 
         [0030]    Therefore, a significant gain in light flux and maximum possible efficiency of LED conversion white light sources can be achieved in all conditions being equal, by directing the radiation coming from the photoluminescent phosphor surface immediately irradiated by LED radiation to the exit aperture of the LED light source to a remote converter. 
         [0031]    Similar technical solution is proposed in U.S. Pat. No. 7,293,908 B2 in which one of the claimed embodiments of the lighting system with side light radiation coupling, which is completed according to this patent, includes a conversion layer which is remote from the LED and located on the light reflector. 
         [0032]    This apparatus is most similar to the apparatus according to the present invention and, therefore, chosen as the prototype. 
         [0033]    The operating principle of the white light source with side light radiation coupling implemented in accordance with the present patent is explained in  FIG. 6 , which shows a cross-section of one of the claimed embodiments of the lighting system with side light radiation coupling. 
         [0034]    The lighting system with side light radiation coupling comprises a LED  402 , the first reflector  404 , the second reflector  406 , an exit aperture  412 , a conversion layer  602 , an additional transparent covering layer  408 , and supporting means which support and separate the second reflector  406  from the first reflector  404 . The supporting means include a flat transparent element  502 , side supports  504  and a base  506 . The side supports  504  are, preferably, transparent or reflective. The first reflector  404  is attached to the base  506 . The second reflector  406  is attached to the flat transparent element  502 . The conversion layer  602  is located on the second reflector  406  surface and converts at least a portion of the primary radiation emitted by the active area of the LED  402  into radiation with the wavelength different from the primary radiation wavelength. 
         [0035]    For the illustrative purposes let us consider light beams  414 ,  415  and  416  which explain the operation of the lighting system with side light radiation coupling. The light beam  414  of the first color is emitted by the LED  402  active area and directed to the LED  402  output surface. The light beam  414  of the first color passes through the LED  402  output surface and is directed to a transparent covering layer  408 . The light beam  414  of the first color passes through the transparent covering layer  408  and is directed to the conversion layer  602  which converts the light beam  414  of the first color in the light beam  415  of the second color different from the first color. Light of the second color can be emitted in any direction from the point of wavelength conversion. The beam  415  of the second color is directed through the transparent covering layer  408  and, then, directed through the exit aperture  412  to the first reflector  404 . The light beam  416  of the second color is reflected by the first reflector  404  and directed to the flat transparent element  502 . The light beam  416  of the second color passes through the flat transparent element  502  and comes out the lighting system with side light radiation coupling. 
         [0036]    The disadvantage of this system is large aperture losses and loss of light at the boundaries of the supporting means and on the reflectors. 
         [0037]    An attempt to overcome these disadvantages was made in another known white light source of searchlight type described in U.S. Pat. No. 7,810,956 B2. 
         [0038]      FIG. 7  illustrating the arrangement and the operating principle of such apparatus, is a cross-section view of a searchlight lamp according to one of the embodiments of the invention claimed in the U.S. Pat. No. 7,810,956 B2. The light source  730  is placed on a fastener  734  and an additional thermal heat sink  736 . The thermal heat sink  736  can be finned, as shown in  FIG. 7 . The light emitted by the source  730  and reflected from the mirror  732  surrounding the light source  730  is radiated in an optical plate  738 . The wavelength conversion layer  742  is separated from the light source  730  and placed so the light from the source  730  can be received. The additional thermal heat sink  744  can cool the conversion layer  742 . The collecting optics  740  collimates the light. As the light source  730 , a LED which generates shortwave light, e. g. blue or ultraviolet light, can be used. The light source  730  can be placed on the additional fastener  734  and attached to the additional thermal heat sink  736 . The optical plate  738  can be formed so that it directs light to the collecting optics  740 . For example, the sides  748  can be inclined or bent so that the total internal reflection directs light to the collecting optics  740 . 
         [0039]    The disadvantage of this system is large aperture losses and loss of light at the boundaries of the optical plate with the light source, mirrors, and the conversion layer, which eliminate its efficiency. Furthermore, the light beam outgoing from the collimating optical system is rather thin, which is not acceptable when using this illuminator as a replacement for standard lamps with small form factor, which have sufficient angular aperture of emitted light beam, even if the halogen lamps are used. 
         [0040]    The proposed invention is based on the objective to provide maximum efficiency of LED white light source with a remote converter, to provide high color uniformity and rendering, as well as wide angular aperture of the light beam when form factor of the illuminator is small. 
         [0041]    An illuminator proposed in the present invention comprises a primary radiation source with one or more LEDs, a heat-removing base with flat peripheral part which the said LEDs are attached to, a conversion layer converting the primary radiation into the secondary radiation, which is placed remotely to the LEDs. The set problem is solved so that the heat-removing base for light radiation coupling has an aperture which edge is in close proximity of the LEDs being placed on the heat-removing base, and the specified surface of the conversion layer, which is irradiated by the LEDs. 
         [0042]    The disclosure of the invention is explained in  FIG. 8  which schematically shows cross-section of the proposed illuminator. 
         [0043]    The illuminator comprises a primary radiation source with one or more LEDs  1 , a heat-removing base  2  with an aperture  3  and a periphery part  4  which the said LEDs  1  are attached to, a conversion layer  5  converting primary radiation  6  into secondary radiation  7 , which has a concave surface  8  facing the LEDs  1 , and a second convex surface  9  wherein the conversion layer  5  is placed remotely to the LEDs  1 . 
         [0044]    The illuminator functions as follows: the primary radiation  6  of the LED  1  reaches the surface  8  of the conversion layer  5 , then reflects partially from the surface  8 , exiting through the aperture  3  of the heat-removing base  2 , reflects partially from the surfaces of photoluminescent phosphor grains, being dissipated in the conversion layer  5 , is absorbed partially by the conversion layer  5  material transforming into secondary radiation  7 ; concurrently, a portion of the primary radiation  6  which has reached the surface  9 , exits outside together with a part of the secondary radiation  7  produced by the conversion layer  5  forming white radiation as their mix. In this case, a certain portion of the primary radiation  6  exits the conversion layer through the aperture  3  of the illuminator and mixes with the secondary radiation  7  to form white radiation. The spectral distribution of white radiation is determined by the properties of the conversion layer materials, primarily, by their composition, the photoluminescent phosphor dispersion and the thickness of the conversion layer. Thickness adjustment of the conversion layer in range from 5 up to 500 .mu.m gives a possibility to make closer each other the color coordinates of white light going through the aperture orifice and leaving the outer surface of the conversion layer  9 . 
         [0045]    Photoluminescent phosphors are usually optical inorganic materials doped with ions of rare earth elements (lanthanides), or, alternatively, ions of the elements, such as chromium, titanium, vanadium, cobalt or neodymium. Lanthanide elements are lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Optical inorganic materials include (but are not limited to): sapphire (Al.sub.2O.sub.3), gallium arsenide (GaAs), beryllium aluminum oxide (BeAl.sub.2O.sub.4), magnesium fluoride (MgF.sub.2), indium phosphide (InP), gallium phosphide (GaP), yttrium aluminum garnet (YAG or Y.sub.3A.sub.15O.sub.12), terbium-containing garnet, yttrium-aluminum-lanthanide oxide compounds, compounds of yttrium, a lanthanide-aluminum-gallium oxides, yttrium oxide (Y.sub.2O.sub.3), calcium or strontium or barium halophosphates (Ca,Sr,Ba).sub.5(PO.sub.4).sub.3(Cl,F), the composition CeMgAl.sub.11O.sub.19, lanthanum phosphate (LaPO.sub.4), lanthanide pentaborate materials ((lanthanide) (Mg, Zn) B.sub.50.sub.10), the composition BaMgAl.sub.10O.sub.17, the composition SrGa.sub.2S.sub.4, compounds (Sr,Mg,Ca,Ba)(Ga,Al,In).sub.2S.sub.4, the composition SrS, the composition ZnS and nitride silicates. 
         [0046]    There are several typical photoluminescent phosphors which can be excited by UV radiation with a wavelength of  250  nm or close to this. A typical red-emitting photoluminescent phosphor is Y 2 O 3 :Eu +3 . A typical yellow-emitting photoluminescent phosphor is YAG:Ce +3 . Typical green-emitting photoluminescent phosphors include: CeMgAl 11 O 19 :Tb&lt; 3 +&gt;, (lanthanide) PO 4 :Ce +3 ,Tb +3  and GdMgB 5 O 10 :Ce +3 ,Tb +3 . Typical blue-emitting photoluminescent phosphors are BaMgAl 10 O 17 :Eu +2  and (Sr,Ba,Ca) 5 (PO 4 ) 3 CI:Eu +2 . For a LED of excitation with a longer wave with the wavelength range of 400-450 nm or close to it, the typical optical inorganic materials include yttrium aluminum garnet (YAG or Y 3 Al 5 O 12 ), terbium-containing garnet, yttrium oxide (Y 2 O 3 ), YVO 4 , SrGa 2 S 4 , (Sr,Mg,Ca,Ba)(Ga,Al,In) 2 S 4 , SrS, and nitride silicates. Typical photoluminescent phosphors for LED of excitation in the wavelength range of 400-450 nm include YAG:Ce +3 , YAG:Ho +3 , YAG:Pr +3 , SrGa 2 S 4 :Eu +2 , SrGa 2 S 4 :Ce +3 , SrS:Eu +2  and nitride silicates doped with Eu +2 . 
         [0047]    Quantum-dot materials are inorganic semiconductor fine particles of less than about 30 nm. Typical quantum-dot materials include (but are not limited to) particles of CdS, CdSe, ZnSe, InAs, GaAs and GaN. Quantum-dot materials can absorb light of one wavelength and then re-emit the light with different wavelengths, which depend on the particle size, particle surface properties, and the inorganic semiconductor material. 
         [0048]    The conversion layer can include both one type of photoluminescent phosphor material or quantum-dot material and a mixture of photoluminescent phosphor materials and quantum-dot materials. Using a mixture of more than one of such material is appropriate if it is desirable to have a wide spectral range of the emitted white light (high color rendering). One of the typical approaches to obtain warm white light with high color rendering index is mixing radiation of InGaN LED with the radiation of the mixture of yellow and red conversion photoluminescent phosphors. The conversion layer can include several photoluminescent phosphors that absorb the light emitted by the LED and emit light with a longer wavelength. For example, for blue LEDs, the conversion layer can include a single photoluminescent phosphor emitting yellow light, or several photoluminescent phosphors that emit red and green light. For ultraviolet LEDs, the conversion layer can include photoluminescent phosphors emitting blue and yellow light, or photoluminescent phosphors emitting blue, green, and red light. Photoluminescent phosphors can be added that emit additional colors, in order to control the color coordinates and the rendering of the mixed light outgoing from the illuminator. 
         [0049]    Transparent host materials can include polymer and inorganic materials. Polymer materials include (but are not limited to): acrylates, polycarbonate, fluoroacrylates, perfluoroacrylates, fluorophosphinate polymers, fluorosilicones, fluoropolyimides, polytetrafluoroethylene, fluorosilicones, sol-gels, epoxy resins, thermoplastics, heat-shrink plastics and silicones. Fluorine-containing polymers are especially useful in the ultraviolet ranges of wavelengths shorter than 400 nm and infrared wavelengths longer than 700 nm, due to their low absorption of light at these wavelength ranges. Typical inorganic materials include (but are not limited to): silicon dioxide, optical glasses, and chalcogenide glasses. 
         [0050]    For providing of necessary mechanical durability of the remote converter the photoluminescent phosphor of the conversion layer can conformally be applied as coating to the inner surface of an auxilary transparent carrying element, e.g., by pulverizing, pasting, deposition or electrical endosmosis from the photoluminescent phosphor suspension in the liquid. One of the problems related to coating the transparent carrying element with photoluminescent phosphor is applying a uniform reproducible coating on the carrying element, especially if the carrying element has a non-planar surface, for example, cylindrical or hemispherical. Liquid suspensions are used to apply photoluminescent phosphor particles to the substrate when the pulverizing, pasting, deposition methods are applied. The uniformity of coating greatly depends on the viscosity of the suspension, particle concentration in suspension, and environmental factors, such as ambient temperature and humidity. Coating defects due to flows in the suspension before drying, and daily changes of the coating thickness are classified as ordinary problems. 
         [0051]    The carrying elements can be manufactured from glass, transparent ceramics or transparent plastics, such as polycarbonate, PET, polypropylene, polyethylene, acrylic, shaped by extrusion. 
         [0052]    The carrying element can be manufactured from a matted material or a transparent material with the matted surface for better illumination uniformity. 
         [0053]    In some cases, it is preferable the homogeneous dispersion of the photoluminescent phosphor inside a material of the carrying element, for example, transparent plastic, such as polycarbonate, PET, polypropylene, polyethylene, acrylic, shaped by extrusion. In this case, the conversion layer can be pre-manufactured in sheets, which are then heat molded into the desired shape. 
         [0054]    The conversion layer of the conformally preformed the inner surface of the carrying element can be attached to it, for example, with a silicone adhesive located between the conversion layer and the inner surface of the carrying element. In this case, the adhesive layer can be thin, thinner, for example, than the conversion layer, and can not maintain a large thermal resistance to the heart removal from the conversion layer. 
         [0055]    In one of specific embodiments of the illuminator, a preformed sheet is used, which is pasted to a glass or polycarbonates cylindrical carrying element. An organic solvent is used to prepare the suspension of photoluminescent phosphor, surface-active agents (surfactants) and the polymer. The suspension can then be formed into a sheet by extrusion or mold casting or it can be poured on a flat substrate, for example, a glass one, followed by drying. The resulting sheet can be separated from the temporary substrate and attached to the carrying element, using a solvent or cyanoacrylate adhesive. 
         [0056]    In a specific example, sheets of different thicknesses, as shown in  FIG. 9 , were formed by extrusion from the suspension of particles of an experimental photoluminescent phosphor based on yttrium-gadolinium-cerium aluminium garnet (Y,Gd,Ce).sub.3Al.sub.5O.sub.12 in a polycarbonate solution in methylene chloride. The conversion layer must be sufficiently thick to achieve the necessary color coordinate values of mixed white light outgoing from the illuminator aperture. The effective thickness is defined based on optical scattering processes in photoluminescent phosphors used and ranges, for example, between 5 and 500 .mu.m, most often between 100 and 250 .mu.m. 
         [0057]    The sheet was secured to a cylindrical carrying element by way of moistening the carrying element inner side with isopropanol and applying pressure to the sheet using a male die of a desired shape. The solvent softens the sheet and allows squeezing out air bubbles from under it to ensure full adhesion of the sheet to the carrying element. The glass carrying element can be coated with a mixture of photoluminescent phosphor with transparent silicone binder; then, the mixture is annealed. In this case, the silicone binder is not removed by annealing. It must be borne in mind that photoluminescent phosphor, which converts blue light in the orange-red one, can degrade until complete uselessness after is has been heated to 480.degree. C. in air. In this case, other polymers with a lower burning-off temperature must be used. In some cases, the burning-off temperature ranges from 260.degree. C. to 540.degree. C. 
         [0058]    The surface of the conversion layer can be additionally coated with a transparent protective layer, which prevents the ingress of moisture and/or oxygen into the conversion layer, as some types of photoluminescent phosphors, e.g., sulfide one, are prone to damage from moisture attacks. The protective layer can be made of any transparent material which prevents moisture and/or oxygen from penetrating into the conversion layer, for example, made of inorganic materials, such as silicon dioxide, silicon nitride or aluminum oxide, and organic polymers or combinations of polymeric and inorganic layers. The preferable materials for the protective layer are silicon dioxide and silicon nitride. 
         [0059]    The protective layer can also optically clear the boundary of a photoluminescent phosphor grain with the atmosphere and can reduce the reflection of the LED primary radiation and the secondary radiation of the photoluminescent phosphor on this boundary, reducing the absorption losses of the photoluminescent phosphor self-radiation in its grains, thereby increasing the efficiency of the illuminator. 
         [0060]    The protective layer can also be applied by finishing surface treatment of photoluminescent phosphor grains, which, among other things, causes to form a nano-sized 50-100 nm film of zinc silicate on the surface of the grains, that clears the boundary of the photoluminescent phosphor grain. 
         [0061]    Where necessary, the aperture can be additionally hermetically sealed with an optically transparent window that protects the conversion layer from moisture and/or oxygen, while the internal volume of the illuminator can be filled with an inert atmosphere, or evacuated. An inert atmosphere and vacuum are optically transparent for radiation of used LEDs and photoluminophores. 
         [0062]    The said protective window can be manufactured from a matted material or a transparent material with the matted surface for better illumination uniformity. 
         [0063]    The surface  8  of the converter  5  and the surface  11  of the carrying element  10  can be shaped as axisymmetric figures (a sphere, ellipsoid, paraboloid, or otherwise), truncated by a plane of the heat-removing base  2 ; in this case, the LEDs  1  are located near and along the line of intersection of the said surface of the heat-removing base  2  with the said surface  8  of the converter  5 . 
         [0064]    The optimization of the converter thickness and surface  8  shape and of the location of the LED based on their radiation directivity diagram allows improving the color uniformity and angular distribution of the radiation coming out of the illuminator due to the incidence of the LED radiation to the converter surface  8  at different angles and redistribution of the reflected radiation inside the cavity of the converter  5  before its exit from the aperture and through the thickness of the converter  5 . 
         [0065]    As known from the specifications, say, for SemiLEDs chips of strong blue LED SL-V-B45AC2 or EZBright1000 family chips manufactured by CREE, the radiation directivity diagram of LED chips can have the Lambertian distribution (a light cone with an angle of obliquity of 90.degree. to the surface of the LED chip) or be limited to a less cone with the angle .alpha. &lt;90.degree., for example, when radiation is coupled out using a quantum-sized lattice structure formed on the surface of the LED chip. 
         [0066]    In this case, the LED can be located on the heat-removing base in such a way that the axis of the LED radiation directivity diagram intersects the axis of symmetry of the converter at an angle β≧90°−α/2. 
         [0067]    However, a certain relatively small part of the LED primary radiation propagates directly outwardly the illuminator aperture; and to avoid the possible user&#39;s direct eye contact with the LED light, the heat-conducting base  2  can comprise a protrusion  12  that screens the direct yield of the primary radiation outwards the illuminator, bypassing the surface  8  of the converter  5 , as it is shown in  FIG. 10 . To ensure a more complete utilization of the LED primary radiation, the said protrusion  12  of the heat-conducting base  2  comprises an additional reflector--a flat mirror part  13  that directs the primary radiation falling on it towards the surface  8  of the converter  5 . 
         [0068]    The illuminator in this embodiment includes an optically transparent carrying element  10 , with the inner side  11 , which contacts to the surface  9  of the converter  5 , and a protruding part  12  with a reflective coating  13  in addition to the elements shown in  FIG. 8  and numbered in the same way as in  FIG. 8 . 
         [0069]    Another specific embodiment of the illuminator with an additional reflector is shown in details in  FIG. 11 , which shows an enlarged sectional view of the illuminator in the area of the base  2  with fixed LEDs  1  where the corresponding components are numbered in the same way as in  FIG. 8  (not to scale). 
         [0070]    The additional reflector is an inclined surface  15  (for example, a truncated conical surface put the base upwards in the case of an axisymmetrically shaped converter) located between the LED chips  1  and the converter  5 , the reflection from which allows almost completely redirecting the portion of the LED chips  1  radiation falling onto it to the opposite side of the converter  5 , which homogenizes the outgoing radiation of the illuminator. 
         [0071]    In this embodiment of the illuminator, the LED chips  1  are located on the base  2  so that the normal to the surface of the LED chip  1  is parallel (or makes a small angle) to the axis of symmetry of the converter  5  made as a film applied by one of the previously mentioned methods to the inner surface of a hemispherical glass cap  17 , glued with an elastic heat-resistant compound  18  to the aluminum hemispherical contacting element  19 , which in its turn glued with an elastic heat-resistant conductive compound  20  to the inner side of heat removing ribs  21 , located between windows for light output from the outer side  9  of the converter  5 , made in a heat radiator untended for heat dissipation in the illuminator. Windows for light output from the outer side  9  of the converter  5 , are also made in the contacting element  19 , located between heat removing ribs of the heat radiator, which act together as the second common electrode for the LED chips  1  that are connected to it in parallel by means of the conductors  14  and the polyimide ribbon  16  with the metallic coating  15 . 
         [0072]    Windows for light output in the contacting element  19  and in the heat radiator, located between heat removing ribs  21  of the heat radiator are shown in the  FIG. 12 . 
         [0073]    To enhance the light reflectivity, the metallic coating  15  on the polyimide ribbon is coated with a thin aluminum layer and serves as an additional reflector in addition to being the electric contact. With this layout of the LEDs, their primary radiation does not directly enter the eye of an observer. 
         [0074]    The first electrode is the base  2 , to which the LED chips  1  are soldered, and ribs of the heat radiator that is in electrical and thermal contact with the base  2 . Electrical power is supplied to the contacting element  19  by the central cylindrical output (not shown on  FIG. 11 ) that is welded (or soldered) to the top of the contacting element  19  in axial alignment with the axis of symmetry of the converter  5  and connected through an electrically insulated hole in the inner surface of the heat radiator to the supply driver located in the corresponding cavity made in the upper part of the heat radiator body (not shown). 
         [0075]    LED chips  1  and wire contacts  14  can be sealed with the optical compound  22  using the known technology applied in the manufacture of LED assemblies. 
         [0076]    The hemispherical cap  17  can also be made of heat-conducting ceramic, or optically transparent plastic, such as polycarbonate, PET, polypropylene, polyethylene, acrylic or another similar material. 
         [0077]    The contacting element  19  can also be made of stainless steel, copper, brass, Kovar, or any similar material. 
         [0078]    The heat radiator  22  can be made of any suitable material, such as copper or aluminum. The heat radiator can be ribbed to increase the heat transfer surface, for example, as shown in  FIG. 12 . 
         [0079]    The sheets shown in  FIG. 9 , using SL-V-B35AK-type LED chips manufactured by SemiLEDs, were used to manufacture samples of white linear semicylindrical illuminators, that achieved the effectiveness of 160-200 Im/W, depending on the sheet thickness. 
       REFERENCES 
       [0080]    1. Yamada, K., Imai, Y. and Ishii K., “Optical Simulation of Light Source Devices Composed of Blue LEDs and YAG Phosphor,” J. Light &amp; Vis. Env. 27(2), 70-74 (2003). 
         [0081]    2. Narendran, N., Gu. Y., Freyssinier, J., Zhu, Y., “Extracting Phosphor-scattered Photons to Improve White LED Efficiency,” Phys. Stat. Sol. (a) 202(6), R60-R62 (2005). 
         [0082]    3. Zhu Y., N. Narendran, and Y. Gu. “Investigation of the optical properties of YAG:Ce phosphor”. Sixth International Conference on Solid State Lighting. Proceedings of SPIE. 6337, 63370S (2006).