Abstract:
The systems and methods described herein relate to solid-state light sources capable of generating radiation beams for, but not limited to, the treatment of surfaces, bulk materials, films, and coatings. The solid-state ultraviolet source optically combines the light output of at least two and preferably as many four independently controllable discrete solid-state light emitters to produce a light beam that has a controllable multi-wavelength spectrum over a wide range of wavelengths (i.e. deep UV to near-IR). Specific features of this light source permit changes in the spectral, spatial and temporal distribution of light for use in curing, surface modification and other applications.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to an apparatus and method of providing substantially improved radiation beams for the treatment of surfaces, thin films, coatings, fluids or objects. More particularly, the present invention pertains to an apparatus and method for optically combining the light output of at least two arrays of solid-state light emitters to produce a light beam that has a selected spectrum chosen for applications requiring a wide range of wavelengths to improve or accelerate a treatment process with a controllable irradiance. 
   2. Description of the Prior Art 
   Radiant energy is used in a variety of manufacturing processes to treat surfaces, films, coatings, over layers, and bulk materials. Specific processes include but are not limited to curing, fixing, polymerization, oxidation, purification, or disinfections. By way of example, the manufacture of components for motor vehicles involves the application of under coatings, paints or clear coatings on vehicle surfaces for various purposes including corrosion resistance, decoration or surface protection (e.g. scratch resistance). The coatings or paints are resins or polymer-based materials that are applied as liquids or powders and require thermal or radiant energy processing to become solids. The processing of coatings or paints by thermal methods is slow and requires times ranging from minutes to hours to complete. In addition, some materials (for example, substrates or coating components) may be heat sensitive and damaged by thermal treatments. 
   Non-thermal curing using radiant energy to polymerize or effect a desired chemical change is rapid in comparison to thermal treatment. Radiation curing can also be localized in the sense that curing can preferentially take place where the radiation is applied. Curing can also be localized within the coating or thin film to interfacial regions or in the bulk of the coating or thin film. Control of the curing process is achieved through selection of the radiation source type, physical properties (for example, spectral characteristics), temporal variation, or the curing chemistry (for example, coating composition). 
   A variety of radiation sources are used for curing, fixing, polymerization, oxidation, purification, or disinfections of a variety of targets. Examples of such sources include but are not limited to photon, electron or ion beam sources. Typical photon beam sources include but are not limited to arc lamps, incandescent lamps, electrodeless lamps and a variety of electronic (that is lasers) and solid-state sources (that is solid state lasers, light-emitting diodes and diode lasers). Selection of a specific radiation source for an application is contingent on the requirements of the treatment process and the characteristics of the radiation source. These characteristics are related to but are not limited to the physical properties of the source, its efficiency, economics, or characteristics of the treatment process or target. For example, arc lamps or radio-frequency or microwave driven “electrodeless” ultra-violet sources efficiently produce high levels of radiated power having applications in many “industrial” processes where rapid treatment using significant levels of irradiance or energy density over large areas are needed. Arc or electrodeless lamps require high voltage, microwave or radio frequency power supplies and in the case of microwave-driven systems, a microwave tube (that is a magnetron). These high-powered lamps also require cooling and heat rejection systems. Such operational requirements limit the application of such photon sources to situations were this need can be met. 
   The spectral emissions of arc and electrodeless lamps are controlled by the conditions under which the lamp is operated, the particular gases used to fill the bulb and the selection of various additives placed in the bulb. Those skilled-in-the-art formulate specific lamp fills meeting curing needs for many photochemical processes, but gaps exist in spectral coverage in certain spectral ranges. 
   Solid-state light sources, such as, but not limited to, light emitting diodes (LEDs), diode lasers, diode pumped lasers and flash lamp-pumped solid-state lasers provide emission sources that can tuned to the needed wavelength or can be combined as arrays to provide a multi-wavelength source for applications needing broadband source. Advances in solid-state source technology provide high-brightness ultraviolet LEDs suitable as sources for radiation treatment. 
   At the present time, commercial UV emitting diodes emitting radiation down to an output of 370 nm. are available from Nichia, Cree, Agilent, Toyoda Gosei, Toshiba, Lumileds and Uniroyal Optoelectonics (Norlux). 
   UV emitting LEDs and laser diodes are constructed using large band gap host materials. InGaN based materials can be used in LEDs emitting at peak wavelengths ranging from 370 to 520 nm (for example, from the ultra-violet (UV-A) to visible green). The band gap of GaN is 3.39 eV and can accommodate luminescent transitions as large as 363 nm. The substitution of In into the GaN host provides localized states that can radiate in the ultraviolet down to 370 nm. 
   Other nitride materials such as InAlGaN can emit ultraviolet radiation in wavelengths as short as 315 nm. InAlGaN is already being used to make high brightness LEDs and laser diodes that operate in the range of 315 to 370 nm. Hirayama et al (Appl. Phys. Lett. 80,207 (2002)) reports devices employing layered structures of InxGa1-xN or quaternary InxAlyGa1-x-yN grown on AlxGa1-xN (x=0.12-0.4) have been used in multiple quantum well structures to produce sources emitting comparable flux at 330 nm to InGaN devices operating at 415-430 nm. Hirayama et al. (Hirayama et al, Appl. Phys. Lett, 80, 1589 (2002)) has also reported a room temperature LED source using an improved multiple quantum well (MQW) structure and InAlGaN materials which emits intense UV radiation at 320 nm and significant emission at 300 nm. 
   Hirayama et al. (Appl. Phys. Lett. 80, 37 (2002)) report that AlxGa1-xN(AlN)/AlyGa1-yN MQWs exhibit efficient photoluminescence between 230 to 280 nm and that the photoluminescence is as high as that of the InGaN-based materials used in the violet diodes now commercially available. AIN-based materials are likely candidates for making ultraviolet LEDs operating in the UV-B or UV-C ranges. Other researchers are studying carbide and diamond materials as hosts for deep-UV based on the fact that their band gaps are as large as AlN. 
   LEDs operating in the blue, violet and UV-A (390 nm) wavelengths are of sufficient radiance to be used in ultraviolet and photochemical curing as “spot” curing sources. U.S. Pat. No. 6,331,111B1 (Cao) and EP 0-780-104 (Breuer et al) describe hand held portable spot curing light systems using solid state light sources consisting of light emitting diodes or diode laser chips. The light source of Cao may contain sources that emit multiple wavelengths so that numerous components in materials whose photo initiators are sensitive to different wavelengths may be cured at once. In the preferred embodiment described in Cao, the light travels directly to the curing surface without going through an optical device like a light guide or optical fiber. Breuer et al. describe a similar device optimized to cure dental resins and also extend claims to apparatus where the irradiator is a stationary curing apparatus whose light source chips are fixed to the walls of the curing chamber. 
   Various light sources have been used for the purposes of curing composite materials. These include plasma, halogen, fluorescent, and arc lamps. Various lasers have been incorporated in curing apparatus. Lasers emitting ultraviolet beams include frequency doubled or re-doubled sources like the 266 nm Nd—YAG systems, argon-ion systems and Nd—YAG pumped OPOs (optical parametric oscillators). Cao cites U.S. Pat. Nos. 5,420,768, 5,395,769, 5,890,794 and 5,161,879 where LEDs have been employed as curing light sources. The application of solid state sources to the curing process are also described in U.S. Pat. Nos. 6,127,447 and 5,169,675. 
   Technology necessary for the application of solid-state sources in the treatment process can be found in the development of LED and laser diode equipped systems for illumination and solid-state displays. These systems include an apparatus for LED illumination that can be incorporated into a hand-held lamp, are battery powered and equipped with electronics that provide pulsed power to control lamp radiance and compensate for the decrease in battery voltage during battery discharge. Published U.S. Patent Application 2002/0017844 A1 teaches the use of optical systems to modify the field of view for LED emitters in displays where the field-of-view is restricted. 
   There are many examples in the prior art of the use of LEDs in arrays to synthesize multi wavelength emissions. U.S. Published Patent Application No. 2001/0032985 A1 teaches the installation of arrays of colored LEDs on a chip to make multicolored or white solid-state illumination sources. U.S. Pat. Nos. 6,016,038 and 6,150,774 disclose the method and electronics needed to generate complex, predesigned patterns of light in any environment. The use of computer controlled LED arrays to provide light sources capable of rapid changes in illumination and spectral selection are detailed in U.S. Pat. No. 6,211,626, which describes a system using sub-arrays of primary colored (red, green and blue) LEDs whose individual elements are addressable and which can be controlled by pulse modulation to emit varying amounts of light to synthesize a third color. U.S. Pat. No. 6,211,626 indicates that such computer-controlled arrays of light emitters are not new but that previous systems had limitations, which reduced the flexibility or efficiency of the illumination system. The use of computer control for lighting networks used in illumination is described in U.S. Pat. Nos. 5,420,482, 4,845,481 and 5,184,114. 
   U.S. Published Patent Application No 2002/0191394 teaches the use of a diffractive optical element (diffraction grating) for mixing light from monochromatic light sources like LEDs and making multicolor or white beams. The monochromatic light sources are positioned relative to the grating where light of that frequency is found in the diffracted order beams higher than the zeroth order. The mixed beam is the zeroth order beam. A white beam will be provided if sufficient frequencies are represented in the first and higher order beams being directed on the grating. Fraunhoffer diffraction is used to mix the monochromatic beams. This is different from the use of Fresnel Zone plates to accomplish the coupling of the multiple radiation sources 
   SUMMARY OF THE INVENTION 
   The present invention provides a solid-state light source and method which optically combines (mixes) the light output of at least two and preferably additional independently controllable discrete solid-state light emitter arrays to produce a light beam that has a selected multi-wavelength spectrum over a wide range of wavelengths such as from deep UV to near-IR to provide irradiance of a target surface with a controlled power level. Optical mixers combine light spectrums which are provided from the light emitter arrays to produce the controllable multi-wavelength spectrum. 
   Specific features of this light source permit changes in the spectral, spatial and temporal distribution of light for use in curing, surface modification and other applications. 
   This light source can be adjusted to precisely match the physical characteristics of the applied light to the chemical properties of materials to provide a means to improve the process at both nanometer and greater length scales by:
         (1) optimizing the degree and rate of cross-linking of polymeric materials;   (2) selecting specific cross-link bonding in polymers;   (3) matching light source characteristics to specific photo-initiators;   (4) controlling the distribution, penetration or rate of light energy deposition in materials to create new morphologies; and   (5) optimizing light source characteristics for surface processing.       

   A preferred embodiment of the invention comprises at least two solid state light emitting arrays which preferably are LED arrays, each of which has a characteristic emitting frequency (wavelength), an optical mixer to mix the radiation from the LED arrays, a reflector to concentrate radiation from the arrays and to provide a two-dimensional energy distribution on the target surface to be treated which is optionally substantially uniform. An optional cooling system may be provided to provide high stability of the spectral output and to improve lifetime of arrays. 
   The invention increases the flexibility of the photochemical processes (especially, but not limited to, UV-curing of inks and the like, plastic, thermal paper, liquid crystal and the like) by either optimizing existing ultraviolet treatment processes and outcomes, or creating entirely new treatment processes or outcomes. The invention performs these tasks by providing a light source whose spectral emissions can be varied to provide changes in the ultraviolet light such as to changes in the brightness, chromaticity, calorimetric purity, hue, saturation and lightness of visible light. Modification of physical characteristics of light provides configuration of a light source to make the best use of the physical and chemical properties of curable materials. 
   Other problems which the invention overcomes include curing applications where the use of technology normally included in light sources cannot be used for technical, process or economic reasons. This includes but is not limited to:
     (1) high voltage cabling, electronics and power supplies;   (2) RF or microwave cabling, wave guides, electronics and power supplies;   (3) gaseous electronic components including electrode and electrode-less bulbs   (4) high power electronics and the needed heat dissipation systems.   

   The invention also provides a solution to the problem of unwanted light emissions such as infrared from curing sources. Ultra-violet solid-state light emitter arrays generate little or no emissions in the infrared. If infrared radiation is needed in the curing process, infrared emitters of the desired wavelength and energy can be configured into the solid state UV generating arrays providing the selected wavelengths which are included in the curing light system to provide the desired missed spectrum. 
   The frequency spectrum of the individual light emitting arrays is chosen either (1) to provide a composite frequency made up of the mixed spectrum from the individual arrays required for the desired application, or (2) each array provides the identical common frequency spectrum to increase the power level of irradiance of the common frequency spectrum. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a side elevational view of a first embodiment of the present invention;  FIG. 1B  is a perspective view of the first embodiment of the present invention; and  FIG. 1C  is an example of a light emitting array which may be used with the practice of the various embodiments of the invention. 
       FIG. 2  is the spectral distribution of irradiance of one of the LED arrays in  FIG. 1  showing ultraviolet radiation source emission near 390 nanometers. 
       FIG. 3  is a spectral distribution of irradiance of the other LED array in  FIG. 1  showing an ultraviolet radiation emission near 410 nanometers. 
       FIG. 4  is a spectral distribution of irradiance produced by optical mixing of the individual spectral radiance distribution of the LED arrays of the embodiment of  FIG. 1  with the spectrums illustrated in  FIGS. 2 and 3 . 
       FIG. 5  is a spectral distribution of irradiance of the individual LED arrays showing the individual components of  FIGS. 2 and 3  and the optical mixing thereof as illustrated in  FIG. 4 . 
       FIG. 6  is a comparison between measured and simulated spectral irradiance showing simulated spectral irradiance as a line and a measured spectral irradiance as diamonds regarding the embodiment of  FIG. 1 . 
       FIG. 7  is a spectral distribution of irradiance of the embodiment of  FIG. 1  operated with the 390 nm LED array operated with a bias voltage of 47 volts and the 405 nm LED array operated at a bias voltage 0, 34, 35, and 38 volts respectively. 
       FIG. 8  shows an embodiment of the invention using two solid state light emitting arrays as illustrated in  FIG. 1 , including a housing with an interior elliptical reflector installed in a lamp enclosure providing cooling to the solid state light emitting arrays and a controller for controlling the power and spectral output produced by the individual solid state light emitting arrays. 
       FIG. 9  is a perspective view of a second embodiment of the invention utilizing three solid state light emitting arrays, three semitransparent mirrors functioning as optical mixers and an interior cylindrical reflector. 
       FIGS. 10A and 10B  show a simulated flux distribution of the second embodiment of  FIG. 9 . 
       FIG. 11  illustrates a perspective view of a third embodiment of the invention in which four solid state light emitting arrays are placed between four semitransparent mirrors functioning as optical mixing elements and an interior cylindrical reflector. 
       FIG. 12  is a perspective view of a fourth embodiment of the invention with four solid state light emitting arrays placed on the edges of four optical mixers and a cylindrical interior reflector. 
       FIG. 13  is a perspective view of a fifth embodiment of the invention with six solid state light emitting arrays placed between six optical mixing elements. 
       FIG. 14  is a perspective view of a sixth embodiment of the invention with six solid state light emitting arrays placed on edges of six optical mixing elements. 
       FIG. 15  is a perspective view of an seventh embodiment of the invention with six solid state light emitting arrays arranged facing eight optical mixers arranged in an octahedron. 
       FIG. 16  is a perspective view of an eighth embodiment of the invention with six solid state light emitting arrays facing eight optical mixers. 
       FIG. 17  is a perspective view of a ninth embodiment of the invention of eight solid state light emitting arrays facing four optical mixers arranged to face a structure with tetrahedral symmetry. 
       FIG. 18  is a perspective view of an tenth embodiment of the invention with four solid state light emitting arrays spaced between four intersecting optical mixing elements contained in an internal ellipsoidal reflector. 
       FIG. 19  shows the flux distribution of the ninth embodiment of  FIG. 18 . 
       FIG. 20  is a perspective view of an eleventh embodiment of the invention with two elongated solid state light emitting arrays facing an optical mixer contained in an internally reflective elliptical reflector. 
       FIG. 21  is a perspective view of a twelfth embodiment of the invention with two elongated solid state light emitting arrays facing a prismatic mixing device contained in an elliptical reflector. 
       FIG. 22  is a perspective view of a thirteenth embodiment of the invention with two elongated solid state light emitting arrays facing one optical mixing element contained in a reflector with an elliptical cross section. 
   

   Like reference numerals identify like parts throughout the drawings. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is a solid state light source and a method of irradiating a target surface with a solid state light source which utilizes solid state light emitting arrays each preferably comprising a plurality of light emitting diodes (LEDs) which are mounted on a flat surface. Each diode emits light away from one side of the flat surface toward a target surface with at least one wavelength which is chosen to satisfy the desired application. At least one optical mixer is provided with each mixer mixing the output of a pair of solid state light emitting arrays. Each optical mixer is positioned symmetrically with respect to a pair of light emitting solid state arrays. Each optical mixer reflects part of the light output from a symmetrically disposed diode array and transmits part of the light output from another symmetrically disposed light array to provide a composite mixed light spectrum to irradiate the target surface with mixed light which has a selected frequency spectrum with the irradiance level of the spectrum being controllable by a variable control parameter such as voltage, but it should be understood that the invention is not limited thereto. The at least one optical mixer may be designed to substantially split (50-50) the light incident thereon from each array into a part which is reflected and a part which is transmitted through the optical mixer. With respect to the portion of the light which is transmitted through the optical mixer from the first light emitting array, the light incident on an opposite surface of the optical mixer from the other light emitting array which is reflected is optically mixed with the portion transmitted through the optical mixer from the first light emitting array. A composite wave front comprised of the mixed components of light from each of the symmetrically disposed solid state light emitting arrays is transmitted toward the irradiated target surface. As is described below, a controller controls the power applied to the light emitting arrays to control the irradiance which is incident on the target surface. Each light emitting array may have a substantially similar frequency spectrum or have a different frequency spectrum. 
   When the frequency spectrums of the symmetrically disposed light emitting arrays are different, the overall frequency of the irradiance on the target surface is a summation of the individual frequency spectrum output by the individual light emitting arrays. In each of the embodiments of the invention, mixing is produced by one or more optical mixers which may be a partially reflective and partially transmissive mirrors which may transmit and reflect substantially equal parts or transmit and reflect unequal parts or a prism which is irradiated by the light from the individual solid state light arrays to provide mixing thereof. 
     FIGS. 1A and 1B  illustrate a first embodiment  10  of the present invention.  FIG. 1A  is an elevational view with a section taken through a curved internally reflective housing  21 ;  FIG. 1B  is a perspective view of the embodiment  10 ; and  FIG. 1C  is an illustration of a suitable light emitting array which may be used with the practice of the invention. 
   The first embodiment  10  is illustrative of a basic solid state light source in accordance with the present invention. Each of light emitting arrays  12 A and  12 B may be manufactured in accordance with any well-known technique. The surface  14  of each of the pair of symmetrically disposed solid state light emitting arrays which, in a preferred embodiment, are LEDs output light rays  16  which pass directly to the target surface  18 . Other light rays  17  produce a combined irradiance produced by optical mixing element  20  on which the rays  17  are incident thereon. As may be seen in  FIG. 1A , the interior of housing  21  has an internally reflective surface  22  which functions to reflect any light output from either of the light emitting arrays  12 A or  12 B toward the target surface  18  to provide controlled irradiance which, in a preferred embodiment, is preferably substantially uniform thereon as described below in conjunction with  FIGS. 2-6  in view of the curvature of surface  22  being elliptical with the optical mixer being near the focal axis of the elliptical curvature. Light rays  16 ,  17  and  19 , which are output from the light emitting arrays  12 A and  12 B and do not pass through the optical mixer  20 , are shown as solid lines and light rays  22  passing through the optical mixer  20 , from either of the light emitting arrays  12 A or  12 B are shown as dotted rays. Parallel solid line rays  19  and dotted rays  22  symbolize the net mixing performed by the optical mixer  20  for the rays emitted from the surfaces of the light emitting arrays  12 A and  12 B which partially transmits and partially reflects the light emitted from the pair of light emitting arrays. The degree of reflection and transmission may be varied from an equal splitting. 
   The housing  21 , while preferably having an elliptical cross section, may utilize other curved cross sections which facilitates converging divergent light rays produced by the solid state light arrays  12 A and  12 B being directed toward the target surface  18  as indicated by arrows  24 . 
   The pair of light emitting arrays  12 A and  12 B are illustrated as square flat panels. The light emitting arrays are comprised of a plurality of devices, such as LEDs, which emit radiation in the ultraviolet range but the invention is not limited thereto with a suitable construction being described below in conjunction with  FIG. 1C . For example, in a high power irradiation apparatus in accordance with the embodiment  10  of  FIGS. 1A and 1B , the arrays  12 A and  12 B may respectively be an array of 40 LEDs as described in  FIG. 1C  which individually emit at 400 mW at 405 nm mounted on an integrated circuit of approximately 1 square cm. The other radiation source  12 B may, without limitation, be an array of 40 LEDs as described below emitting 100 mW at 390 nm mounted on an integrated circuit of approximately 1 square cm. Additionally, the optical mixing element  20  may be semi-reflective mirror which substantially equally splits the emission from the rays  16  into reflected rays  19  and transmitted rays  22  which are mixed as indicated by the aforementioned parallel solid and dotted lines  19  and  22  such that the rays are superimposed onto each other. A semi-reflective mirror, which may be utilized as the optical mixer  20 , may be a UV transmitting quartz plate that is coated with a thin chromium film that reflects and transmits approximately 50% of the incident light. The light emitting diode arrays  12 A and  12 B are symmetrically positioned with respect to the optical mixer  20  such that virtual images of radiation sources are superimposed to create in a preferred embodiment a mixed light source comprising substantially equal amounts of light from each of the light emitting arrays. 
     FIG. 1C  illustrates a suitable construction for the light emitting solid state arrays  12 A and  12 B with a scale of approximately 5:1 for the first embodiment as described above and in the embodiments as described below. The array  60  is comprised of 40 LEDs  62 . A lower bus bar  64  has a group of 8 LEDs mounted thereon. Each of the LEDs  62  mounted on the lower bus bar  64  are in turn coupled by a wire  66  by means of wire bonds  68  which connect the wire extending from the individual LEDs to four upper bus bars  64  on which 4 LEDs are mounted. A lens  70  focuses light emitted by the individual LEDs  62  toward the optical mixer  20 . A thermal sensor  72  is utilized to provide temperature control for the LED array  60 . The LED array  60  is mounted on a hexagonal substrate  74 . Electrical terminals  76  are mounted on the hexagonal substrate  74  to provide suitable electrical contacts for electrical power of the array. 
   The light source represented by the light emitting solid state arrays  12 A and  12 B and the optical mixer  20  is positioned approximately at the focus of the elliptical reflector  22  which is preferably substantially one-half of an ellipse. However, the reflector  22  may be more or less than one-half of an ellipse if desired and may be a non-elliptical surface. Since the reflector  22  is part of an ellipse, the reflector  22  has a major axis, a minor axis, a first focal axis within the reflector, and a second focal axis outside the reflector. The light source comprised of the aforementioned light emitting and optical mixer is preferably positioned on the first focal axis. Light beams from the arrays of diodes  12 A and  12 B are transmitted and reflected by the optical mixer  20  and strike the elliptical reflector  22  that directs the light beams to the second focal axis of the elliptical reflector  22  proximate to the target surface  18 . The target surface  18  is placed substantially at the second focal axis where the light beams are directed to strike the irradiated surface thereof. The location of the target surface  18  at the second focal axis maximizes the irradiance at the second focal axis. The irradiated surface  60  can also be placed beyond the second focal axis such as at the far field to increase the area which is irradiated. 
     FIGS. 2-6  illustrate the optical performance of the radiation on the target surface  18  using the first embodiment  10 . The spectral readings were obtained using an integrated sphere and a spectral radiometer (Ocean Optics model S2000) based on techniques well-known in the field of illumination. The radiation sources were 40 light emitting diodes which are high flux density solid state modules manufactured by Norlux Monochromatic Hex (NHX) emitting either ultraviolet UV-A at a peak emission at 390 nm or ultraviolet UV-B at 405 nm with a peak emission at 410 nm. The LED arrays  12 A and  12 B were independently connected to DC power supplies operated at a constant voltage mode. A forward bias voltage turned the diodes on to produce the UV spectra of  FIGS. 2-6 . 
     FIGS. 2 and 3  show the spectral irradiance of the source  12 A which is a UV-A emitter and the source  12 B which is a UV-B emitter. Radiation source  12 A was operated at forward bias of 15.6 volts and a current of 200 nA. Array  12 A emitted UV-A ultra-violet radiation that peaked at 395 nm and extended from 385 to 405 nm (Full-Width-at-Half-Maximum) (FWHM). Diode array  12 B was operated at a forward bias of 19 volts and a current of 200 mA to produce UV-B ultraviolet radiation that peaked at 410 nm and extended from 400 to 418 nm (FWHM). 
     FIG. 4  shows a measured spectral radiance of embodiment  10  when both radiation sources  12 A and  12 B were operated simultaneously. The composite spectrum peaked at 410 nm and extends from 392 to 418 nm FWHM. The LED array  12 A was operated at 15.6 volt forward bias, whereas the LED array  12 B was operated at 17.5 volts forward bias. The spectrum is a composite of the summed emission from the two LED arrays  12 A and  12 B. 
     FIG. 5  illustrates the simulated spectrum produced by the summation of the individual emission spectra of the diode arrays  12 A and  12 B illustrated in  FIGS. 2 and 3 . 
     FIG. 6  is a comparison of the simulated and measured spectra of the embodiment  10 . The measured spectra are identified by diamonds and simulated spectra are identified by lines. The measured spectrum matched a simulated spectrum over the entire range of emission from the light emitting arrays  12 A and  12 B and shows an excellent mixing of the beams from the two radiation sources. 
   The power levels of the light from the light emitting arrays  12 A and  12 B are controlled by varying the electrical bias applied thereto which changes the forward bias current of the diodes. The variation of voltage or another electrical parameter of the individual light emitting arrays  12 A and  12 B permits the variation of the spectral characteristic of the mixed light by choosing the magnitude and frequency of the spectra that are mixed by the optical mixer  20 . 
     FIG. 7  shows how the spectral composition of a beam from the embodiment  10  can be changed continuously from (1) a spectrum  90  representing the wavelengths from the diode array  12 A, (2) a spectrum  92  with equivalent contributions from the diode arrays  12 A and  12 B, (3) a spectrum  94  with an increased spectrum from the array  12 B, and (4) finally to a spectrum  96  with the dominant contribution from the array  12 B. This demonstrates an important function of the embodiments of the invention including the representation of the spectral composition of  FIG. 1  which permits generation of a spectrum with variable ultraviolet spectral weight. 
     FIG. 8  illustrates a system  120  incorporating the embodiment  10  of  FIG. 1  into a lamp housing  130  which is equipped with a cooling system for the LED arrays  12 A and  12 B. The air cooling system may be by forced air utilizing one or more fans inducting air into the housing and blown past the interior curved reflector  21 . As may be seen, pathways exist for the ingress and egress of cooling air. A controller  170  is coupled via connection  172  to the solid state light source. The curved reflector  21  is mounted in the lamp housing  130  with the reflector being attached to a base of the lamp enclosure that has a rectangular opening  180  from which light rays  182  pass to the target surface  18 . The LED arrays  12 A and  12 B are air cooled by two fans  162  which push air into the lamp enclosure  130 . A slot  190  is cut into the curved reflective surface  21  to permit air to be pushed into the lamp enclosure  192  to allow the air to impinge on heat sinks  194  of the LED arrays  12 A and  12 B which are attached thereto. The fans  162  may be powered from a 12 volt power supply. The LED arrays  12 A and  12 B will suffer a loss of light emitting power if a surface temperature of the substrate to which the LEDs  12 A and  12 B are attached exceeds 40° C. with current commercially available products. The power to the diode arrays  12 A and  12 B and the speed of the fans  162  is adjusted to keep the LED chip surfaces below the maximum temperature, such as 40° C. The controller  170  may be digitally controlled which permits programming of the voltage to be applied to each of the diode arrays  12 A and  12 B in order to produce a variation in the summed output radiation as reflected, for example by the curves  90 - 96  in  FIG. 7  once the frequency spectra is determined by the choice of the individual solid state light emitting elements of the array. 
     FIG. 9  illustrates a third embodiment  230  of a solid state light source in accordance with the invention which is comprised of three LED arrays  232 A,  232 B and  232 C and three optical mixers  250  which intersect at a central point  252  within cylindrical reflector  254 . The three LED arrays  232 A,  232 B, and  232 C produce spectra which are mixed by the symmetrically disposed optical mixer  250  located therebetween. The aforementioned LED arrays and symmetrically positioned optical mixtures  250  perform the same function as described above with respect to the first embodiment  10  of  FIG. 1 . The individual optical mixers  250  which intersect at central point  252  have an occluded angle of 120° between the adjacent optical mixers. The optical mixers  250  preferably are semi-reflective mirrors which split the emission substantially equally from the LED arrays  232 A,  232 B and  232 C into three transmitted and reflected beams of substantially equal intensity which are superimposed onto each other as indicated in  FIG. 1  by the superimposed light rays  19  and  22 . However, this embodiment may use optical mixers which do not transmit and reflect equal parts. The three optical mixers  250  are symmetrical when rotated through an angle of 120°. 
     FIGS. 10A and 10B  show the results of ray tracing simulations to predict the irradiance distribution  272  in the XZ plane as illustrated in  FIGS. 10A and 10B  for the second embodiment  230 . The radiance profiles for traces parallel and perpendicular to the X or Z axis through the center of the irradiance distribution show small asymmetry  272 . The asymmetry is a consequence of a lack of symmetry of the embodiment  230  to rotations 90° along an axis perpendicular to the XZ plane through the center of the embodiment  230 . 
     FIGS. 11 and 12  respectively show a third and fourth embodiment  360  and  400 . The designs respectively differ in the placement of the four LED arrays  232 A- 232 D arrays relative to the intersection  362  of the placement of the optical mixers  350  so that the diode arrays  332 A- 332 D are positioned between the edges  352  in  FIG. 11  and face the edges  350  in  FIG. 12 . In the third embodiment  340 , the LED arrays  332 A- 332 D face the point of intersection  362  while in the fourth embodiment  370 , the light emitting arrays  332 A- 332 D face the edges  352  of the optical mixers  350 . In the third and fourth embodiments, a cylindrical internally reflective housing  360  contains the LED arrays  332 A- 332 D and the four optical mixers  350  centrally disposed relative thereto which are joined together at central location  362  to form a cross. In the fourth embodiment  370  a solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. It should be understood that the connections to a suitable controller and cooling system for the light emitting arrays, such as illustrated in  FIG. 8 , are not illustrated for purposes of simplifying the illustration. 
     FIGS. 13 and 14  show fifth and sixth embodiments  400  and  420  respectively of the invention which have been simplified to only show the LED arrays emitted. The internally reflective curved housing has been omitted along with the controller of the individual LED arrays which is used to produce a controlled application of power to the individual LED arrays to produce a variable spectrum as discussed above. The embodiment  400  of  FIG. 13  has three pairs of LED arrays  432 A and  432 B which are symmetrically disposed relative to optical mixers  440 . Pairs of LED arrays  432 A and  432 B work in concert with their centrally disclosed optical mixer  440  to provide the same function as described above with respect to the first embodiment  10  to produce a controlled mixing of the light emitted from the surface of the pairs of the LED arrays. The difference between the embodiments  400  and  420  resides in the respective placement of the pairs of LED arrays  432 A and  432 B relative to the optical mixers  440 . In the embodiment of  400 , the pairs  432 A and  432 B face the point of intersection  442  of the optical mixers  440  and in the embodiment  420 , the pairs  432 A and  432 B face the edges  444  of the optical mixers  440 . The six optical mixers  440  are joined together at a central location  442  which is centrally disposed relative to the faces of the LED arrays  432 A and  432 B. The light from the three pairs of LED arrays  432 A and  432 B are combined by transmission and reflection of the six optical mixers  440  in accordance with the principal operation described above. While not illustrated, the embodiments  400  and  420  of  FIGS. 13 and 14  may be placed inside of a cylindrical internally reflective housing of the type illustrated in  FIGS. 1 ,  9 ,  10  and  11  so as to cause light to be transmitted toward a target surface  18 . Additionally, a controller and a cooling system, such as that described above with respect to  FIG. 8 , may be utilized to control the emission of light from the LED arrays. The six optical mixers  440  in the embodiments  400  and  420  form a cross at a point of intersection  442  and preferably have the characteristic of reflecting and transmitting substantially equal intensity light. A solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. 
     FIG. 15  shows a seventh embodiment  500  having three pairs of light emitting diode arrays  532 A and  532 B which are symmetrically disposed about eight optical mixers  550  which are triangular semi-transparent mirrors which function to split the irradiation sources  532 A and  532 B into transmitted and reflected beams of substantially equal intensity which are superimposed onto each other in accordance with the mixing function as described above with respect to the first embodiment of  FIG. 1 . The LED arrays  532 A and  532 B are placed at the vertices placed at the edges of the optical mixers  550 . It should be noted that the curved internally reflective housing, controller and target surface have been omitted from the embodiment of  FIG. 15 . A solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. 
   The eighth embodiment  560  of  FIG. 16  utilizes three pairs of LED arrays  532 A and  532 B which are positioned at the vertices of twelve optical mixers  550  which are partially reflective mirrors. Mixing of light from pairs of LED arrays  532 A and  532 B occurs in the manner described above. A solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. 
     FIG. 17  illustrates a ninth embodiment  600  having four pairs of LED arrays  632 A and  632 B which face four optical mixers  650  configured in a structure with tetrahedral symmetry. It should be understood that the connections to a suitable controller and cooling system for the LED arrays, such as illustrated in  FIG. 8 , are not illustrated for purposes of simplifying the illustration. A solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. 
     FIG. 18  shows a tenth embodiment  700  of the present invention having a configuration of four LED arrays  332 A,  332 B,  332 C and  332 D symmetrically disposed about four optical mixers  350  in a configuration similar to  FIG. 11  except that an ellipsoidal reflector  740  is provided as the housing. The ellipsoid  740  has a major access, which is also the axis of rotation of the ellipse that sweeps out the surface of the ellipsoid, a minor axis, a first focus within the ellipsoid and a second focus outside the ellipsoid which are not illustrated. The LED radiation source is positioned on the major axis of the ellipsoid reflector  740  at the first focus. Since the irradiation source is extended, the image of the irradiation source will not be brought into sharp focus. As described above with respect to other embodiments, the internally reflective curved cylindrical housing, controller and cooling system have been omitted. A solid line indicates light rays which are visible to the viewer and a dotted line indicates rays which are occluded from direct view. 
     FIG. 19  shows the simulated irradiance of the embodiment  700  of  FIG. 18  on the irradiated surface  18 . The radiance pattern of the beam shows a ring-like pattern near the peak irradiance. This pattern is due to the placement of the radiation sources  332 A- 332 D in a circle about the optical mixers  350 . As described above with respect to other embodiments, the internally reflective curved cylindrical housing, controller, cooling system and target surface have been emitted. 
     FIGS. 20 and 21  show eleventh and twelfth embodiments  800  and  900  of the present invention that utilize elongated linear arrays of diodes  12 A′ and  12 B′ with the embodiment  800  having elongated optical mixer  20 ′ which is a semitransparent mirror and the embodiment  900  utilizing an optical mixer  902  which is a prism for splitting and mixing beams from the arrays  12 A′ and  12 B′ using internal reflection rather than reflection from a mirror. As described above with respect to other embodiments, the internally reflective curved cylindrical housing, controller and cooling system have been emitted. 
     FIG. 22  shows a twelfth embodiment  1000  which is similar to the embodiment  800  of  FIG. 20  regarding the configuration of the elongated light emitting diode arrays  12 A′ and  12 B′ and the elongated optical mixer  20 ′. The embodiment  1000  differs with regard to the curved internally reflective housing  1002  which is an elliptical reflector with a side reflector as an ellipse with semi-major and semi-minor axis being parallel and perpendicular to the optical mixer  20 ′ or a prism such as  902  used in the embodiment  900  of  FIG. 21  and replacement thereof. The side reflector  1004  is attached to an elliptical plate  1006  to form an elliptical housing. As described above with respect to other embodiments, the internally reflective curved cylindrical housing, controller, cooling system and target surface have been emitted. 
   While the invention has been described in terms of its preferred embodiments, it is intended that numerous modifications can be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. It is intended that all such modifications fall within the scope of the appended claims.