Patent Publication Number: US-2003235800-A1

Title: LED curing light

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to a light transmission system for curing instruments. More particularly, the invention relates to a light transmission system comprising an array of light emitting diodes (LEDs) optically coupled to a light guide arranged as a bundle of drawn optical fibers having a wide diameter at a light receiving end and a narrowed diameter at a light emitting end.  
       BACKGROUND OF THE INVENTION  
       [0002] Dental composites employ well-known materials, and are used in a variety of dental procedures including restoration work and teeth filling after root canal procedures and other procedures requiring drilling. Several well-known dental composites have been sold, for example, under the trade names of BRILLIANT LINE, Z-100, TPH, CHARISMA and HERCULITE &amp; BRODIGY.  
       [0003] These composites are typically formed from liquid and powder components that are mixed together to form a paste. The paste is formed to have a consistency sufficiently workable and self-supporting to be applied to an opening or cavity in a tooth. The liquid component may comprise phosphoric acid and water, while the powder component may comprise ceramic materials such as cordite, silica or silicium oxide. After the composite is applied to a tooth, it must be cured to form a permanent bond with the tooth.  
       [0004] Curing requires the liquid component to evaporate, causing the composite to harden. In the past, curing has been accomplished by air drying, which has had the disadvantage of requiring significant time. This time can greatly inconvenience the patient. More recently, light curing has become popular in the field of dentistry as a means for decreasing curing times. According to this trend, curing lights have been developed for dental curing applications. An example of such a curing light is illustrated by U.S. Pat. No. 5,975,895, issued Nov. 2, 1999 to Sullivan, which is hereby incorporated by reference.  
       [0005] Conventional dental curing lights generally employ tungsten filament halogen lamps that incorporate a filament for generating light, a reflector for directing light and a blue filter to limit transmitted light to wavelengths in the region of 400 to 500 nanometers (nm). Light is typically directed from the filtered lamp to a light guide, which directs the light emanating from a light emitting end of the guide to a position adjacent to the material to be cured.  
       [0006] Composites may be selected to take advantage of curing light properties. For example, for certain polymer composite filling materials, blue light provided by a curing light may be used to excite a camphoroquinine photo intitiator, which has a light absorption peak of 468 nm. This in turn stimulates the production of free radicals in a tertiary amine component, causing polymerization and hardening of the polymer composite.  
       [0007] A problem with conventional halogen-based lights is that the lamp, filter and reflector degrade over time. This degradation is particularly accelerated, for example, by the significant heat generated by the halogen lamp. For example, this heat may cause filters to blister and cause reflectors to discolor, leading to reductions in light output and curing effectiveness. While heat may be dissipated by adding a cooling fan to the light, this fan may cause other undesired effects (for example, undesirably dispersing a bacterial aerosol that may have been topically applied by the dentist to the patient&#39;s mouth). Alternate lamp technologies using Xenon and laser light sources have been investigated, but these technologies tend to be costly, require filtration, consume large amounts of power and generate significant heat. Laser technologies also require stringent safety precautions.  
       [0008] Alternatively, Light Emitting Diodes (LEDs) and Laser Diodes (LDs) appear to be good candidate curing light sources, having excellent cost and life characteristics. In addition, LEDs and LDs can be designed to produce a significant portion of light output having a frequency in the desired range of 400 to 500 nm, thereby eliminating the need to incorporate supplementary spectral filters in the device. For example, much of the spectral radiant intensity for many blue LEDs peaks at 468 nm, producing an almost ideal bandwidth of the required blue light. As a result, LED light sources require no filters and generate little waste heat, and are thereby capable of transferring a greater percentage of applied power to generating blue light than, for example, halogen light sources. Generating little heat, they also present less risk of irritation or discomfort to the patient.  
       [0009] To date, it has been difficult to generate sufficient power levels from LED or LD lamp designs for dental curing applications. A minimum of 800 milliwatts per square centimeter is required. Accordingly, it would be desirable to develop a curing light using LED or LD lamps having sufficient power to support dental curing applications.  
       SUMMARY OF THE INVENTION  
       [0010] These and other deficiencies in the prior art have been remedied by a novel light source comprising an array of LEDs fixedly held in a LED holder such that emitters in each of the LEDs are approximately positioned along a spherical surface defined by a predetermined radius. The radius is selected in order to provided a desired focal length for the LED array.  
       [0011] In a first preferred embodiment of the present invention, the array comprises 36 LEDs and has a focal length of 0.445 inches.  
       [0012] In a second embodiment of the present invention, the LED array is combined with a light guide having a light receiving end positioned near the focal length of the LED array. The light guide comprises a bundle of optical fibers, which have been progressively drawn so that the diameter of the bundle at a receiving end is between 14 and 25 millimeters (mm), and the diameter of the bundle at a light emitting end is between 3 and 13 mm. The large diameter at the receiving end allows the receiving end to capture substantially all of the light emitted by the LED array while being positioned at a minimum distance from the LED array. Minimizing the distance between the LED array and light guide reduces the amount of light energy lost by attenuation over this distance.  
       [0013] In the second embodiment of the present invention, the surface of the receiving end of the light guide may be concave and, preferably, follow a spherical surface. This surface shape reduces reflections of light transmitted by the LED array, thereby capturing more of the transmitted light and reducing light energy losses.  
       [0014] In a third preferred embodiment of the present invention, a convex lens is interposed between the array and light guide of the second embodiment to further focus and curing light emitted by the LED array for transmission through the light receiving end of the light guide.  
       [0015] The aforementioned objects, features and advantages will, in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawing, which forms an integral part thereof. While the description describes the array as comprising a plurality of LEDs, the invention contemplates that a variety of other solid-state light sources may also be employed for this purpose (for example, laser diodes). Additionally, while the description describes applications of the light source relating to the curing of dental composites, the present invention contemplates a variety of other uses (for example, as a focused light source for microscopy applications).  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016] A more complete understanding of the invention may be obtained by reading the following description of specific illustrative embodiments of the invention in conjunction with the appended drawing in which:  
     [0017]FIG. 1 illustrates some general properties associated with light from a light source directed to an optical fiber;  
     [0018]FIG. 2 illustrates a first example of a prior art curing light;  
     [0019]FIG. 3 illustrates a second example of a prior art curing light;  
     [0020] FIGS.  4 A- 4 C show an embodiment of the LED array of the present invention;  
     [0021]FIGS. 5A, 5B illustrate an embodiment of the fiber bundle light guide employed by the present invention;  
     [0022]FIGS. 6A, 6B illustrate positioning of the light guides of FIGS. 5A, 5B with respect to the LED array of FIGS.  4 A- 4 C;  
     [0023]FIG. 7 provides a cross-sectional view of a third embodiment of the preset invention employing a collimating lens for reducing focal distance between the LED array and the fiber bundle ; and  
     [0024] FIGS.  8 A- 8 C illustrate a preferred example of the third embodiment of FIG. 7.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0025] The following detailed description includes a description of the best mode or modes of the invention presently contemplated. Such description is not intended to be understood in a limiting sense, but to be an example of the invention presented solely for illustration thereof, and by reference to which in connection with the following description and the accompanying drawings one skilled in the art may be advised of the advantages and construction of the invention.  
     [0026]FIG. 1 illustrates a standard light transport medium in the form of an optical fiber  10 . Optical fiber  10  includes a fiber core  12 , a fiber cladding  14  and a fiber outer coating  16 . Fiber core  12  typically serves as the portion of the fiber operative to carry light, and has an index of refraction N 1 . Fiber cladding  14  serves to help confine light within the core  12 , and has an index of refraction N 2 , which is typically less than N 1 . Fiber Outer coating  16  provides protection against abrasion and other potential physical damage to fiber  10 . A typical fiber  10  in the present inventive application might have an outer diameter between 0.001 and 0.003 inches in diameter, and have about 83 percent of its cross-sectional area comprising the core  12  and about 17 percent of its cross-sectional area comprising the cladding  14 .  
     [0027] Incident beam  22  from light source  20  moves across air gap  21  to strike receiving end  13  of the fiber  10  at an angle θ 1  with respect to fiber centerline  15 . Incident beam  22  is reflected at face  13  as reflected beam  24 , and is refracted at face  13  as refracted beam  30 . Reflected beam  24  makes an angle θ 3  with respect to centerline  15 , and refracted beam  30  makes an angle θ 2  with centerline  15 . Because face  13  is perpendicular to centerline  15 , angle θ 3  is equal to angle θ 1 . Employing Snell&#39;s law, angle θ 2  can be determined by using the following relationship:  
     N air *sin θ 1 =N core *sin θ 2    [1] 
     [0028] Where N air  is an index of refraction for air, and N core  is an index of refraction for the fiber core.  
     [0029] As illustrated in FIG. 1 by light beam  28 , if θ 1  becomes too large, a portion of refracted light beam  30  will be further refracted at interface  17  between core  12  and cladding  14  to exit the core as light beam  32 . The angle beyond which light will not be fully carried in core  12  is referred to as the critical angle, and can be calculated from the associated indices of refraction. The sine of the critical angle is called numerical aperture, and may be calculated as follows:  
     Numerical Aperture ( NA )={square root}{square root over ( )}(( N   core ) 2 −( N   clad ) 2 )   [2] 
     [0030] Where N core  is the index of refraction for core  12 , and N clad  is the index of refraction for the cladding  14 .  
     [0031] For example, in a common fiber configuration where N core =1.62 and N clad =1.52, NA=0.56, which correspond to critical angle of 34 degrees. As the fiber  10  accordingly accepts light up to 34 degrees off centerline  15  in any direction, the acceptance angle of the fiber  10  is twice the critical angle, or 68 degrees. As optical fibers tend to preserve angle of incidence during propagation of light, light entering a fiber  10  will tend to exit the fiber at an angle equivalent to the angle of entry. Accordingly, the cone of light produced at the exit of the fiber will be limited to the smaller of the acceptance angle of the fiber  10  and an incident angle associated with light source  20 .  
     [0032]FIG. 2 illustrates a conventional curing light  40  as disclosed by U.S. Pat. No. 5,975,895. Curing light  40  includes a halogen lamp  41  that is mounted in a reflector  42 . Light reflected by reflector  42  is contained by a collector  43 , and directed to light receiving end  47  of fiber optic bundle  44  and then to light emitting end  48  of bundle  44 . The light reflected from lamp  41  passes through a corrective filter  49  before entering receiving end  47 . Lamp  41 , reflector  42 , collector  43 , filter  49  and receiving end  47  of fiber optic bundle  44  are each aligned along a centerline of barrel  45 . Lamp  41  is powered by power and control unit  52 , cooled by a fan  46 , and actuated by a switch  50  in handle  51 . Power and control unit  52  includes AC power cord  54  and controls  56 . Controls  56  may be used, for example, to control power output and timing of the curing light  40 .  
     [0033] The curing light  40  of FIG. 2 suffers from a number of the previously discussed difficulties associated with conventional curing lights. Halogen lamp  41  requires use of a costly, externally provisioned power supply  52  to power the light  40 . Filter  49 , lamp  41  and reflector elements  42 ,  43  are each subject to degradations over time. Lamp  41  produces a substantial amount of heat, necessitating both the addition of cooling fan  46  and the positioning of lamp  41  at a substantial distance from receiving end  47  of light guide  44  for patient safety.  
     [0034] U.S. Pat. No. 6,102,696 to Osterwalder et al. discloses an alternative curing light design using LEDs or LDs. As illustrated in FIG. 3, the curing light  60  of Osterwalder includes, for example, a LED light source including a plurality of LEDs  61  arranged along a concave edge  63  of a circuit board  64 , each LED being interconnected to the circuit board  64  at a connecting resistor  62 . In this configuration, the light source produces a focused light beam having a focal point  65 .  
     [0035] While the light source of curing light  60  solves some of the difficulties associated with other conventional curing lights, it exhibits certain other deficiencies. As the small number of LEDs employed in the light source generate a modest power level, the light source is positioned in an application end  66  of the curing light  60  so that the light can be transmitted over a short distance through a window  67 . Because no light guide is employed in curing light  60 , the application end  66  housing the light source must be placed in close proximity to the materials being cured. Application end  66  may be relatively large, and therefore difficult to use in applications having limited physical access such as teeth fillings.  
     [0036] The limitations of the prior art are largely overcome by a novel LED light source  80  illustrated in FIGS.  4 A- 4 C, comprising a plurality of LEDs  81  and a LED holder  82  arranged for fixedly holding the plurality of LEDs  81  so that an emitter in each LED is approximately positioned on a spherical surface having a predetermined radius R. FIGS.  4 A- 4 C respectively illustrate top, side and perspective views of the novel LED light source  80 . The radius R may be defined by the following relationship:  
     R=N*L 2 *f*x   [3] 
     [0037] Where N is equal to the number of diodes, L is a focusing distance of the light source, f is an average distance of each LED emitter from a face of its associated LED diode, and x is a correcting factor. L is preferably maintained between 0.400 inches and 0.600 inches. Applicant has successfully constructed LED light sources of this type that have included between 9 and 99 LEDs  81  in the LED holder  82 . In a preferred embodiment of the present invention:  
     [0038] N=36,  
     [0039] F=0.198 inches  
     [0040] L=0.445 inches, and  
     [0041] X=1  
     [0042] In the preferred embodiment, R can be calculated as 1.199.  
     [0043] LEDs  81  in light source  80  will naturally exhibit a variety of spectral characteristics as a result of variation in associated manufacturing processes. While each of the LEDs  81  are selected to transmit light that is primarily in a spectral range of 430 nanometers (nm) to 490 nm in wavelength, individual ones of LEDs  81  will vary as to characteristic wavelength (wavelength produced with greatest intensity) and spectral range. Accordingly, one aspect of the present invention provides for selectively positioning LEDs  81  within holder  82  in accordance with their spectral characteristics. In one embodiment of the present invention, individual LEDs  81  are grouped according to their spectral characteristics and are randomly selected from these groups and positioned in holder  82 . This scheme provides for a reasonably uniform spectral range and intensity across the full area of the incident light beam generated by light source  80 .  
     [0044] Alternatively, LEDs  81  may be grouped and selected so that LEDs having most desired spectral characteristics (for example, characteristic wavelength of 468 nm) occupy central positions on holder  82 , and LEDs having least desired spectral characteristics occupy peripheral or outer positions on holder  82 . Because peripherally-located LEDs may be positioned at or near the critical angle, this embodiment provides, for example, an incident light beam that maximizes transmission at the desired characteristic wavelength.  
     [0045] Another important element of the present invention comprises a novel light guide for directing light from the LED light source to an application. FIGS. 5A, 5B illustrate two examples of the novel light guide. Light guides  100  comprise a plurality of optical fibers  102  arranged in a bundle. Optical fibers  102  are heated and drawn so that a bundle diameter  104  at light emitting end  106  is substantially smaller than a bundle diameter  108  at light receiving end  110 . Diameters for individual fibers in the bundle may typically range from 0.001 to 0.003 inches in diameter. As a result, light emitting end bundle diameter  104  preferably ranges between 3 and 13 millimeters, while receiving end bundle diameter  108  preferably ranges from 14 to 25 millimeters in diameter. Light receiving end bundle diameter  108  is accordingly substantially larger than bundle diameters found in conventional curing lamp guides.  
     [0046] Emitting end  106  of light guide  100  may be positioned at an angle with respect to receiving end  110  (defined between longitudinal axes of emitting end  106  and receiving end  110  by tip angle θ tip ). Light guide  100  may, for example, have a typical length  112  of between two and eight inches and a typical tip depth  114  of between ½ and 3 inches.  
     [0047] Receiving end bundle diameter  108  has the advantage of enabling light guide  100  to be closely positioned with respect to light source  80  (see, for example, FIGS. 6A, 6B). In FIG. 6A, rays  120  represent an outer edge limit for incident light rays generated by the light source  80 . Given that an associated outer edge angle θ edge  does not exceed a critical angle for the light guide  100 , a minimum distance  130  between the light source  80  and receiving end  110  of light guide  100  is inversely related to the receiving end diameter  108  of the light guide  100 . Thus, light guide  100  having an expanded receiving end diameter  108  can be positioned more closely to light source  80  than conventional light guides. As a result, less light energy is attenuated by air gap  21  as shown in FIG. 1, thereby increasing light transmission through receiving end  108  of light guide  100  of FIG. 6A.  
     [0048] A second example of light guide  100  is illustrated in FIGS. 5B and 6B. In FIG. 5B, receiving end  110  of light guide  100  is formed to have a concave surface  125  that may be, for example, approximately spherical in shape. The concave surface  125  effectively alters the angle of refraction θ 2  shown in FIG. 1 so that the critical angle θ 1  may be enlarged, and the amount of light reflected at angle of reflection θ 3  may thereby reduced. As a result, comparing the light guide of FIG. 5B to the light guide of FIG.  5 A, less light energy from light source  80  is reflected by receiving end  110 , thereby increasing light transmission through receiving end  110 .  
     [0049] A third embodiment of the present invention is illustrated by LED curing light assembly  200  of FIG. 7. FIG. 7 presents a cross-sectional view of assembly  200 , comprising light guide  210 , light source  230 , collimating lens  240  and assembly housing  220 . Lens  240  is fixedly positioned between light source  230  and light guide  210 , and acts to further collimate light emitted by light source  230  in order to reduce the focal distance between light source  230  and light guide  210 . This reduction in focal distance helps to further reduce transmissive losses between light source  230  and light guide  210 . Collimating lens  240  is preferably an anti-reflective fused silica convex lens having a minimum of 98% transmissivity within the operative spectral range (430 nm to 490 nm), as may be commercially obtained, for example, from Thermo Electron Corporation of Waltham, Mass.  
     [0050] Light guide  210  is positioned through recess  225  and cavity  226  of housing  220  such that light receiving end  211  of light guide  210  is held against annular seat  227  of cavity  226 . Recess  228  is arranged to hold an O-ring (not shown) for gripping an outer diameter of light guide  210 . Recess  225  is arranged to engagingly receive a retaining nut (not shown) for applying sufficient lateral pressure to the O-ring in recess  228  to cause an inner diameter of the O-ring to meet and fixedly grip the outer diameter of light guide  210  in order to hold light guide  210  against annular seat  227  of cavity  226 .  
     [0051] With reference to light source  230 , a front annular surface  233  of holder  231  of light source  230  is fixedly held against seat  222  in cavity  221  of housing  220 . A variety of conventional means may be employed to hold surface  233  against seat  222  including, for example, an interference fit between outer diameter  235  of holder  231  and inner surface  219  of cavity  221 . Lens  240  may also be fixedly positioned by a variety of conventional means, including fixedly fitting lens  240  within cavity  224  in physical contact with conical surface  229  and covers of ones of the plurality of LEDs  234  in light source  230 .  
     [0052] Mounting plate  223  is fixedly mounted within cavity  221  by one of a variety of conventional means. Mounting plate  223  includes a variety of apertures (not shown) for receiving terminals  236  of light source  230 , and may further include printed wiring paths (not shown) for interconnecting certain ones of terminals  236 .  
     [0053] FIGS.  8 A- 8 C illustrate a preferred example of the third embodiment of FIG. 7. FIG. 8A provides a perspective view of a light source  230   a  comprising LEDs  234  each individually mounted on facets  237  of holder  231 . Facets  237  are configured so that emitters associated with LEDs  234  are approximately positioned on a spherical surface. As shown in FIG. 8A, light source  230   a  comprises five LEDs  234 . Four of the five LEDs  234  at a periphery of holder  231  are positioned at an angle of approximately 25 degrees with respect to a fifth, centrally-located LED in order to define the approximately spherical surface.  
     [0054] In order to generate sufficient light energy for dental curing applications (an excess of 800 milliwatts of output power), LEDs  234  are high output (high luminous flux) LEDs generating in excess of 160 milliwatts of output power (commercially available, for example, as LUXEON LEDs from Lumileds Lighting, LLC of San Jose, Calif.). To assist with dissipation of heat generated by LEDs  234 , holder  231  is formed from a heat-conductive material (for example, aluminum) and incorporates fingers  238  that effectively operate as a heat sink.  
     [0055]FIGS. 8B and 8C provide cutaway views illustrating assembly  200   a  comprising light source  230  fixedly positioned in housing  220   a.  FIG. 8C presents a cross-sectional view of assembly  200   a  through section A-A of FIG. 8B. As illustrated in FIG. 8C, light source  230   a  is fixedly held at a desired position in housing  220   a  by front cup  235 . Front cup  235  provides a friction fit against a perimeter  230   b  of light source  230   a,  and may comprise a variety of materials including natural rubber and plastic. Light guide  210  is fixedly held in housing  200   a  by bushing  228   a,  which applies force against an outer surface of light guide  210  when compressed by front cup clamp  225   a.    
     [0056] Collimating lens  240   a  is interposed between light source  230   a  and light guide  210 . Light receiving end  211  has a concave surface  211   a  for matingly receiving convex surface  240   c  of lens  240   a.  An opposing surface of lens  240   a  includes pockets  240   b  for matingly receiving dome portions of LEDs  234   a.  In this configuration, a viewing angle of approximately 110 degrees for LEDs  234   a  is collimated by lens  240  into a viewing angle of approximately 15 degrees for light rays leaving lens  240   a  and entering light guide  210 .  
     [0057] Those skilled in the art will recognize a variety of additional embodiments of the present invention are not described, but are contemplated within the scope of the invention. For example, one skilled in the art could readily envision constructing light source  80  with a plurality of LDs rather than a plurality of LEDs.  
     [0058] While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention.