Abstract:
Disclosed is a LED curing light device for curing of photo-polymerization materials. The device comprises of a plurality of LED source and a plurality of Fresnel lenses. The said LED source is powered and controlled by a drive board and batteries providing high power curing light in the range of 300 to 500 nm and optical power in the range of 100 to 800 mW. The said Fresnel lenses couple said curing light efficiently to a focused spot on a curing object.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to provisional patent application Ser. No. 60/593,954, filed Feb. 26, 2005 and patent application Ser. No. 10/906,772, filed Mar. 5, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to the field of light curing device, and more particularly to medical curing device and methods for irradiating and curing photosensitive curing compounds.  
       BACKGROUND OF THE INVENTION  
       [0003]     Photosensitive compounds or adhesives are commonly used in bonding object surfaces together or for filling openings and cavities in an object surface. They are cured by exposure to radiation energy, such as UV with a wavelength between 300 to 400 nm or blue light with a wavelength between 400 to 500 nm. Medical curing light device are commonly used in dentistry, endoscopies, and plastic surgeries. In the field of dentistry, curable adhesives and dental curing apparatus are common practice in restoration and cosmetic procedures using restorative materials, dental sealants and orthodontic adhesives to bond brackets to the surfaces of teeth. Curing light is also widely used in device, component and circuit board packaging using photo-initiator activated composites to bond two different surfaces or protect components.  
         [0004]     Traditionally, curing light apparatus are implemented with bulk lamps such as tungsten-halogen lamps coupled into fiber optic waveguide that delivers light to expose area of adhesives need to be cured. Recent advances in light emitting diodes (LEDs) technologies have enabled a new class of curing light apparatus with smaller size, longer lifetime and lower cost by semiconductor light emitting chips.  
         [0005]     LEDs emit light at selected wavelengths of absorption band of photo-initiators that start the curing process of curable adhesives. Typical wavelength for dental curing is in the range of 400-500 nm. It is highly desirable to have high optical density impinged on the curable adhesives to activate the photo-initiators that allow a quick curing time of between 2 to 10 seconds and a deeper curing depth of between 2 to 6 millimeters. Typical ranges of optical density for a desirable 4 to 5 millimeters curing depth and less than 10 seconds curing time are above 1000 mW/cm 2 . In dental applications, such intensity is exposed to the curing area, typically in the range of 2 to 6 mm dimension, limited by the cavity and bracket size.  
         [0006]     There have been two approaches in the selection of LEDs to achieve such high intensity, namely single high power LEDs or multiple standard single diode LEDs. High power LEDs integrates multiple LED chips in a single package such as LEDs made by Lumiled&#39;s Luxeon product lines that generate optical power as high as 800 mW. Standard single chip LEDs generates optical power below 150 mW. Typical arrangements of more than five LEDs are required to deliver equivalent power at the curing site. Other critical elements of efficient curing are the light delivering system and working distance of the curing apparatus from the curing object for efficient cure.  
         [0007]     U.S. Pat. No. 6,611,110 describes an apparatus using light guides to deliver curing light from a single LED to the curing site. The light guide reduces the deliverable curing light efficiency due to optic coupling, transmission, and diffraction losses from light guide with a typical total efficiency of below 30%. A higher power LED can compensate the loss. Additional use of lens such as total internal reflection (TIR) lens as described in U.S. Pat. No. 6,692,251 can improve the power density. However, they introduce higher cost and more cumbersome system. Additionally, it has been shown that autoclaving the light guide to sterilize the apparatus can reduce the transmission performance of the light guide making them costly to replace.  
         [0008]     U.S. Pat No. 20030133203 describes an apparatus using a bulk aspheric lens to directly focus curing light from a single LED to the curing site. The aspheric lens is molded glass or plastic lens. The benefit of such implementation is a reduced size and cost compared to using of light guide. However, a high power LED is highly non-directional typically following a Lambertian radiation pattern with radiation angles above 120 degrees at half of its maximum intensity. Combined with a source chip size of typically 3 millimeter, the LED radiation incurs collection loss through the aspheric lens and diffracts quickly to lose its intensity due to limited collection angles that aspheric lens offer, which is typically less than 70 degrees. Aspheric lenses with short focal length to collect light from LED source are also thick with aspect ratio of diameter to thickness close to one enlarging the size of the apparatus as well. Working distances of such devices are typically limited to a short distance within a few millimeters. In addition, sterilizing tubes to protect the lens entrance will significantly reduce radiation due to optical diffractions.  
         [0009]     A need exists, therefore, for improved LED curing apparatus that provide efficient light delivery to the curing site at high optical intensity with low cost particularly in the dental applications.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, it is a principal object of the present invention to overcome the disadvantages of prior art methods of dental curing light system. The present invention comprises a method, and resulting apparatus, for highly efficient curing system for curable materials, in particular for dental curing.  
         [0011]     In one embodiment, the optical device of the invention includes a high power LED source and a single Fresnel lens placed at a distance larger than its focal length to the LED source to collect radiation up to 160 degrees and focus the radiation into a curing spot.  
         [0012]     In another embodiment, the optical device of the invention includes a high power LED source. The LED illumination is captured by a Fresnel lens with collection angles approximately between 100 to 160 degrees into diffraction limited collimating beam and then focused into a spot diameter approximately less than 5 millimeters by a second Fresnel lens placed in close proximity to the first lens. The pair of Fresnel lenses is bonded together into an efficient lens with two flat surfaces on the outside and lens grooves bonded in-between with a total thickness approximately between 0.5 to 2 millimeters. The exit window of the lens pair is shielded and protected by a sterilizable and disposable cone-shaped plastic cap.  
         [0013]     In one embodiment of implementing the invention, the optical device of the invention includes a high power LED source placed within a parabolic reflector that works also as a lens mount for a plurality of Fresnel lenses to focus the emitted light from the LED to a small spot size of approximately under 4 mm at the curing composite.  
         [0014]     In one embodiment of implementing the invention, the optical device is integrated with a handpiece containing control electronics and rechargeable batteries that activate the high power LED being focused to a small spot of approximately under 4 mm by the Fresenl lenses at the curing composite.  
         [0015]     It is to be understood that both the foregoing general descriptions and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. Additional features and advantages of the invention will become apparent from the following drawings and description. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which like numerals designate corresponding elements or sections throughout, and in which:  
         [0017]      FIG. 1  shows a prior dental LED curing method using bulk aspheric lens;  
         [0018]      FIG. 2  shows an embodiment of the optical device using current invention;  
         [0019]      FIG. 3  shows another embodiment of the optical device using current invention;  
         [0020]      FIG. 4  shows an embodiment in implementing the optical device using current invention;  
         [0021]      FIG. 5  shows the differences in collection angle using the current invention compared to prior art;  
         [0022]      FIG. 6  illustrates curing light intensity as a function of the distance from the curing apparatus to the curing object using current invention compared to prior art;  
         [0023]      FIG. 7  shows an embodiment of a cordless dental LED curing device using the current invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIG. 1  shows a prior art dental LED curing light device  100  consisting of a LED  130 , an aspheric lens  110 , and a transparent shield  120  all attached to an extension arm  160 . The aspheric lens  110  comprises of a first end  140 , which is substantially flat, and a second end  141  that has an aspheric curvature. The transparent shield has an apex  121  to ease use for insertion into a dental cavity and clips  150  to wrap around and secure the lens  110  in place. The aspheric lens is preferably composed of a transparent material such as glass, aluminum dioxide, sapphire, quartz, acrylic, polyacrylic, polypropylene, and silicone. Apparatus using standard aspheric lenses as described are limited by the performances of the optical parameters of such lens with collection angle typically less than 70 degrees and thickness of more than 3 millimeters due to high curvature required to have short focal length. As a result, they are not efficient in focusing curing light.  
         [0025]      FIG. 2  illustrates an embodiment of the present invention for LED curing source comprising a single high power LED  210  and a Fresnel lens  220  that focuses the illuminating light from the LED to a diffraction limited spot  230 . The high power LED  210  preferably has an optical output power approximately between 400-800 mW, such as that Luxeon V produced by Lumiled at a wavelength in range of 400-500 nm. Higher powers are preferred since they provide faster and deeper curing time. The illumination rays  240 ,  240 ′, and  240 ″ illustrate the function of the lens in collecting radiation and focus to a curing spot.  
         [0026]     The Fresnel lens consists of a groove side  231  and a flat side  230 . The grooves are circular cylindrical portions intersected by conical portions manufactured by standard machine processes such as diamond turning, injection and compression molding. They maintain the contour of the refracting surface of a conventional lens while removing the bulk of material between the refracting surfaces. The groove side of the Fresnel lens is preferably in the receiving direction of the optical illumination and the flat side in the outward direction to avoid surface damage in an assembly. Constant groove spacing or constant groove heights can be used in the design of the Frensnel lens. Compared to aspheric lens, Fresnel lens can be  10  times thinner which is critical to the application for close distance focus. Depending on the shapes of the grooves, a circular, square or narrow line focused spot can be realized at the focus spot  230  using circular or cylindrical lens.  
         [0027]      FIG. 3  illustrates another preferred embodiment of the present invention for LED curing source consisting of a single high power LED  210  and a Fresnel lens pair  320  that focuses the illuminating light to a spot  330 . Compared with a single Frensnel lens in  FIG. 2 , the Fresnel lens pair  320  is more efficient in which it acts as a condenser lens consisting of a collimating lens  321  and a focusing lens  321 ′. The collimating lens  321  is placed at a focal distance from the LED source  310  preferably between 2 to 5 millimeters to collimate the source light to diffraction limited collimation beam. While closer distance from the lens to the LED source chip reduces the size of the lens required, typical LED chips are packaged with glass dome lens with a size in the range of 1 to 3 millimeters limiting the proximity of the lens to the source. Additionally, avoidance of heat dissipated directly from the chip will limit the proximity of the lens to the LED as well.  
         [0028]     The Fresnel lens  321  should maximize collection efficiency while balancing the size limitation of the instrument. A good parameter of the lens performance is described by optical F numbers as defined in:
 
 F  number= f/D 
 
 where the F number is the ratio of the focal length of the lens divided by the beam diameter of the lens. Smaller F number provides higher collection efficiency in angular distributed radiations. The use of Fresnel lens enables a much faster lens with F number below 0.3 that can collect the Lambertian illumination from the LED up to 120 to 160 degrees as compared with typical aspheric lens with F number above 0.5 which collects radiation below 70 degrees. This minimizes loss during coupling as is often encountered in the fiber waveguide coupling and aspheric lens coupling. 
 
         [0029]     The focus lens  321 ′ is placed in close proximity parallel to the collimating lens  321  with a focal length determined by working distance of a particular application. For dental curing applications, the focal length of the focusing lens  321 ′ is preferably between 2 to 20 millimeters optimizing the efficiency at a working distance of 2 to 20 millimeters. The Fresnel lens pair also effectively works as a single lens with very short focal length of below 2 millimeters and very thin thickness as small as 0.5 millimeters, which are critical to both minimizing diffraction loss and making compact devices.  
         [0030]     The Fresnel lens pair  320  is preferably formed by a groove out Fresnel lens  321  and a groove in Fresnel lens  321 ′ bonded together to form a thin sheet lens  320  with flat outside surfaces. Such arrangement eases mounting of the Fresnel lens pair  320  into a lens cell that attaches to the LED mount in addition to improve scratch resistance to the active Fresnel groove surface.  
         [0031]      FIG. 4  illustrates a proposed implementation of the present invention for LED curing source consisting of a high power LED  210  with multiple diode chips packaged in a single diode, directional collimating hyper-spherical lens  210   c , diode heat sink  210   a , a Fresnel lens pair  321  and  321 ′, a mounting parabolic reflector  410 , and a disposable curing cap  420 . The light emitted from LED  210  typically has a divergence angle of over 80 degrees after lens co-packaged lens  210   c.    
         [0032]     The parabolic reflector  410  redirects the high divergence angle beam to a near collimated beam. The reflector  410  is consisted of a mounting thread  410   a , a high reflectance inner surface  410   c , and a cap stopper  410   b . The mounting threads  410   a  mounts the LED illumination optic module to a handpiece head as will be discussed in detail in  FIG. 7 . The high reflectance surface  410   c  provides reflectivity of over 95, for example 98%, at the LED emitted wavelength range. Example materials for  410  are copper, aluminum and plastics. Electroplating of copper with chromium, zinc, aluminum, and silver offers the best reflectivity. Alternatively, high reflective thin films such as multiplayer metal oxide or polymer films can be evaporated or coated and post mounted at the inner surface  410   c  to achieve the desired results.  
         [0033]     The proposed lens  321  and  321 ′ are thin disks of Fresnel lenses. The embodiment of the current invention enables collimation of LED illumination with minimum coupling loss, focus of the beam to a desired spot size limited by diffraction from source chip size and a minimum thickness in the lens assembly. The Fresnel lens consists of circular grooves that refract light with different angle at different radial position to form the function of a lens. They can be formed by either constant grooves spacing or constant groove height. Constant groove height is preferred for the Fresnel lens in curing applications to allow bonding of the two lenses forming the condenser lens into a single lens sheet with grooves facing each other as shown in  FIG. 3  and flat surface on their outside.  
         [0034]     The Fresnel lens can be made of transparent materials such as polycarbonate, acrylic, silicone, rigid vinyl and others that are low cost through compression or injection molding of large piece of materials enabling wafer level productions that make them low cost. The lens pair can be assembled together through standard packaging procedures such as bonding at individual on wafer level. Single lens can be also used in the case of highly collimating beam from the hyper-spherical lens  210   c  and parabolic reflector  410   c.    
         [0035]     The disposable and sterilizable curing cap  420  provides isolation of curing tip from patients or objects under treatment. It is conveniently snapped on the surface of the mounting reflector  410  and stopped by a mechanical edge stopper  410   b . The length of the lens cap  420  is shorter than the focal length of the second Fresnel lens  321 ′. The lens cap  420  provides stray light shield with proper doping of the cap materials to absorb the wavelength of the illuminated light from the LED. It can also be attached with a third Fresnel lens at the exit window to further improve the working distance of the curing light. The lens cap is preferably made of materials that are disposable such as acrylic, polycarbonate and other plastics through standard manufacturing processes such as injection molding. It further provides a means to sterilize or dispose the cap at a minimum cost.  
         [0036]      FIG. 5  compares theoretical performance of the current invention with the prior art dental curing device using a single aspheric lens. The calculation shows the radiation intensity as a function of radiation angle relative to the axis normal to the LED mounting surface. The prior art aspheric lens typically limit collection angle below +/−35 degrees at collection efficiency of 50% indicated by  510  while the current invention can increase the collection angle of the radiation above +/−60 degrees indicated by  520  with collection efficiency up to 90%.  
         [0037]      FIG. 6  shows the curing light intensity (power density) as a function of the distance from the output window of the curing units to the object. Compared with conventional curing units using fiber optic guide  630  and bulk aspheric lens  620 , the current invention  610  maintains and optimizes curing intensity between 2 to 10 mm through minimized diffraction and optimized beam focusing. The light intensity at 10 mm of the current invention is more than five times that of the prior art approaches ensuring maximum curing at desired locations.  
         [0038]      FIG. 7  illustrates an embodiment of the current invention in a high efficiency dental LED curing light consisting of an LED mounting head  710 , a high power LED  210 , a lens mounting reflector  410 , Fresnel lens  321 , a curing cap  420 , a handpiece housing consisting of a control board housing  730 , a rechargeable battery housing  760  and a thread link ring  750  via a thread  730   b . The mounting head  710  and the handpiece portion are preferably made of highly heat conductive materials such as aluminum and copper to enhance heat dissipation speed.  
         [0039]     The LED head mount  710  provides heat dissipation to the LED generated powers through thermal interface  711  bonded by thermal epoxy between the back side of the LED  210  and the surface  711 . The head mount  710  is attached to the handpiece body through threaded retainer  720  that clamps the two together at the interface  730   a . The head mount  710  preferably has an angle from the handpiece to allow an angle of illumination, approximately between 5 to 45 degrees, for ease of access to mouth.  
         [0040]     The LED  210  is powered by a control circuit board  740  by two wire leads  720  feed through a hole inside the head mount  710 . The circuit board  740  resides in the main control housing  730 . It is activated by an on-off switch button through a touch button  740   c  and powered by a plurality of rechargeable batteries  770 . It performs DC-DC conversion to the desired current for the LED  210  in addition to preset exposure timing sequence, thermal protection of LED against high temperatures through a thermal sensor  710  placed in close proximity to the LED through two lead wires  710   a , low battery indicator LED  740   b  and automatic shut off.  
         [0041]     A plurality of rechargeable batteries  570  is housed in the battery compartment  760 . The battery interfaces with two circular board  780   a  and  780   b  via a pair of contact pins  780   c  and hold tight into the compartment  760  by a ring  760   b  via thread  760   a . A clamp ring push the connect ring  750  tight against the ring  760   b  via a connecting ring  760   c . The circuit board  780   a  interfaces with the main control board  740  via a plug-in pin connector  780   a . The circuit board  780   b  interfaces an external power adapter through a small pin connector  790 . The combined circuit board of  740 ,  780   a , and  780   b  also performs the smart charging circuit to safely charge the rechargeable battery  770 .  
         [0042]     The rechargeable battery is preferably lithium ion battery that has 3.7V per battery. A single battery with sufficient energy capacity is typically sufficient for dental curing light operation although multiple batteries can extend the standalone operation time.  
         [0043]     The proposed high efficiency LED curing device enables low cost and efficient curing of photosensitive materials. The device is particularly useful for portable handheld dental curing light. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.