Abstract:
A method and apparatus for curing photosensitive materials uses LEDs and an optical concentrator to generate high optical power intensities. An LED array, comprising a plurality of LED assemblies, generates collimated light. A collection lens functions as an optical concentrator and focuses the collimated light to a desired spot size at a desired location. The LED assemblies may be at least partially disposed in a cooling plenum, where the cooling plenum is at least partially defined by the collection lens. Each LED assembly within the LED array may be detachably coupled to a mounting surface, enabling easy replacement of individual LED assemblies within the LED array. The photocuring assembly may also include a redirecting assembly disposed between the collection lens and the desired location that may further concentrate the light at the desired location. The photocuring assembly may include more than one of the above features.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to a method and apparatus for curing photosensitive materials, and more particularly to a method and apparatus for intensifying and routing light, such as ultra-violet light, generated by light emitting diodes for the purpose of curing photosensitive materials. 
   One typical environment where photosensitive curing technology is encountered is in the curing of ultra-violet (UV) photosensitive materials during the manufacture of electronic components. The photocuring systems found in such environments typically use mercury-arc lamps to flood the UV sensitive material with UV light. While mercury-arc lamp technology is widely used, such technology has several disadvantages. The most obvious disadvantage is the life span of the mercury bulbs used in the mercury-arc lamps. Mercury bulbs have a relatively short life, typically 100-1000 hours. Further, the mercury bulb degrades nonlinearly during its lifetime. As a result, conventional mercury-arc photocuring systems often require means to monitor and adjust the output power as the mercury bulb degrades. Further, mercury-arc lamps are typically powered on even during stand-by periods because they require cumbersome warm-up and cool-down cycles; as a result, much of the life of the mercury bulbs may be lost during these stand-by periods. Another disadvantage involves the broad spectrum of the light radiated by the mercury-arc lamp. A mercury-arc lamp radiates UV and infra-red (IR) light. Typically, UV band pass filters transmit the portion of the UV spectrum required for curing a particular photosensitive material. In addition, heat-rejecting IR filters are usually employed to prevent heating of the cure surface. Because the IR radiation creates a very hot lamp housing, transmission optics proximate to the lamp housing must be made of temperature resistant, UV-transmissive materials. 
   The introduction of UV light emitting diodes (LEDs) has created new alternatives for curing some UV sensitive materials. LED technology offers several advantages over the traditional mercury-arc technology. First, typical LEDs last between 50,000 to 100,000 hours, providing a significant lifespan improvement over mercury-arc technologies. Second, UV LEDs do not emit significant IR radiation, so heat-rejecting IR filtration is not required. As an added benefit, the reduced heat generation allows the use of economical UV transmitting polymers for lenses. 
   LED sources can also be turned on and off as required because LEDs do not require the warm-up and cool-down periods common in mercury-arc lamp systems. Some LED curing systems may implement driver circuits to control the current supplied to the LEDs. These circuits typically use a closed-loop system to monitor and control the output power of the LEDs, by controlling the drive current, to provide a stable and reliable UV source. These circuits may also define different curing cycles for different photosensitive materials, such as emitting a specific output power for a specific length of time. 
   Unfortunately, conventional LED sources and LED systems have relatively low output power compared to traditional mercury-arc lamps. While the lower output power LED photocuring systems have proven to be sufficient for some dental applications, many commercial and industrial UV sensitive materials require higher output powers, such as 0.5 to 3 J/cm 2 , to cure properly. For example, some UV sensitive materials require between 100 to 600 mW/cm 2  of optical intensity to initiate and complete a five second cure. Historically, these intensities have not been achieved with LED-based curing systems. 
   U.S. Patent Application Publication 2001/0046652 to Ostler, et al., entitled “Light Emitting Diode Light Source for Curing Dental Composites,” describes use of UV LEDs for curing of dental composites. The Ostler device increases the output intensity of UV light generated by an array of relatively low-power LEDs by concentrating collimated light generated by the array to a desired spot size at a desired location. While the Ostler system increases the output intensity of a UV curing system, the Ostler approach has several disadvantages. First, the Ostler LED array comprises a fixed array of LED chips and therefore does not allow replacement of individual LED units within the array. As a result, new entire units must be purchased to change the wavelength of the emitted optical power, or to replace one or more damaged or defective LEDs. Second, the Ostler cooling system is both complicated and likely insufficient for cooling the higher power UV LEDs now available on the market. Lastly, the Ostler publication does not discuss any methods or apparatus for capturing and redirecting any stray UV light to further intensify the output light at the desired location. 
   Therefore, there remains a need for high intensity LED-based curing systems that addresses one or more problems outlined above. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method and apparatus for curing photosensitive materials. A photocuring assembly uses LEDs and an optical concentrator to generate high optical power intensities. An LED array, comprising one or more LED assemblies, generates collimated light. An optical concentrator, e.g., a collection lens, focuses the collimated light to a desired spot size at a desired location. 
   In one embodiment, the photocuring assembly includes a cooling plenum at least partially defined by the collection lens. The LED array of the photocuring assembly is at least partially disposed in the cooling plenum. Therefore, a cooling fluid, such as air, cools the LED array by flowing through the cooling plenum defined by the collection lens. 
   In another embodiment, each LED assembly comprises a base and an LED insert detachably coupled to the base. In yet another embodiment, the LED assemblies are detachably coupled to a mounting surface, such as a PCB. Both embodiments enable a user to modify the operating wavelength of the photocuring assembly by replacing one or more LED inserts or assemblies having a first operating wavelength with one or more LED inserts or assemblies having a second operating wavelength. In addition, damaged or defective LED inserts and/or assemblies may be replaced without necessitating the replacement of the entire LED array. 
   In another embodiment, the photocuring assembly includes a redirecting assembly disposed between the collection lens and the desired location. Due to the emission properties of conventional LEDs, the LED assembly may not collimate some minority of the light. As a result, the collection lens does not properly focus the non-collimated light exiting the LED array. The redirecting assembly uses refraction or optical reflection techniques to redirect at least a portion of any unfocused light to the desired location to increase the intensity at the desired location. 
   Other embodiments of the present invention may include photocuring assemblies that comprise one or more of the above embodiments. For example, the photocuring assembly may include the cooling plenum bounded by the collection lens and the detachably coupled LED assemblies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a photocuring apparatus according to the present invention. 
       FIG. 2  illustrates electrical interconnections for the exemplary photocuring apparatus of FIG.  1 . 
       FIG. 3  illustrates an exemplary cooling plenum. 
       FIG. 4  illustrates an exemplary element of an LED assembly. 
       FIG. 5  illustrates an element of an LED assembly collimating LED light. 
       FIG. 6  illustrates a conventional light guide. 
       FIG. 7  illustrates an exemplary redirection assembly. 
       FIG. 8  illustrates exemplary light propagation according to the present invention. 
       FIG. 9  illustrates an exemplary photocuring gun. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to a photocuring system that intensifies light emitted from one or more LEDs. The intensified light may be delivered to a remote location to induce a change in a photosensitive material  12  at the remote location, such as to cure the photosensitive material  12 . Because one application for the present invention is curing UV curable materials, the discussions below use UV LEDs to illustrate the invention. However, it should be understood that the present invention is not limited to UV light or UV photocuring technologies. 
   An exemplary photocuring system according to the present invention, generally indicated at  10 , is shown in  FIGS. 1-8 . The photocuring system  10  includes an electrical assembly  200  and an optical assembly  300 , both enclosed in a suitable housing  100 . In addition to providing the mechanical structure, the housing  100  also provides a safety feature by isolating any potentially hazardous optical energy from a user. As shown in  FIG. 1 , the housing  100  may advantageously include an internal wall  110  that functions as a light baffle to isolate the portion of housing containing the main components of the electrical assembly  200  from any stray optical energy generated in the optical assembly  300 . It should be noted that while  FIG. 1  shows a single common housing  100  for the electrical and optical assemblies  200 ,  300 , these assemblies may be mounted in separate interconnected enclosures, if desired. 
   The electrical assembly  200  supplies power to, and controls the operation of, the photocuring system  10 . Referring to  FIG. 2 , the electrical assembly  200  may include a power supply  210 , a current controller  220 , a timer  230 , and a cooling fan  240 . The power supply  210  performs customary power supply functions, such as converting the incoming AC power to DC voltage and current, providing DC current to the current controller  220 , providing DC power to the cooling fan  240 , and the like. The current controller  220  adaptively controls the power delivered to an LED array  330  in the optical assembly  300  to enable the LED array  330  to generate stable, constant UV light. In addition, the current controller  220  may vary the power supplied to the LED array  330  to vary the optical power generated by the LED array  330  as desired. The timer  230  and optional cycle start switch  232  provide for further control of the operation of the LED array  330  to advantageously allow for triggered starts to the curing cycle, and optionally for adjustable time intervals for the curing cycles. The cooling fan  240  acts to pull cooling fluid, such as air, through the photocuring system  10  to avoid overheating the LED array  330 . While discussed in greater detail below, the air is in general pulled into the housing intake  120 , routed through the optical assembly  300 , through the internal wall  110  to the electronic assembly  200 , and then pushed out of the housing  100  by the cooling fan  240  via the housing exhaust  140 . 
   The optical assembly  300  includes a collection lens  320 , an LED array  330 , a converging chamber  380 , and a cooling plenum  310 , as shown in FIG.  1 . The LED array  330  generates high-power UV light. While an array of LEDs is used herein to illustrate the invention, it will be understood by those skilled in the art that the invention described herein applies equally well to a photocuring system using a single LED. As such, the term “LED array” as used herein is intended to mean one or more LEDs, such as a single LED or a plurality of LEDs arranged as desired. The collection lens  320  intensifies the light generated by the LED array  330  by focusing the light to a desired spot size at a desired location within the converging chamber  380 . An optional redirection assembly  382  may be positioned in the converging chamber  380  to redirect light rays outside of the converging beam to the desired location to further intensify the light at the desired location, as discussed further below. The UV light intensified by the optical assembly  300  may then be delivered to the photosensitive material  12  at the remote location by coupling the intensified UV light into a light guide, such as an optical fiber  384 , secured on one end to the housing  100  with a suitable fitting  150 . 
   The LED array  330 , which is discussed further below, is at least partially disposed in the cooling plenum  310 , as shown in FIG.  3 . For the  FIG. 3  configuration, an electrical substrate  312 , brackets  314 , and the collection lens  320  bound the cooling plenum  310 . Cooling air enters the cooling plenum  310  via intake port  316  and flows along cooling plenum  310  past LED array  330 . The collection lens  320  confines the airflow to the cooling plenum  310  and forces the airflow past the LED array  330 . The airflow exits the cooling plenum  310  via the exhaust port  318 . 
   The LED array  330  comprises a plurality of LED assemblies  340 .  FIG. 4  illustrates an exemplary LED assembly  340  of the present invention. Each LED assembly  340  includes an LED insert  360  coupled to a collimator base  350 . The collimator base  350  includes a heatsink  352  and a reflective cavity  354 . The reflective cavity  354  may be shaped as a curve and functions to generally collimate and direct the diffuse LED light towards the collection lens. In a preferred embodiment, the reflective cavity  354  is shaped as a parabola. The reflective cavity  354  should be fabricated from a metal or metal alloy, e.g., an aluminum alloy, and should be highly polished to efficiently reflect the optical energy radiated at the LED&#39;s operational wavelength. In a preferred embodiment, the collimator base  350  is a single unit formed from a solid piece of material. Alternatively, the heatsink  352  and reflective cavity  354  are separately manufactured and joined together to form the collimator base  350 . 
   The LED insert  360  includes an LED  362 , LED base  364 , thermal conductive adhesive  366 , LED terminals  368 , and a thermal post  370 . An LED die (not shown) emits radiant energy at an operational wavelength preferably within the range of 315 nm to 450 nm. The LED die is typically positioned on a metalized ceramic standoff (not shown) that electrically isolates the LED die from the LED base  364 , although this is not required. The standoff, or its equivalent, raises the LED die above the LED base  364  to maximize the light emitted by the LED  362  and collimated by the reflective cavity  354  of the collimator base  350 . Wire bonds (not shown), insulated from the LED base  364 , electrically connect the LED die to the LED terminals  368 . 
   The LED  362  is fixedly attached to the LED base  364  and inserted into the thermal post  370  to form the LED insert  360 . The LED base  364  is typically fabricated from steel or copper alloys, and plated with gold or silver. Alternatively, the LED base  364  may be fabricated from a ceramic material. Thermal conductive adhesive  366  secures the LED base  364  to the thermal post  370  and improves the thermal conduction from the LED  362  to the heatsink  352 . The thermal post  370 , preferably constructed of an aluminum alloy, includes holes to pass the LED terminals  368  formed along the longitudinal axis of the thermal post  370 . Insulative sleeving, such as plastic, rubber, or fiber (not shown), placed over the LED terminals  368  electrically isolates the LED terminals  368  from the thermal post  370 . Alternatively, a hard-anodized insulating coating added to the internal surface of the thermal post  370  electrically isolates the LED terminals  368  from the thermal post  370 . 
   The assembled LED insert  360  is then inserted in the collimator base  350  to position the LED  362  at a desired location within the reflective cavity  354 . Once the LED  362  is positioned at the desired location within the reflective cavity  354 , the LED insert  360  is either fixedly or detachably coupled to the collimator base  350  by any means well known in the art. For example, the LED insert  360  may include threads (not shown) for threadably coupling the LED insert  360  to the collimator base  350 , with locking compound optionally added to fix the LED insert  360  to the collimator base  350 . Alternatively, adhesive may secure the LED insert  360  to the collimator base  350 . The LED insert  360  coupled to the collimator base  350  forms the LED assembly  340 . 
   In a preferred embodiment of the present invention, each LED assembly  340  is detachably coupled to an electrical substrate  312 , such as a printed circuit board (PCB), to form the LED array  330  (FIG.  1 ). Sockets electrically connected to PCB  312  provide detachable electrical connection points for the LED terminals  368 . Alternatively, a connector may be soldered to the LED terminals  368  for detachably connecting the LED assembly  340  to the PCB  312 . Preferably, the electrical connection points also provide mechanical support for the LED assemblies  340 . For example, the connections may be via known screw lamp base socket (with corresponding threads on the LED base  364 ) for a threaded coupling between the LED assemblies  340  and the PCB  312 , or known bayonet or wedge type lamp base sockets may be used instead. Alternatively, separate mechanical means of connection, such as dedicated screws, clips, or the like, may be used to mechanically couple the LED assemblies  340  to the PCB  312 . As another alternative, L-shaped LED terminals  368  may be used to extend through suitably configured slots in the PCB  312 , with the LED assembly  340  rotated 90° to align the LED terminals  368  with contact pads on the PCB  312  and to seat the LED assembly  340  against the PCB  312  using the inherent spring force generated by slightly-deflecting the LED terminals  368  against the far side of the PCB  312 . Any of these approaches may be used to detachably couple the LED assemblies  340  to the PCB  312 . As should be clear from the above, it is preferred that the coupling be both mechanical and electrical, but some embodiments may merely electrically couple the LED assemblies  340  to the PCB  312 , with the LED assemblies  340  being otherwise mechanically supported. 
   The number of LED assemblies  340  employed determines the size of the LED array  330  and the desired output intensity. For example, five LED assemblies  340  can generate approximately 500 mW/cm 2  of desired output intensity at a wavelength of 400 nm when inserted into an LED array  330  of a photocuring system  10  according to the present invention; forty LED assemblies  340  can generate at least 1,000 mW/cm 2  of desired output intensity at a wavelength of 400 nm, and preferably approximately 4,000 mW/cm 2 . 
   The detachably coupled LED assemblies  340  and/or detachably coupled LED inserts  360  have several benefits. For example, a user can change the operating wavelength of the photocuring system  10  by replacing one or more LED assemblies  340  or one or more LED inserts  360  having a first operating wavelength, i.e., 315 nm, with one or more replacement LED assemblies  340  or LED inserts  360  having a second operating wavelength, i.e., 400 nm. In addition, a user can replace damaged or expired LED assemblies  340  or LED inserts  360  without replacing the entire LED array  330 . Further, a user can easily increase or decrease the output intensity by adding/removing LED assemblies  340  or LED inserts  360  to/from the LED array  330 . 
   Regarding the optical properties of the optical assembly  300 , each LED insert  360 , including LED  362 , emits diffuse light at a predetermined optical power and a predetermined optical wavelength. Exemplary LEDs  362  according to the present invention emit 150-250 mW of optical power at 315-450 nm. The reflective cavity  354  collimates a majority of the diffuse light emitted by the LED  362  when the LED  362  is placed at the desired location within the reflective cavity  354 . A parabolic reflector  354  represents an exemplary reflective cavity  354  that collimates the majority of the light when the LED  362  is placed at or near the focal point of parabolic reflector  354 , as shown in FIG.  5 . It will be understood by those skilled in the art that the collimating means of the present invention is not limited to a parabolic reflector  354 . Other LED collimating means well understood by those skilled in the art may also be implemented in the present invention. 
   The collection lens  320  intensifies the light generated by the LED array  330  by focusing the collimated light to a spot of a predetermined diameter at a predetermined location (FIG.  1 ). In a preferred embodiment, the LED array  330  uses a single collection lens or lens system for the entire LED array  330 . Alternatively, the collection lens  320  may comprise an array of lenses, where each lens in the lens array corresponds to one or more LED assemblies  340  in the LED array  330 . It will be appreciated by those skilled in the art that the collection lens  320  serves as a multifunction device in the present invention—in addition to intensifying the light generated by the LED array  330 , the collection lens  320  also serves as a mechanical boundary of the cooling plenum  310 , as discussed above. 
   As mentioned previously, it is sometimes desirable to deliver the intensified UV light to a remote location via a conventional light guide, such as an optical fiber  384 . The coupling properties of optical fibers  384  are well known in the art, and therefore, are only discussed briefly. As illustrated in  FIG. 6 , the coupling properties of optical fibers  384  are at least partially defined by an acceptance cone angle θ. Generally, only light entering the optical fiber  384  within the acceptance cone  386  couples to the optical fiber  384 . Plastic, glass, and liquid-filled fibers typically exhibit acceptance cone angles θ ranging from 30° to 40°. Therefore, the convergence angle φ of the focusing light should be less than or equal to 30° to efficiently couple the light to the optical fiber  384 . The convergence angle φ of the focusing light is inversely proportional to the focal length of the collection lens  320 . Therefore, a lens designer should evaluate the properties of the selected collection lens or lens system  320  to ensure that the collection lens  320  will focus the light at the required convergence angle φ to the desired spot size. Designing a collection lens or a lens assembly  320  with a preferred convergence angle to focus collimated light to a preferred spot size is well understood in the art. Therefore, for simplicity, the details for designing such a collection lens or lens system  320  are not discussed further. 
   While a properly designed collection lens  320  will couple the majority of the collimated light to the fiber  384 , a small minority of the light emitted by the LED array  330  is not collimated by the parabolic reflector  354 , and therefore, is not properly focused by the collection lens  320 . Most of this stray light converges either too quickly or too slowly to efficiently couple to the fiber  384 . In addition, aberrations caused by the optical components may enlarge the focused spot. Therefore, to increase the intensity and/or to improve the fiber coupling efficiency, the present invention may optionally use a redirection assembly  382 , as shown in  FIG. 7 , to redirect some of the stray light. The redirection assembly  382  can utilize refractive or reflective techniques to couple more radiant energy into the fiber  384 . An exemplary redirection assembly  382  is a reflective cone that intercepts some of the outermost optical energy rays and redirects them towards the fiber  384 . The additional light rays converging on the desired location increases the radiant intensity within the desired spot size and improves optical coupling to the fiber  384 . A properly designed and positioned redirection assembly  382  can couple at least an additional 5% of the light into the fiber  384 . One exemplary implementation can couple an additional 5-15% or more (preferably 10%) of the light into the fiber  384 . As with other optical components of the present invention, the surface of a reflective redirection assembly  382  should be capable of reflecting radiant energy in the wavelength range of the emitted optical energy. 
   To further exemplify the operation of the entire optical assembly,  FIG. 8  illustrates an exemplary ray diagram for a single LED assembly  340 , collection lens  320 , redirection assembly  382 , and optical fiber  384 . It will be understood by those skilled in the art that a similar ray diagram results when the LED array  330  of a single LED assembly  340  is replaced by an LED array  330  of a plurality of LED assemblies  340 . The parabolic reflector  354  of the LED assembly  340  collimates a majority of the diffuse light emitted by an LED  362  located at or near the focal point of the parabolic reflector  354 . The collimated light entering the collection lens  320  focuses to a preferred spot size at the entrance to the fiber  384  and couples to the optical fiber  384 . The non-collimated light entering the collection lens  320  also focuses at a location proximate the entrance to the optical fiber  384  and may or may not couple to the optical fiber  384 . The optional redirection assembly  382  may redirect and couple a percentage of this stray light into the optical fiber  384 . 
   An alternate embodiment of the photocuring system  10  of the present invention is illustrated in  FIG. 9  in the form of a hand-held photocuring tool  20 . The photocuring tool  20  includes the electrical assembly  200  and optical assembly  300  in the head portion  22  of the photocuring tool  20 . Intensified curing light is available at the exit of the fiber  384 , located at one end of the head portion  22 . A handle  24  enables a user to grip the photocuring tool  20  and direct the intensified curing light to a desired location. In this embodiment, the housing near the fiber  384  may be designed to act as the redirection assembly  382  for improving the coupling efficiency of the fiber  384  as described above. 
   As can be seen from the above, the various embodiments of the present invention include a cooling plenum  310  that is at least partially defined by the collection lens  320 , readily replaceable LED assemblies  340  or LED inserts  360 , and/or a redirection assembly  382  disposed between the collection lens  320  and the light guide  384 , either alone or in combination. As a result, the present invention provides an alternative approach to generating intensified light using LEDs with significant advantages over the mercury-arc based approaches or the low-power approach of Ostler. 
   The foregoing description and drawings describe and illustrate the present invention in detail. However, the foregoing disclosure only describes some embodiments of a photocuring system. Accordingly, the present invention may be carried out in other specific ways than those set forth herein without departing from the essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.