OPTICAL IRRADIATION DEVICE

Abstract: An optical irradiation device incorporating a cluster of LEDs (11, 43) arranged so that shaped facets of adjacent LEDs come together to increase the packing density of LEDs in the cluster. A light guide (41) collects light emitted by the LEDs. Two or more light guides (41) and LED clusters (43) may be arranged in series to produce a single light beam. A heat pipe (45) is provided to conduct heat away from the LEDs (43). The heat pipe (56) may be annular and contain an inner storage space for batteries (60) or the like.

Description: OPTICM; IRRADIATION DEVICE fr.r_τmιcAL FIELD This invention relates to an optical irradiation device, especially a compact portable irradiation device suitable for use as a light polymerisation source. it has already been proposed to use light-emitting diodes LEDs in a hand held device to produce a focused beam of light to cure dental materials . Blue light at a peak wavelength of about 470nιcι is used to harden dental polymerε which contain camphorocjuinone as the photoinitator in a methacrylate polymerisation process. However, there is a problem in producing a sufficient level of irradiance even with a clustered array of LEEs , to cure the known dental polymers in the recommended time. At the lower levels of irradiance available generally kelow 300mW/sq.cm, longer curing times have to be allowed, which reduces the efficiency of the dental treatment delivered. DISCLOSURE OF THE INVENTION An object of the present invention is to provide an optical irradiation device that employs LEDs, and thereby has the benefits of compactness, portability, ruςrgedness and long O 99/16136 life, but which also produces improved levels of irradiance at and above 300mW/sq.cm. According to a first: aspect of the invention, LEDs are clustered in an irradiation device by forming shaped facets on adjacent LEDs which allow them to adjoin more closely than they would otherwise with conventional spherical outer surfaces as manufactured currently. According to a second aspect, the invention consists in a tapered light guide for an optical irradiation device, which light guide is tapered from its input end to its output end and has an intermediate region of minimum diameter in which a bend is formed. According to a third aspect, the invention consists in an optical irradiation device employing LEDs and incorporating a heat pipe to cool the LEDs . According to a fourth aspect, the invention consists in a heat pipe comprising inner and outer walls that extend longitudinally from one end of the heat pipe to the other and define an annular space therebetween containing a material that serves to absorb heat by a phase change, the annular space being divided by internal walls into a plurality of fluid flow channels that extend longitudinally between said ends, some of said channels being adapted to conduct the liquid/vapour phase of said material from the hot and of the heat pipe to the cold end, and other channels being adapted to return said liquid phase from the cold end of the pipe to the hot end. According to a fifth aspect, the invention consists in an irradiation device employing LEDs and a tapered light guide to collect radiation emitted by the LEDs and deliver this to an output beam, wherein two or more tapered light guides are arranged in series so that successive guides receive radiation from preceding guides, and a group of LEDs is provided at the input end of each guide, each successive guide preferably being provided with a ring of LEDs around the output end of the preceding guide. The first aspect of the invention means that LEDs occupy more of the available space, and a fixed number produce a higher radiant intensity. Thus, smaller numbers of LEDs can be used to produce a desired level of irradiance, which in turn reduces the power required to drive the device and the heat generated by it. Furthermore, the device can be made more compact. Packing of the LEDs in this way may involve a slight reduction in the output of each LED, but the more effective packin g density produces an overall increase in irradiance. Typically, a central LED might have a polygonal outer surface, and a first ring of LEDs would be arranged around it, each with a flat face to abut a corresponding face of the central LED and possibly each having a pair of radiating side faces which abut adjacent LEDs in the first ring. Furthermore, a second or more rings of LEDs could be arranged concentrically with the first ring, each with respective adjacent flat side faces abutting one another and possibly with inwardly diverted faces abutting respective outwardly directed faces of the LEDs of the inner ring. Alternatively, a single ring or two or more concentric rings of LEDs could be used without a central LED. DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings, in which: Fi g ure ^ is a schematic cross-section through a first embodiment of the invention comprising a cluster of hexagonal section LEDS; F -ff ur ^ 2 s a schematic cross-section through a second embodiment of the invention comprising a cluster of an inner group of LEDS and an outer ring of LEDS; O 99/16136 Figure 3 is a schematic cross-section through a third embodiment of the invention comprising a cluster of two rings of LEDS; F-iσure 4 is a schematic side elevation of a fourth embodiment of the invention; Fiσure 5 is a schematic longitudinal section through a fifth embodiment of the inventio ,- Figure 6 is a schematic longitudinal section through a sixth embodiment of the invention; Figure 7 is a schematic cross-section through a bundle of light guide fibres with modified sections; Ficrure $ is a schematic side elevation of a tapered light-guide according to another embodiment of the invention; and Ficrure 9 is a schematic cross-section through a heat pipe according to the invention. MODE OF CARRYING OUT THE INVENTION In a typical optical irradiation device according to the invention, a plurality of LEDs are clustered together βo as to direct emitted radiation into a single beam. A cluster of LEDs 43 is shown in side view in Figure 4, and in plan view or cross-section in Figures 1 to 3. Each LED comprises a light-emitting semiconductor Pn junction (not shown) which is encapsulated in an outer plastics envelope, the profile of which is shown in the drawings. The sides of the LED envelope are shaped to allow the LEDs to be clustered together more closely at their bases, thereby increasing the ratio of occupied to unoccupied space in the cluster of LEDs. The tips of the LEDs are substantially spherical and transmit the radiation to form the beam. In the embodiment of the invention illustrated in Figure 1 the outer envelope of the LEDs is hexagonal in cross-section, and the LEDs are clustered in the manner of a honeycomb as shown, with adjacent facets abutting one another. In the second embodiment of the invention illustrated in Figure 2, a central LED 21 of hexagonal cross-section has facets which abut adjacent facets of six LEDs 22 in a first ring of LEDs with radially extending side facets that allow adjacent LEDs in the ring to abut one another. A second ring of LEDs 23 is arranged around the first ring of LEDs, and these LEDs 23 have radially extending side facets that allow adjacent LEDs in the ring to abut one another. O 99/16136 7 In a third embodiment of the invention shown in Figure 3 , an inner ring of nine LEDs 31 in a first ring is contained within a second ring of LEDs 32, and radially extending side facets of the LEDs in both rings allow adjacent LEDs in each ring to abut one another. Both the second embodiment of Figure 2 and the third embodiment of Figure 3 may be modified by the addition of one or more further concentric rings of LEDs. Also, the circumferential facets of the LEDs of each ring may be shaped to abut similarly shaped circumferential facets of the adjacent ring of LEDs. In yet another embodiment, the central group of LEDs 21,22 of Figure 2 may be replaced by the same number of LEDs in a honeycomb cluster. Yet another embodiment may consist of the single ring of LEDs 31 shown in Figure 3. It will be appreciated in all three illustrated embodiments, the LEDs are mounted in a substantially flat plane. In modifying the conventional optical sphere shape of the outer plastics envelope of a LED care has to be taken to preserve as much as possible of the focusing effect of the envelope to maxi ' mise the total irradiance. However, because the envelope of existing LEDs have a tapered shape to assist their removal from the mould during manufacture, the shape d O 99/16136 « side facets can be formed around the broader base of the LED to change its cross-section, for example to become hexagonal, but with these facets having a reducing effect on the shape of the envelope towards its tip where the focusing effect of the envelope is concentrated. Thus the invention can employ existing LEDs and modify their shape in a secondary manufacturing process, for example, using jigs, or the invention can employ LEDs which have been specially manufactured with the required outer envelope shape to accommodate better clustering. The shaped facets of the LEDs may be polished to enhance reflection and help reduce any loss of optical powers. Additionally, a reflective metallic film may be applied to the shaped facets to further enhance reflection. The LEDs may also incorporate a microlens or microlens array to aid collimation of the beam. The electrical connections of the LEDs, known as lead frames 44, are connected to respective positive and negative power terminals or bus bars 42. Preferably, these terminals are adapted to serve the dual function of heat sinks to help remove heat generated by the LEDs 43. Thus, the terminals are formed of a good thermal conductor such as copper, and are located in the optimum location relative to the LEDs and the external surfaces of the device. In one particular embodiment most suited to the LED array of Figure 3, the terminals 42 take the form of two concentric rings, each lying adjacent to the bases of one ring of LEDs 31 or 32. Preferably, the negative terminal is the outer one because the negative lead frames 44 of the LEDs generally get hotter than the positive lead frames 44. The typical optical irradiation device according to the invention also preferably incorporates a tapered light guide, shown as guide 41 in Figure 4, to collect light emitted by the LEDs and deliver this as an output beam. It is known to use light guides with adiabatic optical tapers in optical irradiation devices so that there is total .internal reflection of the light as it is conducted from the light source to the output. However, an advantage of the invention is that the more compact cross-section of the LED cluster means that the diameter at the input end of the light guide can be smaller, and thus a smaller angle of adiabatic taper (i.e. the ratio of the diameter of the input end to the output end of the light guide) can be provided in the light guide with the consequent more efficient transmission of radiant energy and increased illuminance. This improvement is most marked compared with a conventional approach of simply increasing the numbers of LEDs in a cluster at ever increasing diameters with decreasing beneficial effect on illuminance and increasing detrimental effect on compactness, heat generation and cost. In another embodiment of the invention, illustrated in Figure 6, two or more adiabatic tapered light guides 41 are arranged in series, each with a corresponding cluster of LEDs 43, but with successive clusters forming a ring around the end of one light guide as it connects to the next. Alternatively, each successive ring of LEDs 43 may be replaced by just one or a fewer number of LEDs. This arrangement allows the overall diameter of the device to be kept relatively small as the LED clusters 43 are arranged in groups along the length of the device . In the preferred embodiment of Figure 4, a single tapered light guide 41 is provided. If required, the light guide can be curved along its length, as shown in Figure 5, to direct the output beam to suit a particular application, this being a known practice with existing light guides. The light guide may be machined from cast acrylic plastic and bent, or could be made from glass or other optically transparent materials. An alternative light guide is illustrated in Figure 8 in which the bend in the light guide 41 is provided at a waisted section 46 in its length which reduces to a minimum d iameter before widening again to a larger diameter towards its output end. By forming the bend at the minimum diameter, the light transmission losses of the light guide caused by the bend are reduced, but the effective cross-sectional area of the output beam is maintained at the required level. Fused fibre-bundle light guides have the advantage of individual fibres being of a relatively small diameter so that they can be bent over a tighter radius without the greater losses associated with larger diameter fibres when bent over the same radius. However, conventional fused- fibre bundles have the disadvantage of a packing fraction loss, that is, the outer cladding of the fibre uses up a significant proportion of the cross-section of the light guide into which light from the semiconductor array is directed, thus reducing the amount of transmitted radiation available from the semiconductor source. Preferably, therefore, in one embodiment of the invention, illustrated in Figure 7, the guide comprises a few shaped fibres 61 placed adjacent to each other and fused together. A guide of this design is manufactured by MicroQuartz Sciences Inc. of Phoenix, Arizona, USA. In this way, the diameter of each fibre is smaller than a single homogeneous guide rod so that they allow greater light transmission on bending around the same bend radius, but also the p acking fraction is also greatly reduced over conventional fibre guides, resulting in a greater than 90% core availability at the input end of the guide. In another embodiment of the invention, a graded-index optical light guide is used. A graded-index light guide has no sudden interface between the cladding and the core. Instead, the refractive index varies either radially or axially. In one embodiment, the gradient of the refractive index of the light guide varies both radially and axially so that the light energy is favourably manipulated. A guide that uses a stepped index could also be used with the same axial and radial variation in refractive index. In this way, the numeric aperture can be varied at either end of the guide to achieve the desired transmission. In other embodiments of the invention, instead of providing a single tapered light guide, each LED or groups of LEDs could be provided with its own light guide fibre incorporating an adiabatic optical taper, and the output ends of these fibres could be collected together to form a single output beam. The input end of the fibre would be moulded optically to the adjacent LED or group of LEDs for efficient transmission of radiation. In this way, the diodes can be spaced more widely to dissipate unwanted heat. In yet another embodiment of the invention , each LED could ba adjusted so that its outer envelope is extended into a fibre light guide which incorporates an adiabatic optical taper. In yet another embodiment, the section of the fibres may be modified so that shaped faces of the fibres fit together to reduce the interstitial space. One embodiment of this design could be as shown in Figure 7. The light guide or light guides used according to the invention may be formed with an outer metallic coating to improve its performanc . It will be appreciated that the irradiance of the device according to the invention can be varied by varying the input power, number of LEDs, or by varying the adi'abatic taper of the light guide. Cooling of the LED cluster can be aided according to another feature of the invention by arranging that the electrical connections of each LED are thermally connected to one or more heat pipes. Conventional LED irradiation devices usually include a heat sink to conduct away the heat from the LED chips. Heat sinks are generally slow and inefficient in conducting heat away from a heat source compared with heat pipes. Heat pipes conduct heat away rapidly by using the latent heat of a substance, such as water, which is vaporised b y the heat from the source. The vapour moves at high speed to the cooler end of the heat pipe and condenses. Heat pipes are unique in their ability to conduct heat rapidly in this way. Figure 5 shows a device according to the invention which incorporates a heat pipe 45 as a single lumen in the main body 46 of the device. The hotter of the LED leads is preferably placed nearer the heat pipe 45 or outer case 47 of the LED cluster so that the heat path of the hotter lead is shorter. A thermal connector 48 may be provided between the LEDs 43 and the end of the heat pipe 45. If required, additional forced cooling means may be used for example, a fan 49 or Peltier device 50 in juxtaposition to the pipe. In addition, a heat sink 51 may be provided. Because of the greater cooling ability of heat pipes, they allow the LEDs to be driven in such a manner as to produce more radiation, and thus allows a more powerful device to be manufactured. For portable use, the LEDs are operated from batteries 52, which are located in a hand grip 53 attached to the body 46, in Figure 5. However, the heat pipe design can be modified as shown in Figure 9 to accommodate batteries. The heat pipe consists of two concentric heat conducting tubes 55,56 with a folded interstitial heat conduction element 57 between these tubes similar in appearance to a length of corrugate d sheet rolled into a tube. This lies within the concentric tubes. The wicks 58 of the heat pipe can then be placed in alternative grooves in the corrugated sheet, while the empty grooves 59 allow for the rapid movement of the vapour formed at the warmer end of the heat pipe. By designing the heat pipe in this way batteries, capacitors, supercapacitors or other energy source 60 can be located within the inner wall 55 of the heat pipe. In some embodiments, for example, where there are a large number of LEDs, a heat sink 51 may be necessary in addition to the heat pipe 45. The intermittent use of an LED irridation device for dental curing, means that with careful design, a heat sink may be omitted. If cooling to below the ambient temperature is required, such as may be the case in extreme environments, a Peltier device 50 may be added to the heat pipe, although a Peltier device will result in a greater consumption of power and a requirement for greater heat dissipation. The wavelengths of the LED used will depend upon the applications of the device. A LED emitting blue light with a peak wavelength of about 470mm is used to harden dental polymers, but a LED emitting red light may be useful for p hotodynamic therapy, for example, cancer therapy. The wavelength of light emitted by the LEDs may be modified in a light guide by doping the material from which it is composed with fluorescent material. This can serve to lengthen the wavelength of the emitted light so as to suit the particular application. The choice of LED is also important in terms of its construction, diameter, irradiance and light angular spread pattern, From a range of known LEDs the best available choice has been determined as that with a 3mm diameter rather than a 5mm diameter and an angular spread of 30 degrees rather than 15 or 45 degrees. Nichia is the manufacturer of these LEDs. It will be appreciated that the rerm "light-emitting diode-LED" as used herein also includes laser diodes. LEDs in the devices according to the invention may be operated in a pulsed mode or modulated mode to vary the output radiation intensity to suit the application, and multiple clusters of LEDs, such as in the embodiment of Figure 6, may each be generated in a different mode. The power supply for the LEDs of the device according to the invention could be mains power, battery power, capaci t or, supercapacitor, solar power, clockwork generator or generator powered by the mechanical effort of the operator or assistant. In one embodiment, a capacitor or supercapacitor could be used to power the array having advantages over conventional rechargeable sources such as batteries. Capacitors can be virtually instantaneously recharged between one or more curing cycles of operation when the unit is connected to a power source. The power supply for the device may be re-chargeable, and may be designed to make automatic electrical contact with the charging means of a base unit when engaged with the latter in the manner of a cordless telephone handset.

Claims: 1. An optical irradiation device comprising an array of light-emitting diodes (LEDs) clustered so that radiation they emit is directed into a beam characterised in that each LED is formed with multiple facets such that the facets of adjacent LEDs adjoin one another in close proximity throughout their length. 2. A device as claimed in claim 1 in which the facets of adjacent LEDs extend substantially parallel to one another. 3. A device as claimed in claims 1 or 2 in which the facets of adjacent LED.s abut one another. 4. A device as claimed in any one of the preceding claims in which LEDs are arranged in a ring with side facets of adjacent LED.s adjoining one another. 5. A device as claimed in claim 4 in which LEDs are arranged in concentric rings with side facets of adjacent LEDs in each ring adjoining one another. 6. A device as claimed in claim 5 in which the LEDs of adjacent rings have radially directed facets adjoining one another. „„, ,, ^ PCT/GB98/02905 WO 99/16136 19 7. A device as claimed in any one of claims 4 to 6 in which a single LED is located within said ring or innermost concentric ring. 8. A device as claimed in claim 7 in which said single LED has radially directed facets that adjoining facets of the LEDs in said ring or innermost concentric ring. 9. A device as claimed in any one of the preceding claims in which the LEDs are regular polygons in cross-section. 10. A device as claimed in claim 9 in which the LEDs are hexagonal in cross-section. 1 1. A device as claimed in any one of the preceding claims in which the facets of the LEDs are polished. 12. A device as claimed in any one of the preceding claims in which the facets of the LEDs are provided with a reflective coating. 13. A diode adapted for use in an optical irradiation device as claimed in any one of claims 1 to 12. 14. A device as claimed in any one of the preceding claims, including a light guide for collecting light from the cluster of light emitting diodes. O 99/16136 20 15. A device as claimed in any one of claims 1 to 13, wherein a light guide is provided for each light emitting diode in the cluster. 16. A device as claimed in any one of the preceding claims including a light guide for collecting light from the cluster of LED.s, the light guide having an index that varies from one part to another so as to manipulate the light. 17. A device as claimed in any one of the preceding claims including a light guide that consists of a few fibres formed individually before being bundled together. 18. A device as claimed in any one of the preceding claims including a light guide consisting of shaped fibres packed together so that the packing fraction is reduced. 19. An irradiation device employing LEDs and a tapered light guide to collect radiation emitted by the LEDs and deliver this to an output beam, characterised in that two or more tapered light guides are arranged in series so that successive guides receive radiation from preceding guides, and an LED or group of LEDs is provided at the input end of each guide. 20. A device as claimed in claim 20 in which each successive guide is provided with a ring of LEDs around the output end of the preceding guide. 21. A device as claimed in any one of the preceding claims in which the heat is removed from the LEDs by a heat pipe. 22. A device as claimed in claim 21 in which a plurality of heat pipes is used to transfer heat from the LEDs. 23. A device as claimed in claim 21 or 22 in which an annular heat pipe is sued so that it can contain energy storage means. 24. A device as claimed in any one of the preceding claim.s, including a Peltier device to cool the LEDs. 25. A device as claimed in any one of the preceding claims having a pistol grip to contain energy storage means. 26. A device as claimed in any one of the preceding claims, including a capacitor or supercapacitor to power the device. 27. An optical irradiation device comprising a plurality of LEDs and a heat pipe to cool the LEDs. 28. A hand-held device for curing dental materials including an optical irradiation device as claimed in any one of the preceding claims. 29. A heat pipe comprising inner and outer walls that extend longitudinally from one end of the heat pipe to the other and define an annular space therebetween containing a material that serves to absorb heat by a phase change, the annular space being divided by internal walls into a plurality of fluid flow channels that extend longitudinally between said ends, some of said channels being adapted to conduct the liquid/vapour phase of said material from the hot end of the heat pipe to the cold end, and other channels being adapted to return said liquid phase from the cold end of the pipe to the hot end. 30. A tapered light guide for an optical irradiation device, which light guide is tapered from its input end to its output end and has an intermediate region of minimum diameter in which a bend is formed.

Inventors: JANDT KLAUS DIETER, MILLS ROBIN WALTER, JANDT, KLAUS, DIETER, MILLS, ROBIN, WALTER
Applicants: JANDT KLAUS DIETER, MILLS ROBIN WALTER, UNIV BRISTOL, JANDT, KLAUS, DIETER, MILLS, ROBIN, WALTER, UNIVERSITY OF BRISTOL
Application year: 1998
Main classification: H01L33
IPC codes: A61C3, A61C13, A61N5, G02B6, H01L25, H01L33