US20080186702A1 - Array of Light Emitting Devices to Produce a White Light Source - Google Patents

Abstract

A device is provided with an array of a plurality of phosphor converted light emitting devices (LEDs) that produce broad spectrum light. The phosphor converted LEDs may produce light with different correlated color temperature (CCT) and are covered with an optical element that assists in mixing the light from the LEDs to produce a desired correlated color temperature. The phosphor converted LEDs may also be combined in an array with color LEDs. The color LEDs may be controlled to vary their brightness such that light with an approximately continuous broad spectrum is produced. By controlling the brightness of the color LEDs, light can be produced with a fixed brightness over a large range of white points with a high color rendering quality.

Description

CROSS REFERENCE TO RELATED APPLICATION
• The present application is a divisional application of and claims priority to U.S. patent application Ser. No. 10/987,241, filed Nov. 12, 2004, entitled “Bonding an Optical Element to a Light Emitting Device”, by Michael D. Camras et al, which is incorporated herein by reference.
FIELD OF THE INVENTION
• The present invention relates generally to light emitting devices and, more particularly, to an array of light emitting devices to produce a white light source.
BACKGROUND
• Adding or mixing a number of different color light emitting devices (LEDs) can be used to produce light with a broad spectrum. The spectrum produced, however, consists of the peaks of the narrow band spectra produced by the individual LEDs. Consequently, the color rendering of such a light source is poor. White light sources with high color rendering, such as that produced by a halogen lamp, have a continuous or near continuous spectrum over the full visible light spectrum (400-700 nm).
• Thus, a white light source with high color rendering that is produced using an array of LEDs is desired
SUMMARY
• In accordance with one embodiment of the present invention, a plurality of phosphor converted light emitting devices may be combined in an array to obtain light with a desired correlated color temperature (CCT). In one embodiment, the phosphor converted light emitting devices produce light with different CCTs. An array of the plurality of phosphor converted light emitting devices may be covered with an optical element that optionally can be filled with a material that assists in light extraction and mixing the light to produce light with the desired CCT. In another embodiment, a plurality of color light emitting devices are combined with the plurality of phosphor converted light emitting devices and the brightness of the color light emitting devices are controlled to produce light with the desired characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1A illustrates a side view of an LED die mounted on a submount and an optical element that is to be bonded to the LED die.
• FIG. 1B illustrates the optical element bonded to the LED die.
• FIG. 2 illustrates an embodiment in which multiple LED dice are mounted to a submount and a separate optical element is bonded to each LED die.
• FIG. 3 illustrates an embodiment in which multiple LED dice are mounted to a submount and a single optical element is bonded to the LED dice.
• FIG. 4 is a flow chart of one implementation of producing such an LED device with wavelength converting material covering the optical element.
• FIG. 5 illustrates an embodiment in which a layer of wavelength converting material is disposed between the bonding layer and the optical element.
• FIG. 6 illustrates an embodiment in which a layer of wavelength converting material is deposited on the LED die.
• FIG. 7 illustrates an array of LEDs, which are mounted on a board.
• FIG. 8 is a graph of the broad spectrum produced by a phosphor converted blue LED.
• FIG. 9 is a CIE chromaticity diagram for the spectrum shown inFIG. 8.
• FIG. 10 is a graph of the spectra produced by phosphor converted LEDs and colored LEDs, which are combined to produce an approximately continuous spectrum.
• FIG. 11 is a portion of a CIE chromaticity diagram that shows the variation in the CCT that may be produced by varying the brightness of the colored LEDs.
• FIG. 12 is a portion of another CIE chromaticity diagram that illustrates variable CCT values for an array of 29 phosphor converted LEDs and 12 color LEDs.
DETAILED DESCRIPTION
• FIG. 1A illustrates a side view of a transparentoptical element102 and a light emitting diode (LED) die104 that is mounted on asubmount106. Theoptical element102 is to be bonded to the LED die104 in accordance with an embodiment of the present invention.FIG. 1B illustrates theoptical element102 bonded to the LED die104.
• The term “transparent” is used herein to indicate that the element so described, such as a “transparent optical element,” transmits light at the emission wavelengths of the LED with less than about 50%, preferably less than about 10%, single pass loss due to absorption or scattering. The emission wavelengths of the LED may lie in the infrared, visible, or ultraviolet regions of the electromagnetic spectrum. One of ordinary skill in the art will recognize that the conditions “less than 50% single pass loss” and “less than 10% single pass loss” may be met by various combinations of transmission path length and absorption constant.
• LED die104 illustrated inFIGS. 1A and 1B includes afirst semiconductor layer108 of n-type conductivity (n-layer) and asecond semiconductor layer110 of p-type conductivity (p-layer).Semiconductor layers108 and110 are electrically coupled to anactive region112.Active region112 is, for example, a p-n diode junction associated with the interface oflayers108 and110. Alternatively,active region112 includes one or more semiconductor layers that are doped n-type or p-type or are undoped.LED die104 includes an n-contact114 and a p-contact116 that are electrically coupled tosemiconductor layers108 and110, respectively. Contact114 andcontact116 are disposed on the same side ofLED die104 in a “flip chip” arrangement. Atransparent superstrate118 coupled to then layer108 is formed from a material such as, for example, sapphire, SiC, GaN, GaP, diamond, cubic zirconia (ZrO2), aluminum oxynitride (AlON), AlN, spinel, ZnS, oxide of tellurium, oxide of lead, oxide of tungsten, polycrystalline alumina oxide (transparent alumina), and ZnO.
• Active region112 emits light upon application of a suitable voltage acrosscontacts114 and116. In alternative implementations, the conductivity types oflayers108 and110, together withrespective contacts114 and116, are reversed. That is,layer108 is a p-type layer,contact114 is a p-contact,layer110 is an n-type layer, andcontact116 is an n-contact.
• Semiconductor layers108 and110 andactive region112 may be formed from III-V semiconductors including but not limited to AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, II-VI semiconductors including but not limited to ZnS, ZnSe, CdSe, ZnO, CdTe, group IV semiconductors including but not limited to Ge, Si, SiC, and mixtures or alloys thereof.
• Contacts114 and116 are, in one implementation, metal contacts formed from metals including but not limited to gold, silver, nickel, aluminum, titanium, chromium, platinum, palladium, rhodium, rhenium, ruthenium, tungsten, and mixtures or alloys thereof.
• AlthoughFIGS. 1A and 1B illustrate a particular structure ofLED die104, the present invention is independent of the structure of the LED die. Accordingly, other types of LED configurations may be used instead of the specific configuration shown. Further, the number of semiconductor layers inLED die104 and the detailed structure ofactive region112 may differ. It should be noted that dimensions of the various elements ofLED die104 illustrated in the various figures are not to scale.
• TheLED die104 is mounted to submount106 viacontacts elements120, such as solder bumps, pads, or other appropriate elements, such as a layer of solder.Contact elements120 will be sometimes referred to herein asbumps120 for the sake of simplicity.Bumps120 are manufactured from Au, Sn, Ag, Sb, Cu, Pb, Bi, Cd, In, Zn or alloys thereof including AuSn, SnSb, SnCu, SnAg, SnAgBi, InSn, BiPbSn, BiPbCd, BiPbIn, InCd, BiPb, BiSn, InAg, BiCd, InBi, InGa, or other appropriate material with a melting temperature that is greater than the temperature that will be used to bond theoptical element102 to theLED die104, but is preferably Au or AuSn. In one implementation, the melting temperature ofbumps120 is greater than 250° C. and preferably greater than 300° C. Thesubmount106 may be, e.g., silicon, alumina or AlN and may include vias for backside connections.
• The LED die104 is mounted to thesubmount106, e.g., using thermosonic bonding. For example, during the thermosonic bonding process, theLED die104 withbumps120 are aligned with thesubmount106 in the desired position while thesubmount106 is heated to approximately 150-160° C. A bond force of, e.g., approximately 50-100 gm/bump, is applied to theLED die104 by a bonding tool, while ultrasonic vibration is applied. If desired other processes may be used, such as thermo-compression, to bond the LED die104 to thesubmount106. As is well known in the art, with thermo-compression higher temperatures and greater bonding forces are typically required.
• In some embodiments, an underfill may be used with the LED die104 andsubmount106. The underfill material may have good thermal conductivity and have a coefficient of thermal expansion that approximately matches the LED die104 and thesubmount106. In another embodiment, a protective side coat, e.g., of silicone or other appropriate material, may be applied to the sides of the LED die104 and thesubmount106. The protective side coating acts as a sealant and limits exposure of theLED104 and thebumps120 to contamination and the environment.
• For more information regarding producingbumps120 from Au or Au/Sn and for submounts with backside vias and bonding LED dice with Au or Au/Sn bumps to a submount, see U.S. Ser. No. 10/840,459, by Ashim S. Haque, filed May 5, 2004, which has the same assignee as the present disclosure and is incorporated herein by reference. It should be understood, however, that the present invention is not limited to any specific type of submount and that any desired submount configuration may be used if desired.
• After the LED die104 is mounted to thesubmount106, theoptical element102 is thermally bonded to the LED die104. In one embodiment, a layer of bonding material is applied to the bottom surface of theoptical element102 to formtransparent bonding layer122 that is used to bondoptical element102 to LED die104. In some embodiments, thetransparent bonding layer122 may be applied to the top surface of the LED die104, e.g., to superstrate118, (as indicated by the dottedlines122 inFIG. 1A). Thebonding layer122 can be applied to the LED die104 prior to or after mounting the LED die104 to thesubmount106. Alternatively, no bonding layer may be used and theoptical element102 may be bonded directly to the LED die104, e.g., thesuperstrate118. Thetransparent bonding layer122 is, for example, about 10 Angstroms (Å) to about 100 microns (μm) thick, and is preferably about 1000 Å to about 10 μm thick, and more specifically, about 0.5 μm to about 5 μm thick. The bonding material is applied, for example, by conventional deposition techniques including but not limited to spinning, spraying, sputtering, evaporation, chemical vapor deposition (CVD), or material growth by, for example, metal-organic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), or molecular beam epitaxy (MBE). In one embodiment, theoptical element102 may be covered with awavelength converting material124, which will be discussed in more detail below.
• In one implementation, the bonding material from whichtransparent bonding layer122 is formed from glass such as SF59, LaSF 3, LaSF N18, SLAH51, LAF10, NZK7, NLAF21, LASFN35, SLAM60, or mixtures thereof, which are available from manufactures such as Schott Glass Technologies Incorporated, of Duryea, Pa. and Ohara Corporation in Somerville, N.J.Bonding layer122 may also be formed from a high index glass, such as (Ge, As, Sb, Ga)(S, Se, Te, Cl, Br) chalcogenide or chalcogen-halogenide glasses, for example.
• In other implementations,bonding layer122 may be formed from III-V semiconductors including but not limited to GaP, InGaP, GaAs, and GaN; II-VI semiconductors including but not limited to ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe; group IV semiconductors and compounds including but not limited to Si, and Ge; organic semiconductors, metal oxides including but not limited to oxides of antimony, bismuth, boron, copper, niobium, tungsten, titanium, nickel, lead, tellurium, phosphor, potassium, sodium, lithium, zinc, zirconium, indium tin, or chromium; metal fluorides including but not limited to magnesium fluoride, calcium fluoride, potassium fluoride, sodium fluoride, and zinc fluoride; metals including but not limited to Zn, In, Mg, and Sn; yttrium aluminum garnet (YAG), phosphide compounds, arsenide compounds, antimonide compounds, nitride compounds, high index organic compounds; and mixtures or alloys thereof.
• In implementations where the LED die104 is configured with the n-contact and p-contact on opposite sides of thedie104, thetransparent bonding layer122 or122′ may be patterned with, for example, conventional photolithographic and etching techniques to leave the top contact uncovered by bonding material and thus to permit contact to make electrical contact with a metallization layer on theoptical element102, which may serve as a lead, as is described in U.S. Ser. No. 09/880,204, filed Jun. 12, 2001, by Michael D. Camras et al., entitled “Light Emitting Diodes with Improved Light Extraction Efficiency” having Pub. No. 2002/0030194, which is incorporated herein by reference.
• In one implementation, theoptical element102 is formed from optical glass, high index glass, GaP, CZ, ZnS, SiC, sapphire, diamond, cubic zirconia (ZrO2), AlON, by Sienna Technologies, Inc., polycrystalline aluminum oxide (transparent alumina), spinel, Schott glass LaFN21, Schott glass LaSFN35, LaF2, LaF3, and LaF10 available from Optimax Systems Inc. of Ontario, N.Y., an oxide of Pb, Te, Zn, Ga, Sb, Cu, Ca, P, La, Nb, or W, or any of the materials listed above for use as bonding materials intransparent bonding layer122, excluding thick layers of the metals.
• The transparentoptical element102 may have a shape and a size such that light enteringoptical element102 from LED die104 will intersect surface102aofoptical element102 at angles of incidence near normal incidence. Total internal reflection at the interface ofsurface102aand the ambient medium, typically air, is thereby reduced. In addition, since the range of angles of incidence is narrow, Fresnel reflection losses atsurface102acan be reduced by applying a conventional antireflection coating to thesurface102a. The shape ofoptical element102 is, for example, a portion of a sphere such as a hemisphere, a Weierstrass sphere (truncated sphere), or a portion of a sphere less than a hemisphere. Alternatively, the shape ofoptical element102 is a portion of an ellipsoid such as a truncated ellipsoid. The angles of incidence atsurface102afor light enteringoptical element102 from LED die4 more closely approach normal incidence as the size ofoptical element102 is increased. Accordingly, the smallest ratio of a length of the base of transparentoptical element102 to a length of the surface of LED die104 is preferably greater than about 1, more preferably greater than about 2.
• After the LED die104 is mounted on thesubmount106, theoptical element102 is thermally bonded to the LED die104. For example, to bond theoptical element102 to the LED die104, the temperature ofbonding layer122 is raised to a temperature between about room temperature and the melting temperature of the contact bumps120, e.g., between approximately 150° C. to 450° C., and more particularly between about 200° C. and 400° C., andoptical element102 and LED die104 are pressed together at the bonding temperature for a period of time of about one second to about 6 hours, preferably for about 30 seconds to about 30 minutes, at a pressure of about 1 pound per square inch (psi) to about 6000 psi. By way of example, a pressure of about 700 psi to about 3000 psi may be applied for between about 3 to 15 minutes.
• The thermal bonding of theoptical element102 to the LED die104 requires the application of elevated temperatures. With the use ofbumps120 that have a high melting point, i.e., higher than the elevated temperature used in the thermal bonding process, the LED die104 may be mounted to thesubmount106 before theoptical element102 is bonded to the LED die104 without damaging the LED die/submount connection. Mounting the LED die104 to thesubmount106 prior to bonding theoptical element102 simplifies the pick and place process.
• Bonding anoptical element102 to anLED die104 is described in US Pub. No. 2002/0030194; Ser. No. 10/633,054, filed Jul. 31, 2003, by Michael D. Camras et al., entitled “Light Emitting Devices with Improved Light Extraction Efficiency”; Ser. No. 09/660,317, filed Sep. 12, 2000, by Michael D. Camras et al., entitled “Light Emitting Diodes with Improved Light Extraction Efficiency; Ser. No. 09/823,841, filed Mar. 30, 2001, by Douglas Pocius, entitled “Forming an Optical Element on the Surface of a Light Emitting Device for Improved Light Extraction” having Pub. No. 2002/0141006, which have the same assignee as the present application and which are incorporated herein by reference. Further, the process of bondingoptical element102 to LED die104 described above may be performed with devices disclosed in U.S. Pat. Nos. 5,502,316 and 5,376,580, incorporated herein by reference, previously used to bond semiconductor wafers to each other at elevated temperatures and pressures. The disclosed devices may be modified to accommodate LED dice and optical elements, as necessary. Alternatively, the bonding process described above may be performed with a conventional vertical press.
• It should be noted that due to the thermal bonding process, a mismatch between the coefficient of thermal expansion (CTE) ofoptical element102 and LED die104 can causeoptical element102 to detach from LED die104 upon heating or cooling. Accordingly,optical element102 should be formed from a material having a CTE that approximately matches the CTE of LED die104. Approximately matching the CTEs additionally reduces the stress induced in the LED die104 bybonding layer122 andoptical element102. With suitable CTE matching, thermal expansion does not limit the size of the LED die that may be bonded to the optical element and, thus, theoptical element102 may be bonded to alarge LED die104, e.g., up to 16 mm²or larger.
• FIG. 2 illustrates an embodiment in whichmultiple LED dice204a,204b, and204c(sometimes collectively referred to as LED dice204) are mounted on asubmount206. The LED dice204 are schematically illustrated inFIG. 2 without showing the specific semiconductor layers. Nevertheless, it should be understood that the LED dice204 may be similar to LED die104 discussed above.
• The LED dice204 are each mounted to submount206 as described above. Once the LED dice204 are mounted onsubmount206, individualoptical elements202a,202b, and202care bonded toLED dice204a,204b, and204c, respectively, in a manner such as that described above.
• If desired, the LED dice204 may be the same type of LED and may produce the same wavelengths of light. In another implementation, one or more of the LED dice204 may produce different wavelengths of light, which when combined may be used to produce light with a desired correlated color temperature (CCT), e.g., white light. Another optical element (not shown inFIG. 2) may be used to coveroptical elements202a,202b, and202cand aid in mixing the light.
• FIG. 3 illustrates an embodiment of anLED device300 that includesmultiple LED dice304a,304b, and304c(sometimes collectively referred to as LED dice304) mounted on asubmount306 and a singleoptical element302 bonded to the LED dice304. The LED dice304 may be similar to LED die104 discussed above.
• The use of a singleoptical element302 with multiple LED dice304, as shown inFIG. 3, is advantageous as the LED dice304 can be mounted close together onsubmount306. Optical components typically have a larger footprint than an LED die to which it is bonded, and thus, the placement of LED dice with separate optical elements is constrained by the size of the optical elements.
• After the LED dice304 are mounted to the submount, there may be slight height variations in the top surfaces of the LED dice304, e.g., due to the differences in the height of thebumps320 and thickness of the dice. When the singleoptical element302 is thermally bonded to the LED dice304, any differences in the height of the LED dice304 may be accommodated by the compliance of thebumps320.
• During the thermal bonding process of theoptical element302 to the LED dice304, the LED dice304 may shift laterally due to the heating and cooling of thesubmount306. With the use of somebumps320, such as Au, the compliance of thebumps320 can be inadequate to accommodate lateral shift of the LED dice304. Accordingly, the coefficient of thermal expansion of the optical element302 (CTE₃₀₂) should approximately match the coefficient of thermal expansion of the submount306 (CTE₃₀₆). With an approximate match between CTE₃₀₂and CTE₃₀₆any movement of the LED dice304 caused by the expansion and contraction of thesubmount306 will be approximately matched by the expansion and contraction of theoptical element302. A mismatch between CTE₃₀₂and CTE₃₀₆, on the other hand, can result in the detachment of the LED dice304 from theoptical element302 or other damage to theLED device300, during the heating and cooling of the thermal bonding process.
• With the use of sufficiently small LED dice304, the thermal expansion of the LED dice304 themselves during the thermal bonding process may be minimized. With the use of large LED dice304, however, the amount of thermal expansion of the LED dice304 during the thermal bonding process may be large and thus, the CTE for the LED dice304 also should be appropriately matched to the CTE of thesubmount306.
• The LED dice304 may be, e.g., InGaN, AlInGaP, or a combination of InGaN and AlInGaP devices. In one implementation, thesubmount302 may be manufactured from AlN, while theoptical element302 may be manufactured from, e.g., SLAM60 by Ohara Corporation, or NZK7 available from Schott Glass Technologies Incorporated. In another implementation, an Alumina submount306 may be used along with anoptical element302 manufactured from sapphire, Ohara Glass SLAH51 or Schott glass NLAF21. In some implementations, abulk filler305 between the LED dice304 and thesubmount306 may be used. Thebulk filler305 may be, e.g., silicone or glass. Thebulk filler305 may have good thermal conductivity and may approximately match the CTE of thesubmount306 and the dice304. If desired, a protective side coating may be applied alternatively or in addition to thebulk filler305.
• In one implementation, all of the LED dice304 may be the same type and produce different or approximately the same wavelengths of light. Alternatively, with an appropriate choice of LED dice304 and/or wavelength conversion materials, different wavelengths of light may be produced, e.g., blue, green and red. When LED dice304 are the same type, the CTE for the LED dice304 will be approximately the same. It may be desirable for the CTE of the LED dice304 to closely match the coefficient of thermal expansion of theoptical element302 and thesubmount306 to minimize the risk of detachment or damage to the LED dice304 during the thermal bonding process.
• In another implementation, the LED dice304 may be different types and produce different wavelengths of light. With the use of different types of LED dice, the CTE of the dice can vary making it difficult to match the CTE for all the LED dice304 with that of theoptical element302 and thesubmount306. Nevertheless, with a judicious choice of theoptical element302 andsubmount306 with CTEs that are as close as possible to that of the LED dice304, problems associated with detachment of the LED dice304 or other damage to thedevice300 during the thermal bonding process may be minimized. Additionally, with the use of relatively small LED dice304, e.g., the area smaller than approximately 1 mm², problems associated with thermal bonding a singleoptical element302 to multiple dice304 may also be reduced. The use of abulk filler305 may also prevent damage to the device during thermal processing or operation.
• As shown inFIG. 3, in one implementation, theoptical element302 may be coated with awavelength converting material310, such as a phosphor coating. In one embodiment, thewavelength converting material310 is YAG.FIG. 4 is a flow chart of one implementation of producing such a device. As illustrated inFIG. 4, the LED dice304 are mounted to the submount306 (step402) and theoptical element302 is bonded to the LED dice304 (step404). After theoptical element302 is bonded to the LED dice304, a layer of thewavelength converting material310 is deposited over the optical element302 (step406). The device can then be tested, e.g., by applying a voltage across the active regions of the LED dice304 and detecting the wavelengths of light produced by the device (step408). If the device does not produce the desired wavelengths (step410), the thickness of the wavelength converting material is altered (step411), e.g., by depositing additionalwavelength converting material310 over theoptical element302 or by removing some of the wavelength converting material by etching or dissolution and the device is again tested (step408). The process stops once the desired wavelengths of light are produced (step412).
• Thus, the thickness of thewavelength converting material310 coating is controlled in response to the light produced by the LED dice304 resulting in a highly reproducible correlated color temperature. Moreover, because the deposition of thewavelength converting material310 is in response to the specific wavelengths produced by the LED dice304, a variation in the wavelengths of light produced by LED dice304 can be accommodated. Accordingly, fewer LED dice304 will be rejected for producing light with wavelengths outside a useful range of wavelengths.
• It should be understood that the process of coating the optical element with a wavelength converting material may be applied to the embodiments shown inFIGS. 1B and 2 as well.
• In another implementation, the coating of wavelength converting material may be placed between the LED die and the optical element, e.g., within, over, or under thebonding layer322.FIG. 5, by way of example, illustrates an LED die502 mounted to asubmount504 and bonded to anoptical element506 viabonding layer508, where a layer ofwavelength converting material510 is disposed between thebonding layer508 and theoptical element506. Thewavelength converting material510 may be bonded to the bottom surface of theoptical element506 bybonding layer509 prior to or during the bonding theoptical element506 to the LED die502. Thewavelength converting material510 may be, e.g., a phosphor impregnated glass or wavelength converting ceramic that is formed independently and then bonded to the LED die502 andoptical element506. In some embodiments, thewavelength converting material510 may be bonded directly to one or both of the LED die502 andoptical element506. In one embodiment, theoptical element506, LED die502 andwavelength converting material510 may be bonded together simultaneously. In another embodiment, thewavelength converting material510 may be bonded first to theoptical element506 and subsequently bonded to the LED die502, e.g., where thebonding layer509 has a higher bonding temperature than thebonding layer508. A suitable wavelength converting material, such as a phosphor impregnated glass, is discussed in more detail in U.S. Ser. No. 10/863,980, filed on Jun. 9, 2004, by Paul S. Martin et al., entitled “Semiconductor Light Emitting Device with Pre-Fabricated Wavelength Converting Element”, which has the same assignee as the present application and is incorporated herein by reference.
• FIG. 6 illustrates another embodiment, similar to the embodiment shown inFIG. 5, except awavelength converting material520 is bonded directly to the LED die502 (and optionally over the edges of the LED die502) prior to or during bonding of theoptical element506. Thus, as shown inFIG. 6, thewavelength converting material520 is placed between the LED die502 and thebonding layer509. If desired, an additional layer of wavelength converting material may be deposited over theoptical element506 inFIGS. 5 and 6, as discussed above.
• In another implementation, the coating of wavelength converting material may be located over the LED die or dice remotely, e.g., on an envelope of glass, plastic, epoxy, or silicone with a hollow space between the envelope and the LED die or dice. If desired, the hollow space may be filled with a material such as silicone or epoxy.
• Related U.S. patent application having Serial No. application Ser. No. 10/987,241, filed Nov. 12, 2004, entitled “Bonding an Optical Element to a Light Emitting Device”, by Michael D. Camras et al, which has the same assignee as the present disclosure, and is incorporated herein by reference.
• FIG. 7 illustrates anarray600 ofLEDs602, which are mounted on aboard604. Theboard604 includeselectrical traces606 that are used to provide electrical contact to theLEDs602. TheLEDs602 may be phosphor converted devices manufactured, e.g., as described above. TheLEDs602 may each produce white light with different CCTs. By mixing the white light with different CCTs inarray600, a light with a desired CCT may be produced. If desired, theLEDs602 may be covered with atransparent element608 of e.g., glass, plastic, epoxy, or silicone. Thetransparent element608 may be filled, e.g., with epoxy or silicone, which assists the extracting and mixing of the light and to protect theLEDs602. It should be understood thatarray600 may include any number ofLEDs602 and that if desired, one or more of the LEDs may produce non-white light. Moreover, if desired, a plurality of theLEDs602 may be bonded to a singleoptical element603, or one or more of theLEDs602 may not includeoptical element603.
• As illustrated inFIG. 7, individual or groups ofLEDs602 may be independently controlled, e.g., bycontroller610, which is electrically connected to thetraces606 on theboard604. By independently controllingLEDs602 or groups ofLEDs602, a high color rendering, e.g., over 85, with a constant brightness may be achieved. Further, the white points produced by thearray600 may be tuneable over a large range of CCT, e.g., between 3000 K and 6000 K. By way of example, a number of phosphor-converted (PC) blue LEDs that produce white light may be used in combination with LEDs with different colors, such as blue, cyan, amber and red to produce a light with a desired CCT. As shown in the graph ofFIG. 8, the phosphor converted blue LEDs generates light with abroad spectrum702 in the green area in combination with a peak in the blue region. The thickness of the phosphor may be tuned to produce approximately equal peak values for both the green and blue parts of the spectrum.FIG. 9 shows a CIE chromaticity diagram for the spectrum shown inFIG. 8, which illustrates the x and y color coordinates752 above theblack bodyline754. Of course, PC LEDs that produce spectra having peaks in other area may be used if desired. Alternatively, if desired, PC LEDs that produce different spectra, i.e., white light having different CCTs may be used together.
• A majority of theLEDs602 in thearray600 ofFIG. 7 may be PC LEDs that generate the spectrum shown inFIG. 8. The remainingLEDs602 shown inFIG. 7 may be color LEDs, e.g., LEDs that produce blue, cyan, amber and red. The brightness of the color LEDs may be adjusted bycontroller610. The combination of fully powered PC LEDs with colored LEDs generates an approximately continuous spectrum, as illustrated inFIG. 10.FIG. 10 shows a graph with thespectrum702 from the PC LEDs along withspectra704,706,708 and710 from the blue, cyan, amber and red colored LEDs combined to formspectrum720. As illustrated in the portion of the CIE chromaticity diagram shown inFIG. 11, by varying the brightness of the colored LEDs, an area that covers part of theblack body line764 can be obtained. By way of example, one embodiment that included 29 PC LEDs and 12 color LEDs, e.g., 3 blue, 3 cyan, 3 amber, and 3 red, is capable of producing a brightness of 800 lumen with a color rendering between 85 and 95 and a CCT between 3200 K and 5800 K.FIG. 12 illustrates a portion of the CIE chromaticity diagram that illustrates variable CCT values for an array of 29 PC LEDs and 12 color LEDs. Of course, any number of PC LEDs and color LEDs may be used.
• Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims (25)

1. An apparatus comprising:

at least one phosphor converted light emitting device that produces light with a broad spectrum;

a plurality of color light emitting devices that produce light with a narrow spectrum; and

a controller for controlling the brightness of the light produced by the color light emitting devices, wherein the combination of the broad spectrum light produced by the at least one phosphor converted light emitting device and the narrow spectra light from the brightness controlled plurality of color light emitting devices produces a resulting spectrum that is more continuous than the broad spectrum produced by the at least one phosphor converted light emitting devices.

2. The apparatus ofclaim 1, further comprising a board, the at least one phosphor converted light emitting device and the plurality of color light emitting devices are mounted on the board.

3. The apparatus ofclaim 1, wherein there is a greater number of phosphor converted light emitting devices than color light emitting devices.

4. The apparatus ofclaim 3, wherein there is over twice as many phosphor converted light emitting devices as color light emitting devices.

5. The apparatus ofclaim 1, wherein the color light emitting devices produce blue, cyan, amber and red light.

6. The apparatus ofclaim 1, wherein the controller individually controls the brightness of the color light emitting devices to vary the correlated color temperature of the resulting spectrum.

7. A method comprising:

providing at least one phosphor converted light emitting device that produces light with a broad spectrum;

providing a plurality of color light emitting devices, each of which produces light with a narrow spectrum;

mixing the light produced by the at least one phosphor converted light emitting device and the plurality of color light emitting devices to produce light with a resulting spectrum that is more continuous than the broad spectrum produced by the at least one phosphor converted light emitting devices;

controlling the brightness of the light produced by the plurality of color light emitting devices to vary the correlated color temperature of the resulting spectrum.

8. The method ofclaim 7, further providing a board and mounting the at least one phosphor converted light emitting device and the plurality of color light emitting devices on the board.

9. The method ofclaim 7, wherein a greater number of phosphor converted light emitting devices are provided than color light emitting devices.

10. The method ofclaim 9, wherein over twice as many phosphor converted light emitting devices as color light emitting devices are provided.

11. The method ofclaim 7, wherein the color light emitting devices produce blue, cyan, amber and red light.

12. The method ofclaim 7, wherein the brightness of the light produced by each color light emitting device is controlled separately to vary the correlated color temperature.

13. The method ofclaim 7, wherein the brightness of each different colored light produced by the color light emitting devices is controlled separately to vary the correlated color temperature.

14. A method comprising:

providing a plurality of phosphor converted light emitting devices that produce light with a broad spectrum, the phosphor converted light emitting devices producing light with different correlated color temperature;

arranging the plurality of phosphor converted light emitting devices in an array; and

covering the array of phosphor converted light emitting devices with an optical element that assists mixing of the light with different correlated color temperatures to produce light with a desired correlated color temperature.

15. The method ofclaim 14, wherein the optical element is bonded to the phosphor converted light emitting devices.

16. The method ofclaim 14, wherein the optical element is a dome mounted over the phosphor converted light emitting devices and filled with an encapsulant.

17. The method ofclaim 14, wherein arranging the plurality of phosphor converted light emitting devices comprises mounting the plurality of phosphor converted light emitting devices to a board.

18. The method ofclaim 14, the method further comprising:

providing a plurality of color light emitting devices, each of which produces light with a narrow spectrum;

arranging the plurality of color light emitting devices in the array with the plurality of phosphor converted light emitting devices.

19. The method ofclaim 18, further comprising controlling the brightness of the light produced by the plurality of color light emitting devices to vary the correlated color temperature of the resulting spectrum.

20. An apparatus that produces broadband light with a desired correlated color temperature, the apparatus comprising:

an array of a plurality of phosphor converted light emitting devices that produce light with a broad spectrum with different correlated color temperatures; and

an optical element disposed over the array of the plurality of phosphor converted light emitting devices, the optical element mixing the light with different correlated color temperatures to produce light with the desired correlated color temperature.

21. The apparatus ofclaim 20, wherein the optical element is bonded to the phosphor converted light emitting devices.

22. The apparatus ofclaim 20, wherein the optical element is a dome mounted over the phosphor converted light emitting devices and filled with an encapsulant.

23. The apparatus ofclaim 20, further comprising a board to which the array of the plurality of phosphor converted light emitting devices is mounted.

24. The apparatus ofclaim 20, wherein the array further comprises a plurality of color light emitting devices.

25. The apparatus ofclaim 24, the apparatus further comprising a controller for controlling the brightness of the light produced by the plurality of color light emitting devices to vary the correlated color temperature to the desired correlated color temperature.

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