Method and apparatus for producing untainted white light using off-white light emitting diodes

[0001] The present invention relates to a method and apparatus for producing untainted white light using off-white light emitting diodes (LED's). More particularly, the present invention pertains to a way of combining different hues of white light from complimentary LED's to create a more “pure” white light.

[0002] A white LED couples two light emitting materials. The first is a blue light emitting diode made of semiconductor material capable of emitting radiation (i.e. the “initial radiation”) when electric current flows through it. The second is a yellow fluorescent or luminescent wavelength-converting material that absorbs a portion of the initial radiation and emits light (i.e., the “second radiation”) of a wavelength different from the initial radiation. The resultant light is the combination of the second radiation with the unconverted portion of the initial radiation. In a white LED, the diode emits blue light for the initial radiation, and the wavelength-converting material emits yellow light for the second radiation.

[0003] One desired resultant radiation in white LED's is untainted white light. There are many different types of white light, e.g. bluish-white, also known as cool-white, and yellowish-white, also known as warm-white. “Pure” white light, i.e. untainted white light, is desired in situations where the equivalent of daylight is needed, such as the flash for an image capturing device. “Pure” white light has been quantified. FIG. 1 is the 1931 CIE (Commission International d'Elchairge) Chromaticity Diagram. The dotted line 110 represents the black body curve. The color of radiation from a black body is dependent only on its temperature. In the lighting industry, it is common to designate a white color with its associated color temperature. Point 100 is “pure,” untainted white light, and its associated color temperature is 6500 Kelvins. This is, incidentally, the reference white for the National Television System Committee. Point 100 is the desired hue for a white light emitting device like an image capturing device.

[0004] Most white LED's employ a common yellow phosphor such as Cerium activated Yttrium Aluminum Garnet (YAG:Ce) as the wavelength converting material. To achieve untainted white light with this phosphor, the blue initial radiation typically has a wavelength which falls between 465 and 470 nanometers. If the initial radiation is too greenish-blue (i.e. greater than 470 nm), the resultant white light will be greenish. If the initial radiation is too purplish-blue (i.e. less than 465 nm), the resultant white light will be purplish. Both of these tainted hues of white can be perceived by the human eye, and are known as “impure” whites. These tainted hues have color coordinates that lie distal from the black body curve in FIG. 1. If either of these impure white lights is used in the flash for an image capturing device, the resulting images will also be tainted.

[0005] The color of blue light emitted by blue semiconductor material as fabricated by epiaxial semiconductor growth processes typically ranges from 460–480 nm despite having a controlled process. Consequently, only about 25% of the available blue semiconductor material fabricated by this process can be used in diodes for “pure” white LED's. The remaining 75% emits blue light which is either too greenish or too purplish and cannot be used for this purpose. Therefore the production cost of “pure” white LED's is very high.

[0006] Thus, there is a need in the industry for a method or apparatus for generating untainted white light from LED's in a more economically feasible manner. The present invention provides a unique, novel solution to this problem.

[0007] A white light emitting device by using off-white light emitting diodes (LEDs) is disclosed. Rather than be limited to using just pure white LED's, the white light emitting device arranges those LED's which exhibit off-white colors in a manner such that the combination of light emanating from these off-white LEDs produces a radiation which appears substantially pure white to the human eye.

[0019] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements in which:

[0020] The result of combining the initial and second radiation in an LED can be illustrated on the Chromaticity Diagram (FIG. 1). A line is drawn between the points representing the color of the initial radiation and the second radiation, in this case blue and yellow respectively. The resultant light has a color coordinate point that lies along this line. Point “B” is blue light. Point “Y” is yellow light. The color coordinate point of “pure” white light lies along line B-Y, in the vicinity of the black body curve. Point “GB” is greenish-blue light. Point “GW,” or greenish-white light, lies along line GB-Y. Point “PB” is purplish-blue light. Point “PW,” or purplish-white light, lies along line PB-Y.

[0021] In one embodiment of the invention, the LED's are chosen such that an LED which has a color coordinate on one side of the black body curve is arranged close to another LED that has a color coordinate that lies on the other side of the black body curve. The resultant color lies along the locus of the straight line joining these color coordinates and therefore lies closer to the black body curve. White LED's with greenish-white and purplish-white colors can be suitably arranged to yield a radiation that appears substantially white to the eye.

[0022] In one embodiment of the invention, the greenish-white LED is made using a blue diode with 480 nm color and the purplish-white LED made using a blue diode with 460 nm color. In an alternative embodiment, the greenish-white LED is made using a blue diode with 475 nm color and the purplish-white LED is made using a blue diode with 465 nm color.

[0023] In this way, the range of blue diodes that can be used is wider than 5 nm. Consequently, the rate of utilization of blue diodes is increased and the cost of production is decreased.

[0024] Using FIG. 1, the color coordinates of the combination of any two points on the Chromaticity Diagram lies along the line connecting these two points. Hence, point 100 lies on line GW-PW in the vicinity of the black body curve and is substantially a “pure” white light. The proportions of the greenish-white light and the purplish-white are suitably chosen such to yield white light at point 100, close to or at the black body curve.

[0025] FIG. 2A illustrates a system of sorting blue diodes according to the wavelength of the light they emit. White LED's are them assembled using the divided blue diodes. Said LED's are then arranged on a device such that their combined light is untainted white. Hence, white LED's can be made using 100% of blue semiconductor material produced by conventional processes. Blue diodes are divided into three groups, PB, B, and GB, depending on the wavelength of the blue light of each. Group PB consists of purplish-blue diodes which emit light in the 460–465 nm range. Group B consists of blue diodes which emit light in the 465–470 nm range. Group GB consists of greenish-blue diodes emitting light in the 470–480 nm range.

[0026] FIG. 2B illustrate an alternative system of sorting off-white and white LED's to be used to create “pure” white light. In this way, white LED's can be made using 100% of blue semiconductor material produced by conventional processes. White LEDs are divided into three groups, PW, W, and GW, depending on the color coordinates of the white resultant light of each LED. Group PW consists of purplish-white LED's made with diodes emitting light in the 460–465 nm range. Group W consists of “pure” white LED's made with diodes emitting light in the 465–470 nm range. Group GW consists of greenish-white LED's made with diodes emitting light in the 470–480 nm range.

[0027] FIG. 3 illustrates a possible arrangement of three white LEDs, one from each group. Array 300 is a 1×3 matrix. In a preferred embodiment, LED 310 is from Group GW, LED 320 is from Group PW, and LED 330 is from Group W (see FIG. 2). The combination of the light of these three LEDs is pure white light, as illustrated on the Chromaticity Diagram. Experimentation has shown that the actual arrangement of LEDs 310, 320 & 330 does not matter, the resultant light of any arrangement is pure white.

[0028] In FIG. 3, the diodes are coupled with a substrate 302 that has electrically conductive traces (not shown). Bond wires 304 are made from one end of the LED terminal to a conductive trace on the said substrate. A layer of encapsulant and phosphor mixture 306 covers the diodes.

[0029] FIG. 4 illustrates an array 400 of four white LEDs arranged in a 2×2 matrix. In a preferred embodiment, LEDs 410 and 420 are from Group GW, while LEDs 430 and 440 are from Group PW (see FIG. 2). In a preferred embodiment, the LEDs are arranged with 410–430 for the first row and 420–440 for the second row, as shown in FIG. 4. Experimentation has shown that the actual arrangement of LEDs 410, 420, 430 & 440 does not matter, the resultant light of any arrangement of these four LEDs is pure white.

[0030] In FIG. 4, the diodes of the LEDs are coupled with a substrate 402 that has electrically conductive traces (not shown). Bond wires 404 are made from one end of the LED terminal to a conductive trace on the said substrate. A layer of encapsulant and phosphor mixture 406 covers the diodes.

[0031] FIG. 5 illustrates a preferred alternative embodiment, wherein the diodes are placed in cavities in the substrate. The cavities act as reflectors, collecting light and directing it out in the desired direction. The said cavity also acts as a receptacle whereby it can be first-filled with the encapsulant/phosphor mixture, and the diode is sandwiched between this and a lens of encapsulant only. A secondary optical lens, advantageously placed over the diode, further controls and enhances the radiation pattern of light. In a preferred embodiment, a fresnel lens is used.

[0032] The encapsulant is preferably an epoxy material, but generally polymers such as thermoplastics and thermosets are suitable. Silicone or glass could also be used. The encapsulant is preferably produced by injection or transfer molding. A casting process could also be used. Alternatively, the encapsulant can be a cap placed over the diodes.

[0033] In FIG. 5, the diodes are coupled with a substrate 502 that has electrically conductive traces (not shown). The said substrate has cavities 501 which act as reflectors. Bond wires 504 are made from one end of the LED terminal to a conductive trace on the said substrate. A layer of encapsulant/phosphor mixture covers the diodes and at least partially fills the cavities. A second, lens shaped layer of encapsulant only 506 at least partially covers each diode. Alternatively, the encapsulant 506 comprises the encapsulant/phosphor mixture. A secondary lens 508 is assembled on the lens side of the LED array.

[0034] The arrangement of LED's is not limited to 1×3 or 2×2 matrices. Any array of m×n rows can be used, so long as LED's from Group PW are complimented by LED's from Group GW, and vice versa (see FIG. 2). The minimum is two LED's arranged in a 1×2 matrix. LED's from Group W do not require a complimentary LED, as they produce pure white light on their own. Furthermore, the invention is not limited to an orderly row-and-column array, but is equally effective in LED arrays of any systematic 2-dimensional or 3-dimensional sort.

[0035] Complimentary white LED's can be arranged on a substrate such as a leadframe, printed circuit board, flexible substrate, glass or ceramic. Alternatively, a plurality of complimentary white LED device components, such as lamps of the through-hole type or chipLED and PLCC of the surface mount type, can be arranged on another substrate, such as a printed circuit board, to create pure white light. These components are typically placed onto the substrate using an automated pick and place equipment and then soldered in place by heat exposure.

[0036] FIG. 6 is a representation of an exemplary embodiment of the invention, an image capturing device. The image capturing device shown here is a cell phone which has a digital camera on it. A 1×3 array 601 (see FIG. 3) provides a flash of light so that photographs can be taken in cloudy, dark, or night-time conditions.