An Apple patent (number 20110069511) for a LED backlight for display systems has appeared at the US Patent & Trademark Office. It relates generally to device display systems, and more particularly to an LED backlight with highly uniform color for illuminating display systems.
The patent is for a LED backlight method for display systems comprising receiving a plurality of light emitting diodes categorized into a plurality of bins, wherein each bin references a separate range of white point colors, and determining an optimal order for mounting the plurality of light emitting diodes at spatially distributed positions, the plurality of light emitting diodes comprising white point colors associated with separate bins, wherein the optimal order of the plurality of light emitting diodes produces a light of a desired white point color when the light outputs of the plurality of light emitting diodes are mixed. The inventors are Wei Chen, Jun Qi, Victor Yin and John Zhong.
Here's Apple's background and summary of the invention: "As computer technology has advanced, the demand for portable computer systems, such as laptops, has increased. Portable computers have dramatically increased the mobility of computing power for the computer user. Since the first portable computer, manufacturers have increased computer mobility by decreasing the size, weight, and power demands of portable computers, increasing battery life, and increasing performance.
"The monitors presently used contribute greatly to the overall size and weight of the portable computer. The monitor must be of a sufficient size, brightness, and clarity to provide the user with readable images. In order to achieve these requirements, monitors place a large demand on available power resources and are therefore a significant contributor indirectly as well as directly to the weight of the portable computer.
"Typically, portable computer monitors utilize a liquid crystal display system. The liquid crystal display systems typically include a top plastic or glass panel and a bottom plastic or glass panel, having a liquid crystal display of thin film transistors and liquid crystal material in between. These systems also utilize a backlight system that typically includes a diffuser for passing light evenly to the liquid crystal display, a cold cathode fluorescent lamp ('CCFL') for producing light, a reflector for directing the light toward the diffuser, and a light pipe located between the diffuser and the reflector to spread light to the entire surface of the diffuser.
"The use of conventional CCFL liquid crystal display systems in the monitors of portable computers, however, creates a limiting factor in the continuing effort to reduce the size and weight of portable computers. CCFL technology has not kept pace with advances in other technologies that have reduced the size and weight of many of the other display components. Today, one of the major limitations in further reducing the thickness and weight of the display is therefore the CCFL illumination system.
"Light-emitting diode ('LED') technology offers attractive alternatives to the CCFL. LEDs are much thinner than the CCFL and do not require many of the weighty power supply systems of the CCFL. Compared to backlights using CCFLs, backlights with LEDs have many benefits, including lighter weight, higher brightness, higher color purity, larger color gamut, longer lifetime, and mercury-free composition. However, while a single CCFL can light an entire display, multiple LEDs are needed to light comparable displays.
"A challenge with utilizing multiple LEDs, particularly when distributed in large arrays, is maintaining uniformity of color throughout the large number of LEDs. The color balance and spectra of the LEDs is governed by numerous factors such as manufacturing variances and the LED phosphorescence. For example, white LEDs are often actually blue LEDs with a complimentary yellow phosphor dot on the front of the LED. Depending upon manufacturing precision (and thus, related manufacturing costs), actual colors may therefore vary from, for example, slightly blue to slightly pink, often following a distribution curve in which many of the LEDs vary from the desired white point color. This can have a negative impact on the color uniformity of LCD displays. Understandably, reducing or compensating for such variability increases costs and complexity significantly as the number of LEDs increases in larger display configurations and environments.
"One solution is to select and utilize only those LEDs that provide the desired white point color. Such a solution, however, causes costs to be very high since only a fraction of the LED production can be utilized.
"Another solution is to compensate for LED color variations, such as by matching LEDs to one another and then filtering the light output to adjust the color to the desired white point color. This reduces the overall cost of the LEDs since most or all of the LEDs can be utilized. However, the additional color filters represent costly additional physical elements, oftentimes requiring a large number of available filter colors of finely varying shades and gradations. It can also increase the thickness of the display. An additional disadvantage is that such filters absorb light, causing the net brightness of the LED light sources to be reduced.
"Thus, if LEDs are to become a viable alternative to CCFLs, an economical and practical solution must be found to utilize a large number of non-matching LEDs while maintaining uniformity of color in the display panels in which the LEDs are utilized.
"In view of ever-increasing commercial competitive pressures, increasing consumer expectations, and diminishing opportunities for meaningful product differentiation in the marketplace, it is increasingly critical that answers be found to these problems. Moreover, the ever-increasing need to save costs, improve efficiencies, improve performance, and meet such competitive pressures adds even greater urgency to the critical necessity that answers be found to these problems.
"Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.
"The present invention provides an LED backlight method and apparatus for display systems. A plurality of light emitting diodes having different white point colors is provided. At least two of the light emitting diodes having different white point colors are selected to produce a light of a predetermined white point color when the light outputs of the selected light emitting diodes are mixed. The selected light emitting diodes are mounted on a display panel in a predetermined order at spatially distributed positions for mixing their light outputs to produce the light of the predetermined white point color to illuminate the display panel with the light of the predetermined white point color."
-- Dennis Sellers
