LED Backlight Structure for AMLCD Monitors with Improved Color Uniformity
Original Publication Date: 2002-Apr-01
Included in the Prior Art Database: 2003-Jun-20
LED Backlight Structure for AMLCD Monitors with Improved Color Uniformity Disclosed is a novel backlight structure for color mixing when Red, Green, and Blue light emitting diodes are used. It is desirable to replace the CCFL (cold cathode fluorescent lamp) light source currently used in flat panel liquid crystal displays with light emitting diodes (LEDs) to improve the color gamut and to avoid the use of Mercury which is contained in CCFLs and is an environmental hazard. The recent development of large size, high power and very efficient LEDs  has made it much more practical to make an LED based backlight for AMLCD (active matrix liquid crystal display) monitors since the number of LEDs required can be reduced from many hundreds to one hundred or fewer. This reduces the total cost for the LEDs and the associated costs for assembly and printed circuit boards. For cell phones, PDA (personal digital assistants), and other small devices, "White" or single color LEDs are used but for a monitor display, individual Red, Green, and Blue LEDs would be desirable for improved color gamut, efficiency, and to permit electronic adjustment of the white point by changing the relative intensity of the different colors. Adjusting the white point by changing the relative intensity of the different colors has advantages over the current methods which reassign the R (Red), G (Green), and B (Blue) data values and reduce the total number of available colors. There are a number of patents (see US Pat. 5,727,862 for example) on using LEDs to uniformity illuminate a display backlight but these generally are for using small low power LEDs which are either White or of a single color. The use of fewer, larger, high power LEDs of three individual colors (R, G B) creates a new problem in that not only must brightness uniformity be considered, but the colors must also be uniformly mixed together in as compact a space as possible. Color uniformity is critical since inadequate mixing of the three colors results in color shading across the display, which is more visible than brightness nonuniformity. Consider an 18.1" diagonal display with a 33% aperture ratio. Approximately 80 LEDs (20 Red, 20 Blue and 40 Green) would be required to provide adequate brightness. If these LEDs were distributed along the top and bottom edges of the display and coupled into a light guide (about 380 mm wide) in an R-G-B-G etc... pattern, the R B LED's would be almost 20 mm apart. The light guide would need to be extended a significant distance to allow the different colors to be uniformly mixed. This invention provides a novel backlight structure which permits the colors to be mixed with a minimal increase in space and cost. In our invention additional light guides are formed behind the display light guide to which the LEDs are coupled to allow for color mixing prior to the light reaching the display light guide. The light would be transferred to the display light guide by using mirrors or total internal reflection (TIR) off angled surfaces as shown below.