Patent Application
20070200121
This invention relates to illumination devices using multi-colored light emitting diodes (LEDs) and, in particular, to techniques for obtaining better color uniformity across the light emitting area of an illumination device, such as a backlight for a liquid crystal display (LCD).
Liquid crystal displays (LCDs) are commonly used in cell phones, personal digital assistants, laptop computers, desktop monitors, and television applications. One embodiment of the present invention deals with a color, transmissive LCD that requires backlighting, where the backlight may contain red, green, and blue LEDs.
The backlight 12 ideally provides homogenous light to the back surface of the display. Providing homogenous white light using physically spaced LEDs is very difficult in a shallow backlight box. The backlight box has diffusively reflective bottom and side walls to mix the red, green, and blue light. The inner surfaces may be painted white. Mixing optics 16, such as a diffuser, improves the color mixing.
Above the mixing optics 16 are conventional LCD layers 18, typically consisting of polarizers, RGB filters, a liquid crystal layer, a thin film transistor array layer, and a ground plane layer. The electric fields created at each pixel location, by selectively energizing the thin film transistors at each pixel location, causes the liquid crystal layer to change the polarization of the white light at each pixel location. The RGB filters only allow the red, green, or blue component of the white light to be emitted at the corresponding RGB pixel locations. LCDs are well known and need not be further described.
As LED technology advances, the light output and efficiency of power LEDs increase, and fewer LEDs are needed to provide the light output needed for an LCD. Using fewer LEDs typically reduces the cost of the backlight. Increasing the pitch of the LEDs makes it more difficult to provide adequate color uniformity across the LCD screen, especially with a relatively shallow backlight box.
Therefore, what is needed are new techniques for improving the color uniformity of a backlight using LEDs across an LCD.
Various techniques are described herein for creating an improved backlight for backlighting an LCD. In one embodiment, the backlight uses an array of red, green, and blue LEDs in a mixing chamber. The mixing chamber has reflective walls, a reflective bottom surface, and a light emitting top area for illuminating the LCD layers overlying the mixing chamber.
The LEDs in the backlight are arranged in clusters. In one example, each cluster has six LEDs with two reds, two greens, and two blues, and the clusters form a 6×5 array for a 32 inch television screen. Various sequences of the RGB LEDs in the cluster are described. Other sizes of clusters and arrays are also described.
In one embodiment, two types of clusters are used, and each cluster has the same number of RGB LEDs so as to have the same white point. All clusters in the same row are the same. The rows alternate between clusters of the first type and clusters of the second type to improve color uniformity. In one embodiment, the sequence in a cluster is symmetrical. In another embodiment, the sequence in a cluster is asymmetrical. Preferably, the number of rows is odd so that each of the four corners has the same cluster.
In another embodiment, there are two types of clusters in each row, and the clusters alternate. The clusters along a column also alternate to produce a checkerboard pattern of clusters. This also improves color uniformity across the LCD.
The arrangement, selection, and control of the multicolored LEDs may be tailored to achieve any desired white point specified by the display manufacturer.
Embodiments of the present invention provide improved color uniformity over a large area. Applications of embodiments of the invention include general illumination and backlighting.
The backlight may be formed of aluminum sheeting, and its inner walls 21 and base 22 are coated with a diffusively reflective material, such as white paint. Various types of reflective material are commercially available and are well known. In another embodiment, the side walls are covered with a specular film. In one embodiment, the depth of the backlight is 25-40 mm.
A first cluster type 24 is formed of a sequence of six LEDs: RGBBGR. The pattern is symmetric. Applicants have found that symmetric clusters with the same number of LEDs of each color provide a color uniformity that is better than asymmetric clusters such as RGBRGB, etc.
In
In the example above, the LEDs in the 2 and 5 positions in a cluster do not change position between the two cluster types. The LEDs in positions 1 and 3 switch, and the LEDs in positions 4 and 6 switch, between the cluster types. This particular change in pattern is advantageous since, in the top row, two reds and two blues are grouped together along the row, while the greens are separated. Placing two LEDs of the same color together is detrimental to color mixing but is unavoidable in a symmetric cluster pattern having equal numbers of the LED colors. In the next row with the clusters 26, the two reds and two blues do not align with the two reds and two blues in the top row, thus preventing concentrations of red and blue.
The rows of clusters 24 and 26 alternate. In the example of a 32 inch TV screen, there are 5 rows (180 LEDs total). The number of rows depends on the particular LEDs used, the size of the backlight, and the light output specifications of the backlight. It is beneficial to have the same cluster type in the four corners of the backlight to cause the color at each corner to be identical. This is achieved by making the number of rows an odd number.
Other cluster types that may be used in the backlight of
Although the examples show LEDs arranged in row and columns, other patterns may also be used. Such patterns include zig-zag, wavy, circular, and polygonal patterns. Each cluster may also be in a shape other than a line, such as circular, polygonal, etc.
The same checkerboard pattern can be made with any of the 6-LED clusters, described above, for a further improvement in color uniformity.
Clusters having six LEDs with two LEDs of the same color provide a higher reliability than clusters with four or five LEDs without redundant LED colors. In a cluster where one of the RGB components is provided by only a single LED, failure of the LED, including a significant diminishing in brightness, has a noticeable effect on the color output of the cluster, leading to nonuniformity of color across the LCD. Failure of one LED in a six-LED cluster will have much less of an adverse effect.
The above embodiments are improvements over the LED layout described in U.S. patent application publication 2005/0001537 A1, assigned to Lumileds Lighting, LLC, where a strip of LEDs forming an entire row is reversed for alternating rows. That technique simply reverses the sequence of LEDs while using the same clusters of LEDs in all rows. In Applicant's arrangements, multiple cluster types are used in the backlight.
The white point of the backlight may be controlled by controlling the current to each LED color. The LEDs of a single color may be connected in a combination of series and parallel and connected to a controllable current source. For the best color uniformity, all LEDs of the same color should have a similar flux and color point so the color output of each cluster is substantially the same. The white point for all clusters can then be adjusted by controlling the current to the red, green, and blue LEDs.
LEDs of colors other than red, green, and blue may also be used in the LCD 50 to create white light.
Having described the invention in detail, those skilled in the art will appreciate that given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.
