Patent Application
20100008070
The present invention relates to light guides for Liquid Crystal Displays (LCDs), and in particular to the use of a diffraction grating to redirect light from a light source.
LCD displays are used in televisions, computer monitors, mobile phones and other devices. LCD displays use a back light module to illuminate the LCD display. There are two popular methods for the placement of the light source. For large screens the light source is placed behind the LCD display. For smaller screens the light source is placed on the side of a light guide. The most common light source used in television is CCFL (cold cathode fluorescent lamp), which requires a high voltage source. A more energy efficient light source is the Light Emitting Diode (LED). However, there are obstacles to using LEDs. Fluorescent light (CCFL) is typically used for large screen TVs, while smaller displays, such as mobile phones, can use LEDs.
There are many examples of improvements on this basic back light module. For example, U.S. Pat. No. 5,392,199 and No. 5,647,655 show a reflecting plate below a light guide, and a diffusion layer above it. The ends of the cylindrical reflector around the light source are bonded to the light guide. U.S. Pat. No. 5,779,337 shows a light guide with a plurality of projections or grooves arrayed on the light emitting (top) surface to cause more light to be emitted. U.S. Pat. No. 6,115,058 shows an LCD using polarizing filters and a Fresnel lens 201 bonded to the back of the LCD. U.S. Pat. No. 6,710,829 shows a press to generate a plurality of recesses of different sizes on the illuminating plate of a back light plate. U.S. Pat. No. 7,160,018 shows a back light plate with an array of prisms on the front or top, and reflective dots on the bottom surface. U.S. Pat. No. 7,270,466 shows a bottom reflecting surface with different sized and spaced protrusions on the two sides of the reflecting surface.
Diffraction gratings have been used to spread the light in light guides to reduce the need for the number of diffusers shown in
Other patents discussing diffraction gratings in light guides include U.S. Pat. No. 7,085,056 “Light guide plate with diffraction gratings and backlight,” U.S. Pat. No. 7,364,340; U.S. Pat. No. 7,242,838, which describes a light source on the side with a reflection layer formed on the base. The reflection layer defines a number of diffraction grating units. Grating constants of the diffraction grating units progressively decrease with increasing distance away from the light incidence surface. This enables the light emitting surface to output highly uniform light. See also published application No. 20070058394.
This present invention provides a back light module which uses first diffractive optics to couple light into a light guide and uses second diffractive optics to extract light from the light guide such that the light emerging from the back light module is uniform. This is accomplished by having an input grating adjacent the light sources at the bottom, or back of the light guide, with the output grating at the top, or front of the light guide. This enables the portions of the output grating to be directly over the light sources, unlike the prior art Nokia U.S. Pat. No. 6,598,987, which puts both the input and output gratings on the same bottom surface.
In one embodiment, input diffractive grating diffracts the entering light so that most of the light reflects off the top surface, and then off the bottom and other surfaces, spreading the light through the light guide before it exits. The top surface has micro diffractive couplers for pulling light out of the light guide. The portion of the top surface directly opposite the light sources has a less dense arrangement of diffractive couplers, so that less of the initial light from the light sources exits upon the first contact with the top surface. The arrangement of diffractive couplers is denser in areas farther away from the light source.
In an alternate embodiment, a modular light guide with angled light guide surfaces to couple in light from the light source is used, instead of the diffractive in-couplers. An array of the modules is formed, with a light source beneath a v-shape formed by the intersection of the modules, with the angled v surfaces coupling light into the light guide.
In one embodiment, the light sources 201 are fluorescent (CCFL) tubes. Alternate embodiments use LEDs. The diffractive couplers are elongated, matching the tubes. The diffractive couplers are designed to diffract the light at an angle greater than the total internal reflections. In addition, the placement of the light source relative to the grating and the length of the diffractive couplers is also optimized. The diffractive couplers, in one embodiment, are an array of micro grooves. Although shown as a separate element attached to the light guide, the diffractive couplers can be laminated to the light guide or can be directly embossed on the light guide surface.
The out couplers 210 are micro arrays, with spacing between them. The arrays are not very dense above the light source, where the 0 angle (undiffracted) light will hit the top surface first. Only enough light is coupled out to match the amount of light that will be coupled out in between the light sources, to give uniform illumination. Because of the variations in materials and light sources, the variations in the pattern of the micro arrays is typically determined through a process of trial and error.
The overall pattern of the input and output diffractive coupler arrays can be generated on a thin film by electron beam lithography or other means. This can be used as a master to impress the pattern on the light guide as the light guide is molded or pressed. This will allow the light guide to be manufactured with integral in-coupling and out-coupling diffractive gratings. This allows for a thinner light guide structure.
In one embodiment, a diffuser layer is used. The diffraction gratings will cause some separation of the light by wavelength, which may affect the desired white color. To maintain the white color, a diffusion layer is added over the diffraction layer.
Two units of structure 400 are connected as shown in
An apex 412 is formed below the top surface of the light guide by the overlapping of the angled top edge of left unit 401 over right unit 401. This prevents light from the light source directly exiting the light guide above the light source, preventing a bright spot. No significant light will go through the apex, instead being directed left or right by the angled edges.
A large back light module can be fabricated by cascading more of the basic module 400. In one embodiment, a series of parallel modules 401 are used, each 2-3 inches wide, with a light source between each one, and optionally light sources at the outer edge of the last modules. Since the modules are relatively narrow, a constant diffraction pattern can be used. Alternately, the pattern can vary, such as by diffraction out less light near the edges, and more near the center. An example of such variation is described in the above-referenced co-pending application. The modules can be assembled by gluing them together, using a press fit with notch patterns, heating the modules to cause the plastic to partially melt together at the joints, or by any other process.
In one embodiment, a diffuser layer is placed over each unit 410 to diffuse the separation of colors caused by the diffraction patterns, as discussed above. Such a diffuser layer can be similar to one of the layers shown in
It is to be understood that the examples and embodiments described above are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. For example, an array of LEDs could be used instead of CCFL tubes. LEDs of different colors are arranged to produce a combined white light, with the diffractive patterns being optimized for the wavelengths of each color. Alternately, instead of parallel modules, a two-dimensional grid could be used. Therefore, the above description should not be understood as limiting the scope of the invention as defined by the claims.
This application is related to co-pending application Ser. No. 12/035,276 entitled “Back Light Module With Micro Fresnel Lens” which was filed on Feb. 21, 2008, which is hereby incorporated herein in its entirety.
