Backlight module with reflection member

A backlight module (20) includes a light guide plate (22), a light source (21), and a reflection plate (23). The light guide plate includes a light incidence surface (221) for receiving light, a light emitting surface (223) for emitting light, and a bottom surface (222). The light source is disposed adjacent the light incidence surface. The reflection plate disposed under the bottom surface includes a base (232), and a reflection layer (233) formed on the base. The reflection layer defines a number of diffraction grating units (231) at an outer surface thereof. 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. Further, if the process of fabrication of the diffraction grating units fails, only the reflection plate need be discarded. Thus the backlight module has a low mass manufacturing cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to backlight modules for liquid crystal displays, and particularly to a backlight module utilizing a reflection plate for controlling light emission.

2. Description of Prior Art

A typical liquid crystal display requires a backlight module in order to be able to provide uniform illumination. The performance of the backlight module greatly depends on a light guide plate employed therein. Means for enhancing the uniformity of light that is output from a light guide plate can be classified into two categories. The first category uses geometrical optics means, such as prisms or micro projections. The second category uses wave optics means, such as diffraction gratings. Light guide plates with multifarious configurations of micro projections and prisms have been developed, and some of these light guide plates can generate quite uniform light beams. However, the uniformity provided by projections is relatively low compared with light guide plates having gratings. This is because the gratings of the latter kind of light guide plate can be precisely configured to correspond to the wavelength band of visible light beams, thereby accurately controlling the uniformity of transmission of the light beams. Nevertheless, there are two main problems associated with gratings. Firstly, a grating is liable to become worn over time. Secondly, a grating generates spectral phenomena.

Referring toFIG. 4, U.S. Pat. No. 5,703,667 issued on Dec. 30, 1997 discloses a backlight module. The backlight module1comprises a light guide plate2having a light incidence surface2c,a bottom surface2band a light emitting surface2a.The backlight module1further comprises a fluorescent tube4disposed adjacent the light incidence surface2c,a reflection plate5disposed under the bottom surface2b,and a diffusing plate6and a prism plate7disposed on the light emitting surface2ain that order from bottom to top.

A plurality of reflective diffraction grating units3is provided on the bottom surface2b.Each diffraction grating unit3comprises a grating part parallel with the fluorescent tube4, and a non-grating part. Because all the grating parts of the diffraction grating units3are arranged in a same direction parallel to each other, the diffraction grating units3provide strong diffraction of light beams received from the fluorescent tube4.

The ratio of a grating part width to a non-grating part width in the diffraction grating units3becomes progressively larger with increasing distance away from the light incidence surface2c.Therefore, light beams that are available in large quantities at locations nearer to the light incidence surface2cundergo weaker diffraction, and light beams that are available only in small quantities at locations more remote from the light incidence surface2cundergo stronger diffraction. As a result, the light emitting surface2aprovides uniform outgoing light beams.

For precision, the diffraction grating units3can be fabricated at the bottom surface2bof the light guide plate2by way of injection molding, laser beam etching, electron beam etching, or another kind of precision process used in the semiconductor field. However, if the process of fabrication of the diffraction grating units3fails, the whole light guide plate2must be discarded. Further, the cost of the light guide plate2is high compared to the cost of other parts of the backlight module1. Defective light guide plates2can significantly increase the cost of mass manufacturing backlight modules1.

It is desired to provide a backlight module which overcomes the above-described problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a backlight module which has a low manufacturing cost and which yields high uniformity of outgoing light.

A backlight module of the present invention comprises a light guide plate, a light source, and a reflection plate. The light guide plate includes a light incidence surface for receiving light, a light emitting surface for emitting light, and a bottom surface opposite to the light emitting surface. The light source is disposed adjacent the light incidence surface. The reflection plate disposed under the bottom surface includes a base, and a reflection layer formed on the base. The reflection layer defines a plurality of diffraction rating units at an outer surface thereof.

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.

Furthermore, the diffraction grating units are fabricated in the reflection plate only. If the process of fabrication of the diffraction grating units fails, only the reflection plate need be discarded. The light guide plate remains intact and is not wasted. The cost of the reflection plate is lower than that of the light guide plate. Therefore the cost of mass manufacturing the backlight module is reduced.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a backlight module according to the preferred embodiment of the present invention is shown. The backlight module20comprises a linear light source21, a transparent plate-like light guide member22having a rectangular cross-section, a plate-like reflection member23, a diffusion plate24, and a prism plate25.

The light guide plate22comprises a light emitting surface223, a bottom surface222opposite to the light emitting surface223, and a light incidence surface221adjoining both the light emitting surface223and the bottom surface222. The light source21is a CCFL (cold cathode fluorescent lamp) disposed adjacent the light incidence surface221. The diffusion plate24is positioned adjacent the light emitting surface223. The prism plate25is disposed on the diffusion plate24.

Referring toFIG. 2, the reflection plate23disposed under the bottom surface222includes a base232, and a reflection layer233formed on the base232. The reflection layer233defines a number of diffraction grating units231at an outer surface thereof. Grating constants of the diffraction grating units231progressively decrease with increasing distance away from the light incidence surface21. Grating constants of the diffraction grating units231are in the range from 2-10 μm, and preferably 5 μm. Since the CCFL light source21mainly emits light beams in a direction perpendicular to the light incidence surface221, a grating direction of the diffraction grating units231is arranged to be parallel with the light incidence surface221for diffracting the light beams with maximum efficiency.

Grating constants of the diffraction grating units231progressively decrease as a function of increasing distance away from the light incidence surface21. Accordingly, diffractive capabilities of the diffraction grating units231progressively increase with increasing distance away from the light incidence surface221. Since the intensity of light beams decreases with increasing distance of propagation, the quantity of light beams received by the diffraction grating units231decreases with increasing distance away from the light incidence surface221. The distribution configuration of the diffraction grating units231in the reflection layer233compensates the light intensity at each diffraction grating unit231with a corresponding diffraction capability. Thus, the light guide plate22assures uniformity of light beams emitting from the light emitting surface223.

The base232is made of a polyester, and the reflection layer233is made of white polyethylene terephthalate. The reflection layer233is formed by way of coating a reflection material on the base232. The diffraction grating units231can be formed in the reflection layer233by way of injection molding, laser beam etching, electron beam etching, or another suitable kind of precision process used in the semiconductor field.

Referring toFIG. 3, in operation, the light source21emits light beams and the light beams are transmitted into the light guide plate22. Part of the light beams, for example the light beam b, emits from the bottom surface222of the light guide plate22, is diffracted and reflected by the diffraction grating units231, and is thus divided into light beams b1, b2, b3. The light beams b1, b2, b3re-enter the light guide plate22, and exit from the light-emitting surface223of the light guide plate22. The light beams b1, b2, b3, then sequentially pass through the diffusion sheet24and the prism sheet25to illuminate a liquid crystal panel (not shown).