A simple iterative optimization method is developed to improve the exiting light uniformity of the edge-lit backlight module. The method uses the relative deviation of the average luminous flux of the entire light guide plate (LGP) and the luminous flux of each region as constraint criterion and the ratio of the luminous flux of each region to the average luminous flux of the LGP as adjustment coefficient to adjust the density of the microstructures in each region. This process loops until the relative deviations of all regions meet a prespecified threshold value. A LGP based on microprism structure is presented to validate the method. The initial values, which represent the size of the microstructures manufactured on the bottom surfaces of the LGP,

Liquid crystal display (LCD) is widely used in our life for its advantages of low cost, low power consumption, no radiation, lightweight, and high video quality. However, the LCD panel is not a self-emissive display device; a uniform illumination backlight is needed to illuminate the LCD panel for image display. Particularly for small and medium sized LCD panels, a high performance light guide plate (LGP) is the most important component of the LCD backlight module. In the past few years, researchers have made efforts to propose new structures of LGPs to increase the efficiency of the LCD backlight module [

Different from the previous works, in this study, we introduce an iterative method for the uniformity improvement of the edge-lit backlight module. The method presented in this work only employs the relative deviation of the average luminous flux of the entire LGP and the luminous flux of each region as constraint criterion and the ratio of the luminous flux of each region to the average luminous flux of the LGP as adjustment coefficient to adjust the density of the microstructures in each region. After a proper initial value of the microstructures across the entire LGP is selected, the method can automatically search for the optimal microstructure distribution on the bottom surface of LGP. Compared with other methods, it is simple and does not need any complex mathematical calculations or a large number of repetitive trial-and-error processes.

Figure

The microprism LGP used to demonstrate the iterative method.

Based on the design principle of LGP, to achieve uniform illumination at the light exiting surface of LGP, the density of the microstructures on the bottom surface of LGP should increase gradually with the increase of distance from the light source. However, it is difficult to directly carry out a mathematical expression for the optimal microstructure distribution on the bottom surface of LGP by mathematical derivation method or analytical method because the microstructure distribution is usually a nonlinear function for a uniform exiting light. A widely used method for LGP uniformity optimization is regional partition method; the method first divides the microstructures on the bottom surface of LGP into several independent regions and then uses proper optimization method to adjust the density of microstructures in each region until uniform illumination is obtained [

According to the contents described above, in this study, the LGP is first divided into

The diagram of the regional partition method for the LGP and the mechanism of the light rays redirected by microstructures.

By using optical simulation software, we could obtain the total luminous flux

For uniform illumination, the luminous flux of each region

Note that (

After the size of the microstructures in each region is calculated by (

The flow chart of the iterative method.

Based on the design procedure described above, a LGP based on microprism structure is optimized to demonstrate the effectiveness of the iterative method. It is a rectangular shape with 2.5-inch size, and the length, width, and thickness of the LGP are 60 mm, 40 mm, and 3 mm, respectively. It is made of PMMA with a refractive index of

The initial values

Initial values

Initial values

The initial values

Initial values

Table

Initial values _{
0}-dependent iterations.

Microprism | |||||||
---|---|---|---|---|---|---|---|

0.05 | 0.06 | 0.07 | 0.075 | 0.08 | 0.09 | 0.10 | |

Iterations | 5 | 6 | 7 | 6 | 8 | 14 | 17 |

According to the discussions above, we note that 0.075 mm is an optimal initial value for a better result. Thus, the nine-point method is used to evaluate the uniformity of the edge-lit backlight module when the initial value of the microprisms is 0.075 mm. The uniformity is defined as the ratio of the minimum luminance to the maximum luminance and is expressed as

Irradiance maps before and after optimization of the microprism LGP.

Before optimization

After optimization

In this study, we developed a simple iterative optimization method to improve the illumination uniformity of the edge-lit backlight module. A 2.5-inch LGP based on microprism structure was constructed to demonstrate the feasibility of the method and the calculation principle of the iterative method was described in detail. The dependencies of the luminous fluxes, microstructure density distributions, and the relative deviations on the initial values were discussed. The result showed that the final luminance uniformity of the backlight is better than 90% with a proper initial value.

The authors declare that they have no competing interests.

This work is supported by scientific research fund of Yunnan Open University under Grant no. 2014Y011 and by Yunnan Province Department of Education Fund under Grant no. 2014Z143.