在3C產品廣泛使用的年代，如何製造好的顯示器是各大廠牌爭相研究的課題之一。目前最廣泛應用於液晶顯示器背光模組的光源為LED，其具備環保、高效等優勢，迅速成為產業研發及科學研究的熱門主題。一般LED的出光場型為Lambertian分布，在本篇論文中，使用了一次光線追跡法，調整光源的出光場型分布，讓光場分布以最理想的方式進入背光模組中，而達到優良顯示器的條件。
經過模擬計算，本篇論文所提出的顯示器背光的均勻性可以達到70%以上，此外，本設計具有相當高的泛用性，可以使用於各種尺寸的顯示器上，不必再因應不同的大小而重新設計。同時，本篇論文所提出的顯示器背光設計中省去增亮膜的使用，可降低背光模組的整體製作成本。

Design of Backlight Lightguide Unit for Various Monitor Sizes Using the One-Time Ray Tracing Optimization Method

Chi Wang
Shu-Chun Chu
Department of Physics, National Cheng Kung University

SUMMARY

This article proposes a new way to fabricate the lightguide for backlight modules of liquid-crystal displayers. Implementing the One-Time Ray-Tracing Optimization Method proposed in earlier studies from our laboratory, we successfully design the lightguide unit. The units are separate pieces of lightguide which can be placed next to one another to replace the ordinary one-pieced lightguide. The units provide fair lighting properties for the backlight, and can be used in different types of monitors.

Key Words: backlight module, lightguide

INTRODUCTION

This article is focused on the design of the lightguide of a backlight module, an example of non-imaging optical design. Since the liquid crystal layer inside a display is not a self-illuminating unit, a backlight module that acts as its light source must be placed below. It is necessary for the backlight behind it to have good lighting properties, so that the displayer can have high performance. In this article, we present a way to optimize the light emission from the backlight module in pursuit of better display performance.

DESIGN METHOD

Figure 1 Side view of the lightguide lighting units

We begins the design process by setting up a general situation for optimization. Fig. 1 is a schematic side view of how we use the lightguide lighting units. The light sources, lightguide plates, and reflectors are placed at the bottom of the box. The lightguide lighting unit is a piece of lightguide coupled with the light source and reflector. With multiple pieces, they replace the standard one-pieced lightguide at the bottom of the module. As seen in the Fig. 1, with our lightguide lighting units, the backlight module will not need a brightness enhancement film.
To enhance the performance of the lightguide lighting units, we use the One-Time Ray-Tracing Optimization Method to adjust the radiant intensity distribution of the light sources. The One-Time Ray-Tracing Optimization Method is a method that optimizes the illumination properties of the target surface by adjusting the light sources illuminating properties. The method works even for complicated lighting systems, such as the backlight module.

Figure 2 Basic layout for One-Time Ray-Tracing

Fig. 2 shows how the optimization will be carried out. We first divide the emission angles of the source into numerous sectors. Then, placing a light receiver above the entire backlight module, we record the optical effects of each sector on the receiver, and compute a special emission pattern using these sectors. This special emission pattern should allow the receiver to obtain desired optical properties. Finally, we come up with a way to fabricate the lightguide lighting units so that light will enter the lightguide and emit light in the way that that fulfills our optimization goal.
Spatial luminance is the primary measure of how the viewer of the displayer directly experiences the light emitted from the screen. With uniform spatial luminance, the viewer will see a uniformly-bright display, thus highly-uniform spatial luminance is a goal that displayers should reach. Spatial luminance uniformity is described by

U= (minimum uniformity on the reciever)/(maximum uniformity on the reciever)×100% (Eq.1)

With our optimization method, we set our primary optimization goal to be a highly-uniform spatial luminance on the receiver.

RESULTS AND DISCUSSION

The lightguide unit is designed to be implemented into various different backlight modules, such as computer screens, tablets, and even televisions. After we optimize and fabricate the lightguide unit, we have tested the lighting unit's performance in differently-sized modules to foresee the performance its displayer will have.

Figure 3 Performance of the units before optimization

Fig.3 shows the spatial luminance above the screen using lightguide lighting units that are yet to be optimized. The picture clearly shows that the spatial luminance is not uniform, with two apparent peaks and valleys on the graph. Thus, we use One-Time Ray-Tracing Optimization to design the lightguide units.
After optimization, we apply the lightguides first on a medium sized backlight, first, as an example, a 9.7-inch tablet. With light-emitting-diodes (LEDs), currently the most popular light source as the sources for our lightguide units, Fig 4 shows the spatial luminance on the receiver above its screen. The picture shows great uniformity, at above 75%.

Figure 4 Using the lightguide units in a tablet's backlight module

After testing the performance for a tablet, we test the lightguide lighting units' performance for larger displayers, such as a 24-inch computer screen. Fig. 5 shows its performance. The results are just as good as the tablet's case, showing that our lightguide units can be used in different backlight modules.

Figure 4 Using the lightguide units in a laptop's backlight module

CONCLUSION

This thesis has proven that the One-Time Ray-Tracing Optimization Method is capable of optimizing optical systems. Former work from our laboratory [1] has also shown the method's strength in optical designing.
We look forward to find extra uses or properties regarding the lightguide unit, and also other fields where the One-Time Ray-Tracing Optimization Method can be applied to success.

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