An integrated LED luminance-uniform device for light ...

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crystal display (LCD), such as LCD TVs, notebook, laptop compute, and many portable information devices such as cellular phones, personal digital assistants ...
An integrated LED luminance-uniform device for light guide plate applications Chao-Heng Chien, Zhi-Peng Chen* MEMS LAB, Dept of Mechanical Engineering, Tatung University, 40 ChungShan North Road 3rd Section Taipei 104, R.O.C ABSTRACT An edge-backlight unit (EBLU) is applied to as the light device to provide uniform light of liquid crystal display (LCD). Generally, cathode cold fluorescent lamp (CCFL) is utilized to be the light source of BLU. With the advantages like long life, no mercury containing and good endurance of heavy impact, the light emitting diode (LED) is well known as a viable device for solid state lighting. To achieve the market requirement of the thin-film liquid crystal display (LCD) and the green-level product, the LED is replaced the CCFL used in monitor to make display thinner, lighter, no Hg containing. In this paper, the integrated LED luminance-uniform device with right angle microprism structure is proposed that it can make the point-like light to distribute propagating-light line pattern successfully. To optimize the distributions and sizes of microprisms, our designed LED-linear device can achieve an optical efficiency more than 85%, and its light output area is 2.5 times the input light source. Therefore, the LED luminous device with microprsims not only can decrease the LED to save the space, but also enhance the luminous efficiency. In future, an integrated LED luminance-uniform device could make displays thinner and brighter for light guide plate (LGP) applications. Keywords: Right angle microprism, backlight unit, luminance-uniform device, light emitting diode

1. INTRODUCTION Usually, liquid crystal display (LCD) is applied to provide a high uniform luminance as well as thinness, low weight, low electric energy consumption. Edge-lit backlight unit (BLU) are utilized for many medium and small size liquidcrystal display (LCD), such as LCD TVs, notebook, laptop compute, and many portable information devices such as cellular phones, personal digital assistants (PDAs) and digital video cameras , and so on. A conventional edge-lit backlight unit (BLU) is generally comprised of cold cathode fluorescent lamp (CCFL), a light guide plate (LGP), two prism sheets, two diffusive sheets and a reflective sheet. For example, the reflector sheet under the light guide plate (LGP) can decrease the optical loss to enhance the luminance. The LGP is key component in backlight unit (BLU), which could guide the light emit into the front of the LGP. The typical LGP is not designed control the luminous angle; therefore, it presents the poor luminance and luminous uniformity. In order to solve the uniform problem of distribution, some optical plate such as diffusive sheet, prism sheet are combined into the typical LGP to make the light emit in a direction perpendicular toward the top surface of BLU and to increase the brightness and luminous uniformity. In general, the cold cathode fluorescent lamp (CCFL) is used as the light source for backlight unit (BLU), which is placed behind the LCD. Additionally, the light emitting diode (LED) is beginning to replace conventional cold cathode fluorescent lamp (CCFL) as light source for LCD backlight unit (BLU) due to their desirable characteristics such as a large color gamut, color temperature adjustability, long lifetime, and environmental safety (i.e. mercury free), as shown in Fig. 1. In recent year, in order to satisfy the strong market requirement for mobile phones and displays panels, it is desirable to have a slim size, lighter weight, longer battery life, low energy consumption and high luminance. For the small size penal increasing, many researchers have devoted to improve the LED light guide plate design to make it thinner, lighter and brighter [1, 2, 3]. Park et al reported a novel LED backlight unit implementing a light guide plate (LGP) with a two-dimensional array of grating micro-dots [4]. Miyamoto et al [5] demonstrated a novel light guide where they have applied a special diffraction grating technology which provides a high performance with a thinner backlight. Ye et al [6] provided the diffraction transmission characteristics of submicrometer gratings (SMGs) designed for coupling tricolor light out of a light guide plate (LGP). Parikka et al proposed the new system eliminates the images * [email protected] ; phone +886-2-25973036; fax +886-2-25973036

New Developments in Optomechanics, edited by Alson E. Hatheway, Proc. of SPIE Vol. 6665, 666506, (2007) 0277-786X/07/$18 · doi: 10.1117/12.735649 Proc. of SPIE Vol. 6665 666506-1

of light sources, which appear as bright lines emanating from each source in the conventional diffractive approach [7]. Nagasawa fabricated an ultra slim backlight system using optical-patterned film, which has a lot of advantages to be composed of a single component [8]. Feng et al [9] approved a design concept of integrated LGP based on microstructures using polymethylmethacrylate (PMMA) material, to control the illuminative angle. Chien [10, 11] recommended the integrated light guide plate on microstructure-based by using Microelectromechanical Systems (MEMS) technique for BLU application. There are also many patents, which have been proposed for small size LCD backlight panel [12, 13, 14].

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Light guide plate Light emitting diode Reflector Fig. 1. Schematic diagram of LED backlight unit of cellular phone

In comparison to previous papers and patents, their commended light guide plate (LGP) is not concerned the LED incident light uniformity, dark band problem and luminous efficiency, etc. In order to overcome these problems, we develop an integrated LED luminance-uniform device, which can make the incident light guided uniformly into the LGP. The right angle microprsims are constructed on the luminance-uniform device, which can make the point-like LED to appear a propagating-light line pattern and to increase the optical efficiency. Thus, our integrated LED luminance-linear device could diminish the LED and achieve the demand of luminous uniformity. A new LED output-linear system is demonstrated successfully for cellular panel in this research.

2. DESIGN CONCEPT The edge-light type of the backlight unit (BLU) for cellular phone is comprised a transparent PMMA light guiding plate (LGP) with diffusive printed dots, some optics-controlling components and light source. Usually, three or more light emitting diodes (LEDs) are placed beside the edge of the LGP and the total internal reflection of light inside LGP will be occurred. The diffusive dots are necessary printed at bottom of the LGP to emit the light out of the plate. The LGP doesn’t redirect the incident light from the side to the front direction itself; therefore, the diffusive sheets, prism sheets and a reflection sheet should be used that can redirect the emitted light from the light guide to front direction. 2.1 Design principle and conception of the LED luminance-uniform device Snell’s law is applied to calculate the light direction. If the light is from air to an interface material, either refraction or total reflection should be occurred, as shown in Fig. 4, the critical angle for total reflection is expressed as

π n fra θ cri = sin −1 (sin )( ) 2

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θ fra = sin −1 (sin θ i )( Incident Light

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Three or more light emitting diodes (LEDs) are generally placed inside the backlight unit (BLU). The dark band will be produced the in the gap of between the LEDs to cause the emitting light, which is unable guided uniformly into the light guide plate (LGP) as shown in Fig. 3. To overcome illuminative uniformity, it is necessary to design a luminanceuniform device to diminish the dark band occurred, as shown in Fig. 4. Therefore, the right angle principle is utilized to establish the LED luminance-uniform device in this study.

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Fig. 3. Simulation of LED light rays guided into the LGP (a)

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Right angle prisms, most commonly used in optical instrumentation, are used as single prism or in combination with other prisms [15]. Geometrywise, the base angles of these prisms are made 450 because it makes for a compact size. The reflective surfaces of the prisms, generally, function in total internal reflection (TIR) mode. A right angle prism is used as a mirror to deviate light through 90 degree, and also as a retroreflector to deflect light through 180 degree by total internal reflection, as shown in Fig. 5. (a)

(b)

I-\

Fig. 5. Simulation of the light rays guided into the right angle prisms, (a) 900 deflection, (b) 1800 deflection.

2.2 The design for the LED luminance-uniform device Our designed LED luminance-uniform device is made of PMMA with a refractive index of n =1.49. The right angle microprisms are constructed in the luminance-uniform device such as a mirror to deviate light through 90 degree. Three kinds of the light-output components are designed in this study. In first case, the 13.9 degree right angle microprisms are placed on the hypotenuse of the light coupler, as shown in Fig. 6. To design the second case, its edge surfaces are the right angle microprisms that are the same shape and are positioned parallel to one another, but their densities and sizes are varied with distance from light source, as shown in Fig. 7. In final case, we propose a ladder-shaped light output component with the 45 degree microprisms, as shown in Fig. 8.

Fig. 6. Schematic diagram of the 3rd light coupler, (a) 3-D structure, (b) design dimension.

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V Fig. 7. Schematic diagram of the 1st light coupler, (a) 3-D structure, (b) design dimension.

Fig. 8. Schematic diagram of the 2nd light coupler, (a) 3-D structure, (b) design dimension.

3. SIMULATION RESULTS AND DISCUSSION 3.1 Geometries and optical properties of optomechanical component set up The optical analysis, simulation, and design of LED luminance-uniform device, using APILUX optical modeling software, are simulated in this research. Fig. 9 shows a typical process of an APILUX simulation, which is composed three phases, i.e. pre-procedure, procedure, and post-procedure. The pre-procedure phase involves creating models of the light coupler and light source. The ray tracing program is the core solver in the procedure phase. In the post-procedure phase, various analyses can be performed. The first, three-dimension of the luminance-uniform device is built by using the SOLIDWORKS software. The luminance-uniform device is formed a powerful built-in International Graphics Exchange Specification (IGES) translator and is imported into the APILUX. In APILUX, the components such as the light source, the optical component and the detector are inserted in the system. Each component should be defined the optical properties. To define the light source, which is based on the LED specification, the present simulations assume that the luminous flux of a single LED is 1.5 lm and that the angle of half intensity is 13 degree (LED lamp) or 55 degree (side view LED). To define the optical component, interfaces are used to import from SOLIDWORKS through IGES file. The material is used as the PMMA and the aluminum is coated on the hypotenuses for three kinds of LED luminanceuniform device. To define the detector, the luminance meter is applied to detect the luminous distribution. We set the dimensions to a rectangle 16 luminous area, and the resolution will be 100*100 pixels. After the completion of optical property assignment, and the light source generation, the optical computation can be able to determine the interactions between light beams and luminance-uniform device. The calculation is called the ray tracing. By completing the ray tracing, the simulation will enter the post-procedure phase, where the various optical characterizations can be retrieved and viewed. The luminance of LED luminance-uniform device is concerned and demonstrated in this article.

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Fig. 9. Schematic diagram of the process of APILUX simulation

3.2 Optimization of LED luminance-uniform device Three different LED light-linear devices are simulated and analyzed in this section. Two types of LED sources are considered to analyze the half emission angle of 13 degree (LED lamp) and 55degree (side view LED). The first, the 8.2mm*5mm area is chosen to measure the luminance, as shown in Fig. 10. In 13 degree case, the output luminance is simulated in different integrated half angle, as shown in Fig. 11(a). Fig. 11(b) shows the luminance of different integrated half angle for 55 degree case.

Fig. 10. The luminous measuring method for 8.2mm*5mm Half angle of emmission=55

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The analysis results are shown that the 1st and 3rd case, their output illuminative areas are about two times the input light source. Thus, the LED luminance-uniform device is designed and demonstrated by optimizing the distribution and sizes of right angle microprisms in this research. Our proposed LED luminance-uniform device not only improves the dark band problem, but also increases the illuminative efficiency.

4. CONCLUSION The right angle microprisms are utilized to demonstrate the LED luminance-uniform device successfully in this research. From the analysis results, the output area is twice as the input LED light source to decease the LED and the average

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output optical efficiencies of the LED luminance-uniform device has achieved more than 80%~90%. A new lightuniform device not only saves the space to diminish the cost, but also enhance the luminous efficiency to guide light uniformly into the light guide plate. In future, the integrated LED luminance-uniform device is applied in light guide plate to make display thinner and brighter.

5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided to this study by the National Science Council, Republic of China, under Grant No. NSC95-2221-E-036-040.

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