Micro Structures Design of OLED Tail Lamp Abstract 1. Introduction

Transcription

1 Micro Structures Design of OLED Tail Lamp Yao-Min Ho, Jih-Tao Hsu Automotive Research Testing Center No.6, Lugong S.7th Rd.,Lugang,Changhua County 50544,Taiwan(R.O.C) Abstract Organic Light-Emitting Diode (OLED) advantages like as thin, flexible, transparent, long life, environmental friendly and it s optical efficiency is better than conventional light source. However, OLED are planar source in combination with a Lambertian angular light distribution causes central brightness decline. In this paper, in order to control OLED light viewing cone to focus within ± 30 degrees and improve the center s view of the brightness, we design the optical element (micro-structure and reflector) by ASAP software simulate the enhancement effect and optimize the micro-structure parameters. With the use of an optimized micro-structure, the center s view of the brightness increase about 110%. We develop the OLED tail lamp prototype to verify this design can effectively enhance the brightness about twice and the light pattern consistent with the needs of tail lamp. Keywords:Organic Light-Emitting (OLED), micro-structure, tail lamp 1. Introduction OLED is a technology that organic and emission materials can emit light biased with current and accomplish the display. OLEDs are most applied for small display about 1 inch in early periods. In comparison with LCD (liquid crystal display), OLEDs have many advantages: ultra compact and thin (less than 1mm), high illumination, large view angle (up to 170 o ), no backlight, low power consumption, fast response time, high resolution, low heat generation, excellent vibration resistance, low manufacture cost, flexibility and plane light emission [1] [2]. These advantages can also be applied to taillight lighting. LED taillight owing to high directivity of LED easily caused the spotty effect [3], as shown in Fig. 1, to discomfort a driver in rear vehicle. The light-emitting characteristic of OLED is a uniform plane light-emitting source, which can reduce discomfort and prettify the appearance of a vehicle. Using traditional lens will cause poor light-concentrating efficiency owing to the characteristic of plane light-emitting source for OLED. Therefore, the micro structures are employed in this work to enhance the illumination of OLED taillight and reshape the light distribution.

2 Fig. 1 LED taillight 2. Study Method Philips rectangle Red OLED is used in this work, which the light-emitting efficiency is 7lm/W. The light-emitting efficiency of OLED today is far unable to compete against that for LED. Therefore, we will make use of micro structures to enhance the illumination and modify the appropriate light distribution of the taillight. Because the way to emit the light for OLED is similar with light guide module for LCD, it is the light-emitting type of a uniform plane. In LCD module, there is a layer of brightness enhancement film (BEF) to converge rays and enhance illumination of central view angle[4, 5]. In this work, we utilize micro prisms similar to BEF in LCD as design basis together with ASAP for analysis simulation. In order to meet the test regulations, the receiving plane is set at 4m to investigate the light distribution and illumination due to the structure of micro prisms under the different distances and angles. 3. Design Principle of Micro Prisms CCFLs are most applied for light source for backlight module. After a diffusing film is attached, the light-emitting characteristic, the same as the plane light source of OLED, is a uniform and diffusing plane Lambertian source. Therefore, we employ the technique such as BEF to design the light distribution of the taillight. The main purpose is to enhance illumination and conform to the light distribution for the taillight. The light distribution of a brake lamp is designed according to ECE-R7 [6]. The simulation of light source is based on Philips rectangle Red OLED, which the light-emitting dimensions are 37.5mm 30.5mm. The Lambertian light source is set here. The micro structures in this OLED taillight can be designed as OLED light source, reflecting mirror and micro-prism film, as shown in Fig. 2.

3 Periodic prisms Reflecting mirror OLED Fig. 2 Cross-section of micro structures for OLED taillight The plane light source can be seen as convergence of many point sources. Therefore, we employ the periodic design to converge plane light source via periodic prism. With the light-emitting dimensions of OLED, the periodic interval is appropriately adjusted to achieve the optimal convergence. As shown in Fig. 3, D is the periodic interval, Z is the interval between OLED and micro prisms, and θ is the focusable angle. Illustration of periodic micro prisms D Z θ Red OLED Fig. 3 Illustration of periodic micro prisms If Z is fixed at 10mm and the focusing angle θ is changed, the optical focusing angle will be 56.3 o. Fig. 4 shows the light distributions at different angles. When the distance between OLED and micro prisms is put closer, the optimal focusing angle will decrease with Z.

4 Fig. 4 Light distributions at different angles The method to design micro prisms is via refraction and total internal reflection. The divergence angle of rays is controlled within ±30 o. If the divergence angle is greater than 30, rays will deflect back to the direction of light source. Rays can exit at appropriate angles (±30 o ) after multiple reflection by a reflecting electrode on the back of light source, where the design principle of θp is the same as that for Fresnel lens, as shown in Fig. 5 [7]. Fig. 5 Illustration of micro prisms The design of θp is based on geometrical optics and mathematic calculation to derive related slope angle by the ray path, as shown in Fig. 6. θp θi 光源 θ Fig. 6 Illustration of micro prism model

5 where θi, θp and θ are the incident angle of point light source, the Fresenl angle and the slope angle. The relationships to each angle are as follows: sin( θi + θ ) = nsinθ...(1) -1 sinθi θ = tan...(2) n - cosθi θp = 90 -θ...(3) Some rays with large angle or emitting from the edge can be reflected by the reflection plane (consisting of parabola) to achieve convergence, as shown in Fig. 7. Fig. 7 Illustration of reflection plane 4. Results and Discussion OLED is a uniform plane light source, which presents uniform and diffusive Lambertian one prior to attaching micro structures, as shown in Fig. 8. (a) Simulated light distribution (b) Simulated contour Fig.8 Simulated light distributions for OLED After micro structures are attached, the light distribution that originally diffuses will centrally concentrate. The half angle of view is converged from 60 o to 30 o, in compliance with the range of light distribution for the taillight, as shown in Fig. 9.

6 (a) Simulated light distribution (b) Simulated contour Fig. 9 Simulated light distribution of taillight Owing to the design of micro structures, the illumination at the center can enhance about one time. Fig. 10 shows the comparison of luminous intensity for OLED with and without micro structures. Fig. 10 Luminous intensity with and without micro structures Fig. 11(a) shows a prototype of OLED taillight. Fig. 11(b) shows the comparison of OLED and conventional taillights, where the thickness of OLED and conventional taillight is 15mm and 120mm. The OLED taillight not only can save boot space, but also reduces the complicated structure of lamp case.

7 (a) Prototype of OLED taillight (b) Comparison of taillight thickness Fig. 11 Prototype of OLED taillight Light measurement tester (LMT) and a photometer are used in this work, where LMT can rotate left and right about 180 o as well as up and down about 90 o. The luminous intensity is measured once per rotation. The measured light distribution is similar to the simulated one, as shown in Fig. 12. (a) Simulated light distribution (b) Measured light distribution Fig. 12 Measured and simulated light distributions The results measured in each distribution point are as shown in Table 1, the same as the simulated ones. The illumination at the center is enhanced about one time.

8 Table 1 Candela distribution 5. Conclusions The characteristic of plane-emitting source for OLED can improve a light-concentrating efficiency of traditional lens. For the study of OLED taillight here, micro-structure lens is designed to enhance illumination on lighting area. Using refraction of transmissive light and total internal reflection, plus the special structure of periodic prism, light from OLED is distributed within ±30 o. With the reflecting mirror, illumination not only enhances one time but also the requirement of light distribution for taillight meets. Moreover, OLEDs have the thin and compact characteristic. The thickness of conventional taillight is about 120mm. The thickness of OLED taillight in this work is about 15mm, which can save boot space. The flexible and transparent characteristics of OLED, using plastic or flexible lens, can be taken into account in the future. Combining OLED encapsulation with the windscreen will present personal style on taillight. 6. References [1] Malte Hagemann, Matthias Brinkmann (University of Applied Sciences Darmstadt, Germany), Optical and Electronic Luminance Homogenization for OLED Devices, ISAL, Volume 13, Page(s): [2] Jin-Xin Chen, Jin-Di Chen, and Chong-Zhi Wu, White OLED Lighting, Wunan Publication, [3] Eugeniy Neytchev, Technical Innovations in Rear Lighting, ISAL, Volume 13, Page(s): ,2009

9 [4] Yu-Bin Fang, Optical Optimization of LCD Backlight Module, Master Thesis, Engineering Science Department of National Cheng Kung University, Tainan, Taiwan, [5] 3M Innovative Properties Company (US), Variable pitch structured optical film, US Patent, No , July [6] [7] Entire Technology CO., LTD., Direct Backlight Module Structure, Taiwan Patent No