評估散射片對於有機發光二極體之影像品質影響

Abstract

隨著顯示器產品的日新月異，有機發光二極體顯示器(OLED)也因為它的高對比度、大觀賞角度、低成本等優點而漸漸受到更多關注。有機發光二極體顯示器也因為其特性(可製成透明顯示器、可彎折等等)能有很多應用，例如展示櫥窗、電子紙、互動式鏡面等等，吸引許多人的關注並且投入研究。然而，有機發光二極體顯示器仍有一些議題需要解決，其中最引人注目的一項就是環境光反射問題。在反射式有機發光二極體顯示器中，由於電極的高反射率，環境光受電極反射進入使用者眼睛，與OLED顯示影像同時被使用者所觀察，影響到使用者對於OLED顯示影像的觀賞品質。為了解決這個現象，現在市售的OLED產品，主要是利用偏振片(polarizer)將入射OLED的環境光偏振化，接著利用1/4玻片(quarter-wave plate retarder film)將其偏振方向改變後將光由偏振片(polarizer)擋下。這樣的方法能有效地消除環境光反射，使反射率剩下約5%。另外也有其他方法，如色彩過濾片(color filter)以及灰玻片(gray film)也有不錯的效果。然而，在強光環境下，不可避免地反射環境影像依然會被使用者觀察到，加上OLED像素結構的關係，反射影像會有繞射產生。既然在強光環境下反射影像無法避免，我們希望其繞射能被消除，讓使用者被其干擾的程度減少。為了達到此目的，我們在OLED表面加上了一層散射片，打亂環境光的光層差，以達到消除反射環精光的繞射，使觀賞品質上升。由於加上了散射片，OLED顯示影像的品質也會受到影響，因此散射片的各項參數對於顯示影響品質的影響也是這篇研究的重點之一。最終結果可以達到利用散射片消除反射環境光的繞射問題，同時維持OLED顯示影像在一定的標準之上。

With the rapid development of display technology, organic light emitting diode (OLED) display is getting more and more attentions due to its high contrast ratio, wide color gamut, large viewing angle, and so on. Many people are dedicated in the researches of OLED display because of its properties (transparent, flexible, foldable etc) and particular applications, such as showcase window, head-up display, smart window and so on. However, some issues still need to be solved for OLED displays, and one of the most eye-catching ones is the ambient contrast ratio issue. In an OLED display, because of the high reflectance of the electrodes, ambient light is reflected by the display and seen by users along with OLED displayed image simultaneously, which influences the viewing quality of OLED displayed image for users. In order to solve this issue, for modern commercial products, the main solution is using a linear polarizer to make ambient light polarized and placing a quarter-wave plate retarder film to make the polarization of the reflective ambient light opposite to the polarizer and further eliminate it. This solution can reduce the reflectance of ambient light to approximately 5%. In addition, there are some other methods, such as color filter and gray film, having the similar effect. However, when the ambient environment is very bright, the reflective ambient image will inevitably be observed by users. Moreover, due to the periodic pixel structure of an OLED display, the reflective ambient image will be diffracted. The reflective ambient image under bright ambient environment is not avoidable, so we want the diffraction to be eliminated in order to reduce the degree of interference for users. To do so, a scattering film is applied on the surface of an OLED display to ruin the optical path difference of ambient light. The quantified relation among the haze of the scattering film and the luminance change of the diffraction pattern is obtained by using various different scattering films with different specifications. Combining the contrast sensitivity function, the specification of scattering film that can eliminate diffraction in reflective ambient image for human eyes can be determined and finally enhance the viewing quality. Because of the scattering film, the sharpness of OLED displayed image will also be influenced. By researching the MTF curve corresponding to different scattering films with different configurations, the optimized structure of the OLED display (thickness, resolution etc.) can be determined. Eventually, the result shows the reflective diffraction can be eliminated by the scattering film and the OLED displayed image can be kept beyond a certain criterion under the influence of scattering film.