Refond Optoelectronics Mini LED technical paper published in internationally renowned academic journals

Mar 20, 2022

Not long ago, the Mini LED team of Refond Optoelectronics, together with Southern University of Science and Technology and the State Key Laboratory of Advanced Displays and Optoelectronics Technologies of Hong Kong University of Science and Technology, published an academic paper online in the internationally renowned academic journal Nanomaterials(2021, 11(12), 3304). The paper is titled Investigation of Enhanced Ambient Contrast Ratio in Novel Micro/Mini-LED Displays.

(This article belongs to the Special Issue Quantum Dots and Micro-LED Display 2.0)


In recent years, ambient contrast ratio (ACR) has become very critical for advanced outdoor displays, including transparent displays, portable displays, and so on. In this work, the ACR of typical flat panel displays was introduced, while LED-based displays showed distinctive advantages. Micro-LED displays with a different pitch of 10 μm, 15 μm, 30 μm, and 60 μm were fabricated and characterized. Various mini-LED and micro-LED panels were systematically investigated in the aspect of brightness, reflection phenomenon, and ACR to reveal their enormous potential for outdoor applications. Through a series of experiments and comparisons, three methods were proposed to further improve the ACR of LED-based panels, including optical method, antireflection coating, and structure optimization.

Keywords: mini-LED and micro-LED display; emissive display; ambient contrast ratio


Recently, LED-based displays have attracted tremendous attention in many fields because of their superior properties such as self-emission, high brightness and efficiency, low power consumption, long lifetime, and wide operating environment. They are regarded as promising candidates for next generation display, including some advanced display applications such as head mounted display (HMD), head-up display (HUD), wearable displays such as smart watches, AR/VR glasses, and so on. Considerable efforts have been carried out by industry and academic institutes with fast-evolving technologies and prototypes.

The LED-based displays can be classified into three categories: fine-pitch LED displays, mini-LED displays, and micro-LED displays. Fine-pitch LEDs are mostly used for backlight units (BLU) with a new concept of local dimming, in which the backlight is divided into several parts and selectively driven. Mini-LED and micro-LED are applicable in emissive displays. For mini-LED display, large screen products are the mainstream, while transparent, portable applications are also very promising in the future. The mini-LED display has a large application area and has different requirements. For outdoor display or cinema screens, high brightness is very important. While automotive screens and portable displays such as a smart watch require low power consumption and high reliability. The innovative technologies and products of high pixel per inch (PPI) micro-LED display were also witnessed and had a notable impact on the micro-display field. The most attractive application is AR/VR, which requires high ACR, reliability, and low power consumption. As self-emissive devices, mini-LED and micro-LED can provide higher brightness, contrast ratio and efficiency, which are more suitable for low power consumption applications.

In recent years, displays do not restrict to the form of a flat panel. Many future applications, such as immersive stage shows, AR glasses, and smart watches, all require good outdoor display quality and stability. Ambient contrast ratio (ACR), which means the contrast ratio in the presence of ambient light, becomes a significant factor for emissive display. ACR has already been widely applied to estimate the sunlight readability of transflective liquid crystal displays (LCDs). However, the results are usually not good enough because of the low light efficiency of LCD-based products. Then this concept is also extended to other flat panel displays (FPDs) such as OLED. However, OLED is not suitable for outdoor applications because of the relatively lower efficiency and faster degradation under high-current density, which may result in insufficient brightness. Moreover, the instability of organic material makes it undesirable to be exposed to the ambient environment such as high temperature or high humidity conditions. Mini-LED and micro-LED displays, with the significant advantages of high brightness and good stability, are expected to be superior for outdoor applications. Figure 1 show the development of mini-LED and micro-LED displays. It is obvious that the panel screen of the mini-LED display is gradually increased, while the pixel pitch of the micro-LED display tends to be reduced.

Figure 1. Development of mini-LED displays and micro-LED displays.
Figure 1. Development of mini-LED displays and micro-LED displays.

In this paper, the fabrication and characterization of micro-LED displays with a pixel pitch of 10 μm, 15 μm, 30 μm, and 60 μm, were reported. Micro-LED prototypes were finally achieved through monolithic integration, with a brightness of 40,000 nits for blue and 120,000 nits for green. The total power consumption was about 0.94 W. Compared with other good prototypes in recent years, we have higher brightness and lower power consumption owing to the optimized fabrication process of micro-LEDs with a sapphire substrate. The far-field luminance distribution and full width at half maxima (FWHM) are comparable and even better with others. In the future, we will also focus on the light extraction efficiency to further improve the quality of the prototypes.

The principle of ACR was illustrated to form the specific model for LED-based display. Micro-LED and mini-LED (provided by Refond Optics) panels were systemically analyzed and compared to evaluate the outdoor performance. Three methods, including optical method, anti-reflection coating and structure optimization, were proposed to improve the ACR of LED-based display.

The basic information of the paper

Journal Name: Nanomaterials

Author : Ke Zhang1,2Tingting Han3Wai-Keung Cho2Hoi-Sing Kwok2Zhaojun Liu1,2,*

Unit: 1 Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China

2 State Key Laboratory of Advanced Displays and Optoelectronics Technologies, The Hong Kong University of Science and Technology, Hong Kong 999077, China
3 Shenzhen Refond Optoelectronics Co., Ltd.,Shenzhen 518055, China

Academic Editor: Iván Mora-Seró

Title: Investigation of Enhanced Ambient Contrast Ratio in Novel Micro/Mini-LED Displays

Paper link: (Click on the link to view the paper directly!)