Qi Ouyang¹, Yang Cheng¹,*, Chi Chen², Danyan Yu³
(1. School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China;
2. Institute of Optical and Laser Metrology, National Institute of Metrology, Beijing 100029, China;
3. Xiamen Institute of Product Quality Supervision and Inspection, Xiamen, Fujian 361004, China)
Abstract: With the rapid development of LED display technology, users have increasingly higher demands for screen performance. Thanks to its high resolution and high contrast, Mini LED has gradually become a key research focus for compact projection and display devices. Response time is a crucial parameter for evaluating display performance. This paper proposes and establishes a testing system for measuring the response time of Mini LED direct-view displays. Experiments were conducted to measure the gray-scale response time of Mini LED screens under different refresh rates and color displays. The testing setup enables stable real-time control and accurate acquisition of gray-scale response times. Test results show that when displaying primary colors, the screen has the shortest response time for red, a moderate response time for green, and the longest for blue. Moreover, as the refresh rate of the Mini LED display increases, the gray-scale response time decreases.
Keywords: Mini LED direct-view display; gray-scale response time; refresh rate; real-time testing
LED (Light Emitting Diode) is a type of semiconductor solid-state light-emitting device, and its core component is a chip composed of P-type and N-type semiconductors. A PN junction is formed between the P-type and N-type semiconductors, and when a forward voltage is applied to the PN junction, it conducts and converts electrical energy into light. When a reverse voltage is applied to the PN junction, minority carriers are difficult to inject, thus it cannot emit light [1]. Before the 1980s, LEDs could only produce red, orange, yellow, and green light, and were mainly used for indicating information [2]. Based on the principle of the three primary colors, the addition of phosphors enabled the development and application of LEDs into the full-color general lighting stage [3].
With further research and development of LED displays, they have been widely used in outdoor commercial advertising, digital entertainment in cinemas, monitoring and command systems, and military technology, thanks to their advantages such as being environmentally friendly, highly energy-efficient, long-lasting, compact, and having high luminous efficacy [4–5]. As science and technology continue to advance, projection displays are evolving toward miniaturization, high brightness, energy efficiency, and fast response, and small-sized display devices are receiving increasing attention. Common display technologies include LCD (Liquid Crystal Display) [6], LED display technology, and OLED (Organic Light Emitting Diode) [7]. Among them, LCD relies on a backlight source and is less suitable for the development of high-definition, miniature display devices. OLED, though self-emissive, faces limitations in further improving pixel density and suffers from short device lifespans. Both LCD and OLED technologies face obstacles in enhancing overall display performance. This study focuses on Mini LED display technology [8], a new type of self-emissive display device that integrates both lighting and display functions. With pixel sizes reaching the micron level, it can be designed to achieve high-resolution and low-loss micro-projection imaging systems. With the emergence and rise of new display technologies, various projection display devices and other consumer electronics have appeared on the market. However, the labeling of product specifications is inconsistent, testing methods vary, and a unified standard is lacking [9–10]. Therefore, this paper focuses on building a real-time testing system for the gray-scale response time of Mini LED direct-view displays and experimentally analyzing the variation of gray-scale response time under different display conditions.
Response time refers to the speed at which each pixel of a display reacts to input signals [11], that is, the time it takes for a pixel to change from dark to bright or from bright to dark. It is a key performance parameter for all types of displays. Response time includes black-and-white response time and gray-scale response time. In early LED displays, which featured monochrome outputs, black-and-white response time was sufficient to roughly indicate the screen’s responsiveness. As displays evolved into full-color output with multiple brightness levels, the more grayscale levels a display supports, the finer the image it can present. Therefore, dynamic display performance is more accurately evaluated by gray-scale response time. Generally, displays with shorter gray-scale response times have better motion image performance, while longer response times can lead to motion blur or “ghosting” effects. The content of this paper is structured into four sections: the first section describes the construction of the Mini LED direct-view display response time control and testing system; the second section outlines the experimental procedure; the third section presents the test experiments and analyzes the gray-scale response time of Mini LED displays at different refresh rates and color outputs; and the fourth section concludes the paper.
1 Mini LED Direct-View Display Response Time Testing System
1.1 Principle of Gray-Scale Response Time Testing
The key factors determining display quality include resolution (number of pixels), pixel density (PPI), and viewing distance. Generally, Mini LED refers to chips sized between 100–300 μm, with a pitch between 0.1–1 mm, commonly used in direct-view displays [10] or as LCD backlights [12]. Mini LED backlighting offers advantages such as low cost, broad application range, and long lifespan, making it highly promising for use in smartphones, automotive displays, and VR systems. Mini LED direct-view technology continues the path of narrow-pitch development by further miniaturizing the chips, bridging the gap between traditional LED displays and Micro LED technology. Mini LED self-emissive solutions are more commonly used in public display fields such as advertising, traffic control, security, and nighttime economic activities. After more than two decades of development, LED chip sizes have continued to shrink, pixel pitch has decreased, and pixel density has improved. As Mini LED direct-view technology continues to mature and its applications expand, it becomes increasingly important to conduct real-time calibration of the electrical performance parameters of Mini LED displays. When using a display, users can intuitively perceive its smoothness during operation, which can be quantitatively analyzed by measuring response time. Therefore, this paper focuses on the measurement of gray-scale response time for Mini LED displays.
Whether it’s an LCD, OLED, or Mini LED display, the core light-emitting principle can be simplified to individual pixel emission. Each pixel contains red, green, and blue (RGB) subpixels, and by independently adjusting the brightness levels of these subpixels, full-color image rendering can be achieved. The greater the number of brightness levels, the finer the image appears. Different brightness levels correspond to different gray levels, and the time it takes to shift from one brightness level to another is known as the gray-scale response time. To control pixel brightness, a brightness signal must be transmitted. Using binary “0” and “1” to represent different electrical signals: if there are 2¹ signal levels, “0” represents fully off (black) and “1” represents fully on (white), resulting in two brightness states and a response time referred to as black-to-white response time. If there are 2² signal levels, the pixel can represent four brightness states: fully off (00), low brightness (01), high brightness (10), and fully on (11). As the number of input signals increases to 2³, the pixel can reach eight brightness states, as illustrated in Figure 1. With exponential increases in brightness control signals, the pixel gains more levels of brightness control, enabling smoother color transitions. For example, an 8-bit display panel can present 256 brightness levels, also known as 256 gray levels [13].
When an LED display transitions from one gray level to another, the electrical signal, as shown in Figure 2(a), appears as a square wave excitation signal. For the LED chip within a pixel, its response process is illustrated in Figure 2(b), where the time it takes for brightness to rise from 10% to 90% is defined as the rise time, denoted as Tr. After the voltage signal is removed, the time it takes for the brightness to fall from 90% to 10% is the fall time, denoted as Tf. The sum of the rise time and fall time represents the gray-scale response time for that particular brightness transition range[14]. Specifically, when the gray level changes from 0 to 255, this represents the black-to-white response time, thereby aligning the concepts of gray-scale response time and black-and-white response time.
1.2 Construction of the Gray-Scale Response Time Testing System
A gray-scale response time testing system for Mini LED direct-view displays was built to study the impact of refresh rate and color on gray-scale response time, thereby providing technical support for the metrology of response time. A high-speed photodetector was used to measure the gray-scale response time of the Mini LED direct-view display under different refresh rates and color conditions. The design scheme of the real-time control system for gray-scale response time testing of the LED direct-view display is shown in Figure 3.
The testing system primarily consists of the Mini LED direct-view display under test, a controller, a DC regulated power supply, and PC-based control software. The Mini LED display used in this study is model YFO.9, manufactured by Shenzhen Leyard Optoelectronic Co., Ltd., with physical dimensions of 3300 mm × 337.5 mm (length × width) and a pixel pitch of 0.9375 μm. The controller includes both the receiving card and the sender, which are managed via software on the PC. The control interface is shown in Figure 4. The overall control process can be summarized as follows: (1) Power on the regulated power supply to simultaneously supply power to both the controller and the Mini LED display. The display will light up upon successful activation. (2) Connect the PC to the controller and launch the LED control software. Pre-configured gray-scale files can be loaded into the software, allowing for easy control over color and gray-level output during the experiment. The refresh rate can also be modified within the screen configuration page.
The gray-scale response time testing process also requires an optical test system, which mainly includes a microscope objective and a high-speed photodetector, as shown in Figure 5. Considering that the response time of Mini LED displays is on the nanosecond scale, this study utilizes a silicon-based high-speed photodetector from Sorebo, featuring a bandwidth of 150 MHz and a minimum detectable rise time of 2.3 ns. The testing principle is as follows: the Mini LED display is powered by the regulated power supply; the optical signal from the display is captured by a high-magnification microscope objective; the high-speed photodetector converts this light signal into an electrical signal, which is then transmitted via BNC coaxial cable to an oscilloscope for signal acquisition. In Figure 5, the oscilloscope displays the real-time waveform corresponding to a particular gray level. Combining the diagrams of the real-time control system and the optical test system, the assembled physical test setup is shown in Figure 6.
2 Gray-Scale Response Time Testing Procedure
For the Mini LED direct-view display, software control allows it to display a standard 255-level gray-scale brightness range, where level 0 represents completely dark and level 255 represents fully bright. In this study, the measured characteristic gray levels are 0, 63, 127, 191, and 255. Additionally, the rise time and fall time of the LED display at each of these gray levels are measured. The control software used in this study modifies the display’s refresh rate by adjusting the pixel clock signal (DCLK) parameters. The primary refresh rates tested in this experiment are 2160 Hz, 2880 Hz, and 3420 Hz. The complete procedure for measuring the gray-scale response time of the LED display at a specific color and refresh rate can be described as follows:
(1) Measure the rise time between successive gray levels, such as the time it takes to transition from level 0 to levels 63, 127, 191, and 255. Each transition is tested at least five times, and the average is taken to reduce error.
(2) After completing the rise time tests, measure the fall time in reverse order, i.e., from level 255 to 191, 127, 63, and finally back to 0. These are also repeated and averaged.
(3) For a fixed refresh rate and a specific display color, the test covers 10 increasing gray-level intervals and 10 corresponding decreasing intervals. After collecting the data, the average rise and fall times for each gray level are calculated, and the gray-scale response time of the Mini LED display under that specific display condition is derived. To meet the experimental requirements, the gray levels, display colors, and refresh rates of the Mini LED direct-view display can be flexibly adjusted.
3 Gray-Scale Response Time Test Results
3.1 Gray-Scale Response Time Under Different Colors
Based on the real-time gray-scale response time testing system described in Section 1 and the experimental procedure in Section 2, the Mini LED direct-view display was controlled via PC to operate at a refresh rate of 3420 Hz, and experiments were conducted while the screen continuously displayed white. The gray levels were gradually adjusted to ensure smooth brightness transitions on the Mini LED display. The position of the high-speed photodetector was fine-tuned to ensure that the oscilloscope could capture a pulse signal resembling a square wave. When the measured signal frequency matched the 3420 Hz reading from the controller, the signal was confirmed to be the one under test. The oscilloscope’s horizontal time base was adjusted so that one full cycle of the signal could be clearly observed on screen, allowing for optimal measurement of rise and fall times. As shown in Figure 7, the table displays the gray-scale response time results for the Mini LED display emitting white light at a 3420 Hz refresh rate. Keeping the refresh rate constant, the Mini LED display was then set to display red, green, and blue light separately for individual testing. The corresponding gray-scale response time for each color was measured and is illustrated in Figure 8.
The test results show that at a refresh rate of 3420 Hz, the gray-scale response time of the Mini LED direct-view display varies depending on the displayed color. Specifically, the gray-scale response time is longest when the display shows white. Among the three primary RGB colors, red yields the shortest gray-scale response time at approximately 526.21 ns, followed by green, while blue has the longest response time at approximately 700.10 ns.
3.2 Gray-Scale Response Time at Different Refresh Rates
The refresh rate of an LED display—also referred to as “refresh frequency” or “visual refresh rate”—is defined as the number of times per second the screen image is redrawn, measured in Hertz (Hz). A higher refresh rate results in a more stable image display and less noticeable flicker to the human eye. In general, when the refresh rate exceeds 960 Hz, flicker becomes difficult to perceive with the naked eye. However, it can still be detected using a smartphone camera or high-definition video recording device. A practical example is that screens with a standard 960 Hz refresh rate exhibit visible scan lines or ripple effects when filmed with a smartphone, whereas high-refresh-rate screens do not.
Considering that the Mini LED direct-view display used in this study falls under the category of high-refresh-rate LED displays, the characteristic refresh rates selected for testing were 2160 Hz, 2880 Hz, and 3420 Hz. The effect of these different refresh rates on gray-scale response time was analyzed. Figure 9 presents the gray-scale response time data for white light under the three different refresh rates. To minimize experimental error and enhance measurement accuracy, further experiments were conducted to analyze how different colors and refresh rates affect the gray-scale response time of the Mini LED display. The results are shown in Figure 10.
The test results demonstrate that as the refresh rate of the Mini LED display increases, the gray-scale response time for white light decreases. This same trend holds true for displays showing red, green, or blue. These findings confirm that the gray-scale response time testing system developed in this study effectively supports real-time, efficient, and stable experimental measurement.
4 Conclusion
This study proposed a gray-scale response time control and real-time testing system for Mini LED direct-view displays and conducted experiments to evaluate how gray-scale response time varies with different colors and refresh rates. The key findings can be summarized as follows: (1) When the LED display shows the three primary colors (RGB), the shortest gray-scale response time occurs when displaying red light, followed by green, with blue resulting in the longest response time. When the Mini LED display shows white light, the gray-scale response time is relatively longer. This can be attributed to the fundamental principle of LED emission: when testing RGB colors individually, the light detected comes directly from the red, green, or blue subpixels, each of which has a distinct response time. In contrast, white light is produced through the combined emission of RGB subpixels, leading to a longer overall response time in measurement. (2) As the refresh rate of the display increases, the measured gray-scale response time decreases accordingly. A higher refresh rate results in faster image updates on the display, which in turn accelerates the transition speed between gray levels.
The experimental results confirm that the proposed gray-scale response time testing system is simple in structure, supports real-time operation, and produces stable results that are unaffected by external environmental conditions. Therefore, the testing method described in this paper can serve as a practical reference for the calibration and measurement of gray-scale response times across various LED display products available in the market.