What Is the Visual Impact of LED Viewing Angle on the Brightness Uniformity of Display Screen

1. Introduction

The electronic industry standard SJ / t11281-2003 LED display screen test method issued in 2003, in which the measurement of brightness uniformity is based on the colorimetric theory. The measurement data has nothing to do with human vision and does not involve the changes of observer's spatial position and observation angle. As a large screen display product, the visual effect of LED display screen can be imagined. Therefore, it is necessary to observe the LED display screen from different angles and qualitatively analyze the impact of different angles on the brightness uniformity of LED display screen, so as to give a more objective evaluation of LED display screen.

2. Led viewing angle

The shape of the light emitting surface used in LED packaging and the position of the LED chip from the top lens determine the LED viewing angle and light intensity distribution. Generally speaking, the larger the viewing angle of the same led, the smaller the maximum luminous intensity, but the cumulative luminous flux on the whole three-dimensional hemisphere remains unchanged.

The half value angle (or half power angle) of LED refers to the angle between the direction where the luminous intensity value is half of the intensity value in the optical axis direction and the optical axis direction. Two times the half value angle is called the viewing angle of the LED.

The luminous intensity of LED changes with the change of observation angle. When observing LED light-emitting tubes from different angles, its luminous intensity is very different. The luminous intensity is determined by the luminous intensity angular distribution curve of the LED luminous tube itself, and its distribution curve is shown in Figure 1. The relative luminous intensity in the optical axis direction of LED (i.e. the ratio of luminous intensity to maximum luminous intensity) is 1, i.e. the luminous intensity is the largest. The greater the angle away from the optical axis, the smaller the relative luminous intensity. The half value angle of the LED can be estimated from Figure 1, and the viewing angle value is.

In order to meet the requirements of large viewing angle display effect of LED display screen, led with large viewing angle is generally selected. In this paper, the LED light emitting tube with circular light emitting surface is analyzed.

3. Brightness uniformity measurement of LED display screen

The measurement of brightness uniformity specified in the test method for LED display screen is only for the brightness measurement in the optical axis direction of LED light-emitting tube. Figure 2 is the front view of brightness measurement of LED display screen. For convenience, only three areas of LED display screen are analyzed. See Fig. 2 for the positions of display area 1, display area 2 and display area 3. The luminous intensity angular distribution curve of display area 1, display area 2 and display area 3 and the detection position 1, detection position 2 and detection position 3 are shown in Fig. 3. The brightness of the three display areas is measured according to the test method for LED display screen. The detection position 1, detection position 2 and detection position 3 are located in the optical axis direction of display area 1, display area 2 and display area 3 respectively. The measured brightness value is the maximum brightness value of each area. This paper assumes that the luminous intensity angular distribution curves of the three display areas are the same, so it is obtained that the luminous intensity of the three display areas of LED display screen is the same, and the brightness uniformity of LED display screen reaches the national C-level standard.

4. Influence of LED viewing angle on brightness uniformity of display screen

Due to the problem of the viewing angle of the LED itself, the brightness of the LED display screen will change with the change of the viewing angle, which affects the display effect of the display screen. When the observer looks at the display screen from different angles, the change of brightness will have a negative impact on the brightness uniformity of LED display screen.

In the actual use of LED display screen, the observer will observe the display screen within a certain range. Taking Fig. 4 as an example, suppose the observer observes the LED display screen at observation position 1, observation position 2 and observation position 3, and the visual axes of the observer at observation position 1, observation position 2 and observation position 3 coincide with the optical axes of display area 1, display area 2 and display area 3 shown in Fig. 2.

When viewing position 1, the brightness of display area 1 is the largest, the brightness of display area 3 is the smallest, and the brightness of display area 2 is between the two; When viewing position 2, the brightness of display area 2 is the largest, and the brightness of display area 1 and display area 3 are basically the same, but less than that of display area 2; When viewing position 3, the brightness of display area 3 is the largest, the brightness of display area 1 is the smallest, and the brightness of display area 2 is between the two. Although the angular distribution curves of LED luminous intensity in the three display areas are the same, the brightness values of the three areas are different in the same observation position due to the influence of LED viewing angle.

In the display area 1, the brightness is the maximum in the observation position 1, the brightness is the minimum in the observation position 3, and the brightness is between the two in the observation position 2; The display area 2 has the maximum brightness at the observation position 2, and the brightness is basically the same at the observation position 1 and the observation position 3, but less than that at the observation position 2; In the display area 3, the brightness is the maximum in the observation position 3, the brightness is the minimum in the observation position 1, and the brightness is between the two in the observation position 2. Therefore, the brightness of the same area of the LED display screen will change with different observation angles.

According to the change law of LED luminous intensity angular distribution curve, the whole screen brightness distribution of LED display screen is simulated at observation position 1, observation position 2 and observation position 3. The brightness distribution of LED display screen is shown in Figure 5. Fig. 5 (a) is a simulation of the brightness distribution of the LED display observed at the observation position 1. Fig. 5 (b) is a simulation of the brightness distribution of the LED display observed at the observation position 2. Fig. 5 (c) is a simulation of the brightness distribution of the LED display observed at the observation position 3.

In a certain observation range, the brightness change of adjacent areas of LED display screen is determined by the slope of luminous intensity angular distribution curve. The larger the curvature of the angular distribution curve of luminous intensity, the greater the change of LED brightness in adjacent areas; The smaller the curvature of the luminous intensity angular distribution curve, the smaller the change of LED brightness in adjacent areas. The brightness takes the projection of the observation position on the display screen as the center of the circle and is distributed in a concentric circle. The center of the circle is the brightest. The brightness decreases with the increase of the radius. The brightness change is related to the curvature of the luminous intensity angular distribution curve.

By analyzing the brightness distribution of the three cases, it can be seen that the difference between the maximum brightness and the minimum brightness in different display areas of the LED display screen in observation position 1 and observation position 3 is relatively large. Therefore, the brightness uniformity of the LED display screen observed in observation position 1 and observation position 3 is poor. The difference between the maximum brightness and the minimum brightness in different display areas of the LED display screen observed at observation position 2 is small, so the brightness uniformity of the LED display screen observed at observation position 2 is good. However, the brightness uniformity of LED display screen observed at observation position 2 is much lower than that measured by LED display screen test method.

In an ideal state, the observer observes the LED display screen at different positions to obtain the brightness uniformity data of the display screen observed at different positions, which constitutes the three-dimensional distribution diagram shown in Fig. 6. Through the analysis of Figure 6, when the observer observes in the central area of the LED display screen, the brightness uniformity of the LED display screen is the best, but the brightness uniformity of the LED display screen observed in the central observation area is much lower than that measured by the LED display screen test method. Observe the LED display screen at the edge, and the brightness uniformity is poor. The farther the observation position is from the central observation area, the worse the brightness uniformity of LED display screen.

5. Discuss

In the practical application of LED display screen, the relationship between the brightness uniformity of LED display screen and the observation position can be simulated through the angular distribution curve of LED luminous intensity. The objective evaluation standard can be used to complete the evaluation previously only by human eyes, and the best observation range can be estimated. Its advantage is that the final display effect can be predicted in the R & D stage, so as to reduce risk and save time and money. As the evaluation of display screen is a cumbersome process and there are many uncertain aspects, this paper only briefly introduces the visual impact of LED perspective on the brightness uniformity of display screen, and does not take into account other LED packaging forms, the color lamp arrangement of full-color LED display screen, the non-uniformity of module luminous distribution and color mixing effect.

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Generally speaking, it is to make a giant color TV, which has been worked hard by many domestic manufacturers for a long time, and has made considerable progress, but it is still far from our ideal goal, and there is still a big gap compared with foreign full-color large screens.At the beginning of this year, the commercial street of Hunan Road in Nanjing invested heavily in the construction of a full-color large screen with an area of 114 square meters. The screen was built by UNITEK company in the UK and has been running for more than half a year. From the effect, it can be called the best large screen in China. The author participated in the formulation of the technical indicators of the screen and the final acceptance test, and felt that many aspects were worthy of learning and imitation by domestic peers.As a large full-color screen, its key technical indicators include: high enough brightness, viewing distance and viewing angle; Good color reduction; Sufficiently high grayscale capability; High enough scan refresh rate. High enough video signal processing capability, especially good consistency, structure and technology. The technical indicators set by Hunan Road Daping during bidding are:The white balance brightness is greater than 5000 CD / m2;White balance color temperature 6500k;The number of video processing bits is greater than 10bit;The gray level of each primary color is greater than 10bit, which can display 1.07 billion colors;Refresh frequency is greater than 400Hz;The display screen has no perceptible color difference, and the color difference of each primary color of the full screen is less than 4nm;There is no perceptible brightness difference on the display screen, and the brightness difference of adjacent pixels is less than 5%. There is no perceptible mosaic when displaying monochrome gray images of red, green and blue;MTBF is greater than 5000 hours and MTTR is less than 30 minutes;Objectively speaking, not only at that time, but also today, the products of domestic manufacturers have not been fully achieved. In many cases, due to the improvement of the level of LED, the brightness of the normally designed large screen is no longer a problem and can meet the requirements. However, brightness is only an important indicator, not all. Now we generally feel that the large screen brightness of domestic manufacturers is more than enough and the softness is insufficient. The overall effect is not as good as that of foreign well-known manufacturers. The reason is that there are more technical problems that have not been solved. The author believes that from a technical point of view, it is necessary to make efforts in the following aspects.(1) Color matching and blendingThe large screen is composed of a large number of light-emitting diodes. The photoelectric parameters of the light-emitting diodes must be designed, calculated and tested in order to achieve a good white balance. Simple estimation and judgment with the naked eye can not have good results. This involves the selection and driving design of LED, which not only affects the photoelectric performance of the screen, but also affects the reliability. Especially in the design of indoor full-color screen, thermal design should be paid enough attention, otherwise the effect and reliability will be a problem.
System Design of LED Display Screen
System Design of LED Display Screen
With the rapid development of science and technology, the development of traditional TV, newspaper and magazine media has lagged behind the development of outdoor media. Here we will introduce the system design of dot matrix LED dot matrix display.Composition of LED dot matrix displayComposition of LED dot matrix display: it is a new display device, which is composed of multiple LED's encapsulated together. In order to explain the principle of LED dot matrix display, a display system including hardware and software is designed and manufactured with single chip microcomputer as the control core. The circuit can realize various display forms such as rolling shutter, moving up and moving left of any number of Chinese characters through key control. It can be widely used in public places such as airports, shopping malls and hospitals for text advertising and information release. Moreover, it has the characteristics of small volume, simple circuit structure and easy implementation.LED dot matrix displaySystem hardware circuitThe hardware circuit of the display controller is mainly composed of three parts: serial communication interface, expansion of external memory, LED dot matrix display driver and interface circuit. Generally speaking, there are many contents to be displayed, so the external data memory must be expanded. Static data memory has the advantages of low cost, low power consumption and fast reading and writing speed. We use two pieces of static ram62256 data memory (64KB in total). Store all the content codes to be displayed in 62256.System software programSoftware program is the core of the whole control system. After the system main program starts, the system environment is initialized first, including setting serial port, timer, interrupt and port. The whole software design is mainly divided into two parts: display part and communication part.Communication part: the upper computer software adopts visual basic programming, and VB is used to provide a powerful communication control MSComm, which controls the message format and protocol setting of serial port and serial port communication, and directly uses the serial port of PC to send data. In order to realize the reliable communication between MCU and PC, both sides must have the same data format and baud rate. The use of the software is relatively simple. When using, you can use Microsoft Word to handle the content to be displayed (note that the width of the page setting should be the same as that of the LED dot matrix display), then select the select all command and copy command in the edit menu, and then run the LED display Download Software v1.1. Select the "clipboard" button in the interface, and the content to be downloaded can be displayed in the preview box, After checking, select the baud rate and connect to the serial port, click the "download" button, and you can download the display. Repeat the above operation when you need to modify or display new content. It is very convenient to use.Display part: use dynamic scanning to realize the control and display functions of Chinese characters, characters, images and data transmission on the display screen. The program divides the data memory into three areas: display buffer, data storage area and receiving buffer. The single chip microcomputer receives the data from the PC through the serial port interrupt, temporarily stores it in the receiving buffer, analyzes and processes it, saves it in the data storage area according to certain rules, and then extracts the data from the data in the memory according to the display mode sequence to display it in the display buffer. The display adopts progressive scanning. The real-time communication part of PC mainly uses the MCU serial port to interrupt the reception of data information and realize the data information transmission with the computer.
Composition and Working Principle of LED Display Screen
Composition and Working Principle of LED Display Screen
In this paper, LED display screen is composed of several display units (unit display board or unit display box) that can be combined and spliced, plus a set of appropriate controller (main control board or control system). Therefore, a variety of display boards (or unit boxes) with different control technologies can form many kinds of LED displays to meet the needs of different environments and display requirements.1 Main composition of LED display screen (taking the more complex synchronous full-color screen as an example)1. The metal structure frame is used to form an inner frame and carry various circuit boards such as display unit board or module and switching power supply2. Display unit: it is the main part of LED display screen, which is composed of LED lamp and driving circuit. The indoor screen is the unit display board of various specifications, and the outdoor screen is the module box.3. Scanning control board: the function of the circuit board is data buffer to generate various scanning signals and duty cycle gray control signals.4. Switching power supply: change 220V AC into various DC to supply various circuits.5. Transmission cable: the display data and various control signals generated by the main controller are transmitted to the screen body by twisted pair cable.6. Main controller: buffer the input RGB digital video signal, transform the gray level, reorganize it, and generate various control signals.7. Special display card and multimedia card: in addition to the basic functions of computer display card, it also outputs digital RGB signals and line, field, blanking and other signals to the main controller. In addition to the above functions, multimedia can also change the input analog video signal into digital RGB signal (i.e. video acquisition).8. Computer and its peripheralsFigure 1 LED display system composition2 Analysis of main functional modules1. Video broadcastThrough multimedia video control technology and VGA synchronization technology, various forms of video information sources can be easily introduced into the computer network system, such as radio and satellite TV signals, camera video signals, VCD video signals of video recorders, computer animation information, etc. Therefore, the following functions can be realized:Support VGA display and display various computer information, graphics and images.Support various input modes; Support pal, NTSC and other systems.Display color video images in real time to realize on-site broadcasting.Broadcast radio and television, satellite television and cable television signals.Real time playback of video signals such as TV, camera and video disc (VCR, VCD, DVD and LD).It has the function of playing pictures and words with different proportions on the left and right at the same time2. Computer broadcastGraphic stunt display function: it has the functions of editing, zooming, flowing and animation.Display various computer information, graphics, images and 2D and 3D computer animation and overlay text.The broadcasting system is equipped with multimedia software, which can flexibly input and broadcast a variety of information.There are a variety of Chinese fonts and fonts to choose from. At the same time, you can also input English, Spanish, French, German, Greek, Russian, Japanese and other languages.There are many broadcast modes, such as single / multiple line translation, single / multiple line up / down, left / right pull, up / down, rotation, stepless zoom, etc.Editing, broadcasting and instant release of notices, announcements, announcements and news, and there are a variety of fonts to choose from.3. Network functionEquipped with standard network interface, it can be connected with other standard networks (information query system, Municipal Publicity network system, etc.).Collect and broadcast real-time data of each database to realize remote network control.You can access the Internet through the network systemIt has a sound interface, which can be connected to audio equipment to achieve audio-visual synchronization.4. Screen controllerThe screen controller controls the display effect of its corresponding display screen. It can automatically run a continuous cycle program to obtain relevant display data according to the given program list, or it can intervene manually to produce the display effect on the screen. The pixels on the screen are directly mapped with the pixels on the corresponding area of the screen controller display.The screen controller can manually adjust the brightness of the display screen and select the gray nonlinear correction data suitable for the current environment. The screen controller can also automatically adjust the brightness of the display screen and select different gray nonlinear correction data according to the ambient brightness through the network system.The contrast and chroma of image display can be adjusted through the screen control machine. The screen controller transmits the information to be displayed to the video processing and control unit for video processing.5. Video processing and control unitThe video processing and control unit completes the image processing of the display data, including:The second level of gray nonlinear correction can be adjusted automatically or manually.Automatic or manual stepless brightness adjustment.The gray level control system can control the display screen to display 24 bit data of 8 bits of each primary color, that is, 256 gray levels of each primary color.Image noise reduction processing to ensure that the image is cleaner and smoother.Image enhancement technology, image edge enhancement technology.Motion compensation technology, chroma adjustment function.6. Visual processingIn the system, the brightness of each primary color of a single module can be adjusted respectively, the color balance can be adjusted, and the mosaic phenomenon can be basically eliminated. The brightness consistency of all modules and display modules is good, and the monochrome display has no visual difference.The video processing and control unit transmits the processed display information to the communication module for long-distance transmission to the display screen.7. Communication moduleThe communication module ensures that the display content of the screen control machine is accurately transmitted to the display screen for a long distance. The communication module encodes and decodes the transmitted information, and processes the long-distance transmission and reception data. The communication module transmits the display information to the data distribution and scanning unit.The LED display screen is usually composed of the main controller, the scanning board, the display control unit and the LED display screen body. The main controller obtains the color brightness data of each pixel of a screen from the computer display card, and then assigns it to several scanning boards. Each scanning board is responsible for controlling the dry lines (columns) on the LED display screen, and each line (column) The LED display signal on the is transmitted in series through each display control unit of the bank, and each display control unit directly faces the LED display body. The work of the main controller is to convert the signal of the computer display card into the data and control signal format required by the LED display screen. The function of display control unit operation is similar to that of image display screen. It is generally composed of shift register latch with gray level control function. However, the scale of video LED display is often larger, so we should use integrated circuits with larger integration scale. The function of the scanning plate is the so-called connecting link. On the one hand, it receives the video signal of the main controller, on the other hand, it transmits the data belonging to the current level to its own display control units, and also transmits the data not belonging to the current level to the next cascade scanning plate. The differences between video signals and LED display data in space, time, sequence and other aspects need to be coordinated by a scanning board.
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