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1 Color Consistency of LED Full Color Display Screen
Led full-color display is a new type of indoor and outdoor large-scale electronic communication media, which has the characteristics of large-scale, good environmental adaptability, high brightness, dynamic play, etc., and is unique in the field of indoor and outdoor large-scale electronic communication media. With the further improvement of cost performance, it is widely used in advertising, stage, signboard, transportation facilities, public places and other fields. In recent years of Spring Festival Gala, led full-color display screen is used as background and decoration. The stage design of the 2008 Spring Festival Gala, especially from the main scenes such as the front background, the side background and the ceiling background, uses a large number of LED full-color display screen, which makes the stage beautiful and the atmosphere first-class. The application of stage is only one of the applications of LED full-color display, and LED full-color display is entering into various application fields at an unprecedented speed.
2 Performance parameters of LED full color display
In recent years, China has become the manufacturing center of the global LED full-color display screen, and the technical quality level has been continuously improved. With the improvement of technical level, users and manufacturers have higher and higher requirements for various performance parameters of LED full-color display screen. About 70% of all performance parameters of LED full-color display depend on the performance parameters of LED devices. Therefore, LED devices become the most important key component of LED full-color display. Enhancing the understanding of the performance parameters of LED devices and selecting high-quality LED devices play an important and decisive role in improving the performance parameters of LED full-color display screen. The influence of LED device parameters on the performance parameters of LED full color display are shown in the following.
There are dozens of performance parameters of LED full-color display screen, including nearly 20 key parameters. The following describes the parameters of LED devices and the corresponding performance parameters of LED full-color display screen one by one.
(1) Brightness
Brightness is the basic index of LED full color display. In the industry, it is generally believed that the outdoor display screen should be no less than 5000nits, and the indoor display screen should be no less than 1000nits. The brightness of the whole screen depends on the superposition of the brightness values of the red, green and blue LEDs that make up each pixel. The brightness values of the red, green and blue are approximately in the proportion of 3:6:1. Set different current values of red, green and blue based on the factory nominal value of device current to achieve the above proportion. For example: 1800 MCD for green tube, 900 MCD for red tube and 300 MCD for blue tube of outdoor P20 full color display screen. The brightness of each pixel point is 3000 mmcd, and the maximum brightness of the whole screen is 7500nits which is the result of 2500 points plus 3000 mcd. Finally, the brightness required in the specification can be achieved by adjusting the current value. For the brightness, it is not the higher the better, and appropriateness is enough.
(2) wavelength
Led full-color display is composed of red, green and blue LEDs. Each color is measured by wavelength. The color of the display is closely related to this wavelength. The average wavelengths of red, green and blue LEDs determine the dynamic color gamut range of LED full-color display. Generally speaking, the wavelength of red LED is about 625nm, green is about 525nm and blue is about 470nm. LED full color display of high quality should have a wide range of color gamut.
(3) Consistency
① Brightness Consistency
The brightness consistency of LED full-color display depends on the brightness consistency of LED devices. Generally, it is required that the maximum and minimum luminance ratio of each color of LED device shall not be higher than 1.3, and then the luminance difference between two adjacent LEDs of the same color shall be less than 10% according to the scientific PCB lamp insertion mode.
② Chromaticity Consistency
The LED devices that make up the LED full-color display screen all need a certain wavelength range. If the wavelength range is too wide, it will affect the color consistency of the display screen. Generally speaking, the wavelength range of red, green and blue is required to be within 5nm.
③ Angle Consistency
Led full-color display screen is composed of LED devices with angles. The consistency of angles of LED devices will seriously affect the consistency of brightness and chroma of different angles of display screen. The consistency of red, green and blue brightness curves of LED devices (especially elliptical lights) with the change of viewing angle also seriously affects the consistency of brightness and chromaticity of different angles of the display screen. Among all the parameters of full-color display screen, this parameter is the most difficult to achieve. The difference of high-grade led full-color display is mainly due to this.
(4) Power Consumption
LED is a very energy-saving light-emitting device. The power consumption of LED devices determines the power consumption of LED full-color display. Taking P20 full-color outdoor display as an example, its average power consumption is about 250W per square meter in the video playing state. For some 24-hour broadcast advertisers, because of the large area and long playing time, power consumption is an indicator of their concern. Led with higher brightness can be used to make the display, so low current can be used. Generally speaking, the nominal brightness of red, green and blue LEDs is increased by 20%, and the power consumption of the display screen will be reduced by 15% when the brightness of the display screen remains unchanged.
(5) Perspective
The perspective of LED full-color display depends on the perspective of LED devices. At present, most of the outdoor display screens use elliptical led with horizontal viewing angle of 100 ° and vertical viewing angle of 50 °, while the indoor display screens use chip LED with horizontal and vertical viewing angle of 120 ° because of its particularity. The display screens on highways generally use circular led with viewing angle of 30°. Some high-rise display screens require high vertical viewing angle. The angle of view and brightness are contradictory to each other. Large angle of view will inevitably reduce the brightness. The choice of perspective needs to be determined according to the specific purpose.
(6) Failure Rate
An important index of LED full-color display screen is the blind spot rate of LED, which is completely determined by the quality of LED devices (excluding the influence of faulty welding, missing welding and poor contact in the production process of display screen). Before leaving the factory, LED devices shall be subject to various sampling tests, including normal temperature lighting, high temperature and humidity, cold and heat shock and other series tests, to ensure the stability, consistency and continuity of products. Generally speaking, the failure rate of high-quality LED devices for full-color display screen in 1000 hours should be less than 1 in 10000.
(7) Attenuation Characteristics
After working for a long time, the LED full-color display screen will appear the phenomenon that the brightness drops and the color of the LED screen is inconsistent, which is mainly caused by the brightness decay of LED devices. The attenuation of LED brightness will reduce the brightness of the whole screen. The inconsistent attenuation amplitude of red, green, blue LED brightness will cause LED display color inconsistency, that is the phenomenon of the blurred display what we often say. LED device with high quality can control the brightness attenuation amplitude well. According to the standard of lighting 20mA at room temperature for 1000 hours, the red attenuation should be less than 2%, and the blue and green attenuation should be less than 10%. Therefore, the blue and green LEDs should not use 20mA current as much as possible when designing the display screen, and it is better to use only 70% to 80% of the rated current.
Besides the related characteristics of the red, green and blue LEDs, the attenuation characteristics are affected by the current used, PCB heat dissipation design, and the ambient temperature of the display screen.
(8) Size
The size of LED device affects the pixel distance of LED full-color display screen, that is, the resolution. The oval lamp of 5mm is mainly used for outdoor display screen above p16, The oval lamp of 3mm is mainly used for outdoor display screen of p12.5, p12 and P10, and 3528 patch LED is mainly used for indoor display screen of P6 and P8, and 5050 patch LED is mainly used for semi outdoor display screen such as p16 and p12.5. On the premise of constant point spacing, the size of LED devices increases, which can increase the display area and reduce the particle sense. However, due to the decrease of black area, it will reduce the contrast; on the contrary, the size of LED decreases, which reduces the display area, increases the particle sense, the black area and the contrast.
(9) Life Expectancy
The theoretical life of LED device is one hundred thousand hours, which is much longer than the working life of other parts of LED full color display screen. Therefore, as long as the quality of LED device is guaranteed, the working current is appropriate, the PCB heat dissipation design is reasonable, and the production process of display screen is rigorous, LED device will be one of the most durable parts in the display screen.
LED device is the most important component of LED full-color display screen. Its performance parameters play a decisive role in the performance parameters of the display screen, and its cost also accounts for half of the total cost of the display screen. It is very necessary for the design and manufacture of display screen to select, use and understand the influence of LED devices on display screen parameters, especially for high-quality LED full-color display screen. At present, the domestic LED industry is developing rapidly. The technology level of LED used in full-color display screen is comparable to that of foreign brands, and the price is appropriate. It has been widely used in the 2008 Olympic Games project and other major projects, and its performance is not bad, which will promote the transformation of China from a large LED display manufacturing country to a strong LED display manufacturing country.