The influence of LED devices on the performance parameters of LED full-color display screens

1. OverviewLED

full-color display is a new type of indoor and outdoor large-size electronic communication media. It has the characteristics of large size, good environmental adaptability, high brightness, dynamic playback, etc. It is unique in the field of indoor and outdoor large-size electronic communication media. With the further improvement of cost performance, the application field is becoming wider and wider, and it is widely used in advertising, stage, signboard, transportation facilities, public places and other fields. In recent years, the Spring Festival Gala has used LED full-color display as background and decoration. The stage design of the 2008 Spring Festival Gala used a large number of LED full-color display screens from the main scenes such as the front background, side background, and ceiling background, making the stage beautiful and the atmosphere setting effect first-class. The application of the stage is only one aspect of the application of LED full-color display screens. LED full-color display screens are entering various application fields at an unprecedented speed.

2. Performance parameters

of LED full-color display screens In recent years, China has become the global manufacturing center of LED full-color display screens, 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 screens. Of all the performance parameters of LED full-color display screens, about 70% are determined by the performance parameters of LED devices. Therefore, LED devices have become the most important key components of LED full-color display screens. Strengthening the understanding of LED device performance parameters and selecting high-quality LED devices play an important and decisive role in improving the performance parameters of LED full-color display screens.

III. The influence of LED device parameters on the performance parameters of LED full-color display screens

There are dozens of performance parameters of LED full-color display screens, among which there are nearly twenty key parameter indicators. The following is a description of the LED device parameters and the corresponding influence on the performance parameters of LED full-color display screens.

(1) Brightness

Brightness is a basic indicator of LED full-color display screens. The industry generally believes that outdoor display screens should not be less than 5000nits and indoor display screens should not be less than 1000nits. The brightness of the entire screen depends on the superposition of the brightness values ​​of the three LEDs, red, green and blue, which make up each pixel. The brightness values ​​of red, green and blue are approximately in the ratio of 3:6:1. Different current values ​​of red, green and blue are set based on the factory nominal value of the device current to achieve the above ratio. For example: the outdoor P20 full-color display uses 1800mcd for green tube, 900mcd for red tube, and 300mcd for blue tube, then the brightness of each pixel is 3000mcd, and the maximum brightness of the whole screen is 2500 points × 3000mcd = 7500nits. Finally, by adjusting the current value, the brightness required by the specification can be achieved. The higher the brightness, the better, as long as it is appropriate.

(2) Wavelength

The LED full-color display is composed of three types of LEDs: red, green, and blue. Each color is measured by wavelength. The color of the display is closely related to this wavelength. The average wavelength of the red, green, and blue LEDs of the display determines the dynamic color gamut range of the LED full-color display. Generally speaking, the wavelength of the red LED is selected at around 625nm, the green at around 525nm, and the blue at around 470nm. A high-quality LED full-color display should have a wider color gamut range.

(3) Consistency

① Brightness consistency

The brightness consistency of the LED full-color display depends on the brightness consistency of the LED device. Generally, the ratio of the highest to the lowest brightness of each color of the LED device is required to be no higher than 1.3. Then, according to the scientific PCB lamp insertion method, the brightness difference between two adjacent LEDs of the same color is guaranteed to be less than 10%.

② Chromaticity consistency

The LED devices that make up the LED full-color display screen all require a certain wavelength range. If the wavelength range is too wide, it will affect the chromaticity consistency of the display screen. Generally speaking, the wavelength range of each color of red, green and blue is required to be within 5nm.

③ Angle consistency

The LED full-color display screen is composed of angled LED devices. The angle consistency of the LED device will seriously affect the brightness and chromaticity consistency of the display screen at different angles. The consistency of the red, green and blue brightness curves of LED devices (especially elliptical lamps) with the change of viewing angle also seriously affects the brightness and chromaticity consistency of the display screen at different angles. Among all the full-color display screen parameters, this parameter is the most difficult to achieve. The difference of high-end LED full-color display screens mainly lies in 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 screens. Taking the P20 full-color outdoor display screen as an example, its average power consumption is about 250W per square meter in the video playback state. For some 24-hour advertising clients, power consumption is an indicator of concern due to the large area and long broadcast time. Higher brightness LEDs can be used to make display screens, so that low current can be used. Generally speaking, if the nominal brightness of red, green, and blue LEDs is increased by 20%, the power consumption of the display screen will be reduced by 15% when the brightness of the display screen remains unchanged.

(5) Viewing angle

The viewing angle of LED full-color display screens is determined by the viewing angle of the LED device. At present, most outdoor display screens use elliptical LEDs with a horizontal viewing angle of 100° and a vertical viewing angle of 50°, while indoor display screens use SMD LEDs with a horizontal and vertical viewing angle of 120°. Due to their special nature, display screens on highways generally use circular LEDs with a viewing angle of 30°. Some display screens on high-rise buildings have higher requirements for vertical viewing angles. Viewing angle and brightness are contradictory. A large viewing angle will inevitably reduce brightness. The choice of viewing angle needs to be determined according to the specific purpose.

(6) Failure rate

An important indicator of LED full-color display screen is the blind spot rate of LED, which depends entirely on the quality of LED devices (excluding the effects of false soldering, leaking soldering, poor contact, etc. in the display screen production process). LED devices should undergo various sampling tests before leaving the factory, including a series of tests such as normal temperature lighting, high temperature and high humidity, and cold and hot shock tests to ensure the stability, consistency, and continuity of the product. Generally speaking, the failure rate of high-quality LED devices used for full-color display screens should be less than one in ten thousand after 1,000 hours.

(7) Attenuation characteristics

After long-term operation, the LED full-color display screen will experience a decrease in brightness and inconsistent display color, which is mainly caused by the brightness attenuation of the LED device. The attenuation of LED brightness will cause the brightness of the entire display screen to decrease. The inconsistency of the attenuation amplitude of the red, green, and blue LED brightness will cause the color of the LED display screen to be inconsistent, which is what we often call the phenomenon of the display screen being mottled. High-quality LED devices can well control the brightness attenuation amplitude. According to the 20mA standard for lighting 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, when designing a display screen, try not to use a 20mA current for blue and green LEDs. It is best to use only 70% to 80% of the rated current.

In addition to the characteristics of the red, green, and blue LEDs themselves, the attenuation characteristics are also affected by the current used, the heat dissipation design of the PCB board, and the ambient temperature of the display screen.

(8) Size

The size of the LED device affects the pixel distance of the LED full-color display screen, that is, the resolution. 5mm elliptical lamps are mainly used for outdoor displays above P16, 3mm elliptical lamps are mainly used for outdoor displays of P12.5, P12, and P10, 3528 type SMD LEDs are mainly used for indoor displays of P6 and P8, and 5050 type SMD LEDs are mainly used for semi-outdoor displays such as P16 and P12.5. Under the premise of keeping the pixel pitch unchanged, increasing the size of LED devices can increase the display area and reduce the graininess, but the contrast ratio will be reduced due to the reduction of black area. On the contrary, decreasing the size of LED devices will reduce the display area, increase the graininess, increase the black area, and increase the contrast ratio.

(9) Lifespan

The theoretical life of LED devices is 100,000 hours, which is much longer than the working life of other components of LED full-color display screens. Therefore, as long as the quality of LED devices is guaranteed, the working current is appropriate, the PCB heat dissipation design is reasonable, and the display screen production process is rigorous, LED devices will be one of the most durable components of the display screen.

LED devices are the most important components of LED full-color display screens. Their performance parameters play a decisive role in the performance parameters of the display screen, and their cost accounts for half of the total cost of the display screen. It is very necessary to select, use and understand the impact of LED devices on display screen parameters for the design and manufacture of display screens, especially for high-quality LED full-color display screens. At present, the domestic LED industry is developing rapidly. The technical level of LED used in full-color display screens is comparable to that of foreign brands, and the price is appropriate. It has been widely used in major projects such as the 2008 Olympic Games and has performed well. This will surely promote China's transformation from a major LED display screen manufacturing country to a strong LED display screen manufacturing country.