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Why is the contrast of different LED displays so different?


  1. For LED display screens, the illuminance is one of the important parameters. The greater the illuminance, the clearer the image and the brighter the color. The contrast of the LED display screens on the market is very different, and the value is often different by several digits.

       Data comparison between an SMD LED display screen on the market and a full-flip Mini COB display screen

       Why is the difference so big?

    According to the calculation formula of illuminance: illuminance = brightness when emitting light (luminous brightness)/brightness when not emitting light (reflected brightness). It can be seen that if you want to improve the illuminance of LED display screen, the main methods include increasing the brightness of the display screen to make " "Brighter and brighter", and reduce the light reflection coefficient of the external surface of the display screen to make "blacker and darker".

       Then, do you know the reasons for the difference in illuminance of the small-pitch LED display screen in practical applications?

      Different chip sizes

       Generally speaking, the smaller the chip used, the higher the specific illuminance of the LED display screen.

       Under the same dot pitch, the smaller the size of the LED chip, the larger the black area, and the lower the brightness when no light is emitted. Although the light-emitting area is reduced, the specific illuminance has been greatly improved. This is currently the most direct and effective way to improve the illuminance.

       Under the premise of the same dot pitch, the smaller the chip, the larger the "leave black" area, which can make the overall display screen "blacker and darker"

      Industry pioneer Sony is the world's first manufacturer to launch a large-size high-end display system based on Micro LED technology. The excellent display effect of its Crystal LED "black crystal" display system has left a deep impression on countless people in the industry. This product using Micro LED chips can theoretically achieve a dot pitch below P0.4, but Sony’s dot pitch is P1.26. Industry insiders analyzed that the purpose of keeping a large dot pitch is to save enough black areas, so that the product will reach the ultra-high contrast ratio of 1,000,000:1.

       Formal chip is different from flip chip

       On the other hand, the contrast ratio of flip chip will be better than that of formal chip.

    In the formal LED chip, the electrode and the bonding wire are on the light-emitting surface, occupying a part of the area that should be light-emitting, which will affect the light-emitting angle and luminous intensity of the LED chip to a certain level; and the metal components such as the electrode and the bonding wire It will also form a sense of reflection, which will make "the bright time darker" and reduce the specific illuminance of the display screen.

      Flip chip, no electrodes and bonding wires to block and reflect light, large light-emitting area, luminous efficiency is significantly better than that of formal-mounted chips, making "brighter and brighter", the specific illuminance is usually higher than that of formal-mounted chips.

        Under the same COB packaging method, the flip-chip structure has a larger light-emitting area

      SMD and COB packaging methods are different

      The specific illuminance of COB packaged products is often much higher than that of traditional SMD packaged products. This is because:

      SMD LED bracket structure problem

      SMD LED display screen requires a support structure to support chip devices. However, due to the size limitation of the bracket, the package body using the SMD bracket cannot continue to decrease, and there is a physical limit for RGB light-emitting pixels. The large area of the light-emitting point means that the "black area" on the external surface of the LED display screen is small, and it cannot make the "black blacker", resulting in low LED display screen's specific illuminance.

       Since the COB packaging method does not have a bracket structure, with the support of Mini/Micro LED chip technology, display screen manufacturers can use smaller chips according to different viewing needs. The smaller the chip area on the PCB board, the larger the black area, the clearer the bright black area of the LED display screen, which can present a darker black field, brighter brightness, and make the LED display screen higher than the illuminance. .

      The structure of COB and the distance between light-emitting pixels are better than SMD package under the condition of the same size of the chip.

      SMD LED bracket color problem

    Due to the lack of brightness of early LED chips, when the chip is applied to the SMD display screen, a white bracket is required to strengthen the focus reflection and improve the brightness of the display screen, but this also loses the black area, and the color performance of the display screen is lost and low contrast. The question of degree.

    Later, as the luminous intensity of LED chips became higher and higher, display screen manufacturers began to use black bracket SMD, or add black layers on the luminous surface to make "black blacker" and improve the contrast illuminance to meet the audience’s fidelity of the image. demand. However, there is also a color difference between the black of the bracket itself and the black when the LED is turned off, and the overall contrast enhancement effect is limited.

       Since the COB packaging method does not have a bracket structure, its specific illuminance is not affected by this factor and can be maintained at a high level.

      The difference between black light SMD and white light bracket SMD

      Different measurement methods

       Different measurement methods of contrast illuminance will also have a great impact on the contrast illuminance data. Fundamentally, the illuminance is also divided into static illuminance and application illuminance.

       Static contrast is the highest contrast of the screen measured in a dark room environment. Generally speaking, as the brightness of the LED display screen is very high under the background of the darkroom environment, the static contrast illuminance is often as high as tens or even millions to one.

    The applied illuminance refers to the illuminance tested in accordance with the "Light-Emitting Diode (LED) Display Screen Test Method" issued by the Ministry of Industry and Information Technology of the People’s Republic of China-the highest illuminance of the screen measured under an environmental illuminance of 10±10%lx degree. According to industry standards, the application of LED display screen with a specific illuminance greater than or equal to 5000:1 is the highest level C. The application ratio of the unique ink color "Ice Jade Black" of Zhongqi Optoelectronics is as high as 15000:1, which is 3 times the industry standard, and the display effect is smart and lifelike.

       In summary, it can be seen that the separation of the miniaturized LED chips of the full flip-chip and COB packaging is of great value for improving the contrast ratio of the display screen. At present, with the blessing of Mini/Micro LED chips, a full-flip COB display screen will be the best processing plan for high-contrast illuminance and small-pitch LED display screens.

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