Technology|Contrast, why is there so much difference between different LED displays?

As we all know, for LED display, the contrast ratio is one of the important parameters. The higher the contrast, the sharper the image and the more vivid the color. The contrast ratio of LED displays on the market varies greatly, and the value is often different by several digits.

According to the calculation formula of contrast ratio: contrast=brightness when emitting light (luminous brightness)/brightness when not emitting light (reflection brightness), it can be seen that if you want to improve the contrast of LED display, the main methods include increasing the brightness of the display to make the "brighter" Brighter", as well as reducing the light reflectance of the display surface to make "blacker blacker". So do you know what are the reasons for the difference in contrast of small-pitch LED displays in practical applications?

1.different chip sizes

Generally speaking, the smaller the chip used, the higher the contrast ratio of the LED display.

Under the condition of the same point spacing, the smaller the LED chip size, the larger the black area, and the lower the brightness when no light is emitted. Although the light-emitting area is reduced, the contrast is greatly improved. This is the most direct and effective way to improve contrast at present.

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

2.Form chip is different from flip chip

On the other hand, flip-chip contrast will also be superior to full-chip. In the front-mounted LED chip, the electrodes and bonding wires are both on the light-emitting surface, occupying a part of the area that should emit light, which will affect the light-emitting angle and luminous intensity of the LED chip to a certain extent; and metal components such as electrodes and bonding wires are also It will cause a sense of reflection, make it "dark when it should be bright", and reduce the contrast of the display.

The flip-chip has no shielding and reflection by electrodes and bonding wires. The light-emitting area is large and the luminous efficiency is obviously better than that of the regular chip, making it "brighter", and the contrast is usually higher than that of the regular chip.

In the case of the same COB packaging method, the light-emitting area of the flip-chip structure is larger.

3.SMD and COB packaging are different

Products in COB packages tend to have a much higher contrast ratio than products in traditional SMD packages because:

SMD LED bracket structure problem

The SMD LED display needs to use a bracket structure to support the chip device. However, due to the size limitation of the bracket, the package body using the SMD bracket cannot continue to shrink, and there is a physical limit to the RGB light-emitting pixels. The large area of the light-emitting point means that the "black area" on the surface of the LED display is small, which cannot make the "black blacker", resulting in a low contrast ratio of the LED display.

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

In the case of the same size of the positive chip, the structure of the COB and the distance between the light-emitting pixels are better than the SMD package.

4.Measurements are different

Differences in how contrast is measured can also have a large impact on contrast data. Basically, contrast is also divided into static contrast and application contrast.

Static contrast ratio is the highest contrast ratio of the screen measured in a completely dark room environment. Generally speaking, due to the very high brightness of LED displays in the dark room environment, the static contrast ratio is often as high as tens or even millions to one.

The application contrast ratio refers to the contrast ratio tested according to the test method of light-emitting diode (LED) display screen in the "Electronic Industry Standard of the People's Republic of China" issued by the Ministry of Industry and Information Technology - the highest contrast ratio of the screen measured under the ambient illumination of 10±10%lx. According to industry standards, the application contrast ratio of LED display screen is greater than or equal to 5000:1, which is the highest level of C-level.