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Hispider tells you the shortcomings of OLED cell phone screen at this stage

May 26 , 2023

The advantages of OLED screens are extreme, but so are the disadvantages.
The first disadvantage:
Short life span. OLED stands for organic self-emitting diode. The luminous part of the LCD screen is the backlight layer, and the night layer is only responsible for deflecting and controlling the amount of light output, while the filter layer is only responsible for changing the color of light and does not participate in luminescence. The voltage is applied to the LCD screen that does not participate in luminescence, while the OLED screen voltage is directly applied to the self-luminous diode, which will lead to frequent electron migration in the OLED luminous layer.

The luminous layer itself is organic matter, organic matter is very easy to aging compared with inorganic matter. Organic matter and frequent electron migration and self-luminous, directly lead to OLED screen life is significantly shorter than LCD screen. The LCD screen backlight is a block to age, all the pixels with a block aging. Each pixel of an OLED screen emits light independently, which means that different areas of the screen age at different rates depending on how much they are being used. The essence of screen burning is not to say that the physical layer of the screen burned, but the uneven aging of pixels caused by the screen color difference.


The second disadvantage:
Dimming mode. At present, there are two main ways to control brightness: PWM and DC. DC dimming is very simple, directly controls the voltage to change the brightness of the lamp, high voltage lamp light, low voltage lamp dark, DC dimming because the light source is always on, will not cause eye injury phenomenon.

But PWM is different, PWM is used to adjust the duty ratio to control brightness. To control the brightness is simply: turn on and turn off. When the frequency of switching is frequent, it appears to the naked eye that the light is always on. Brightness control, just need to adjust the switch time ratio can be. For example, if a light can be switched on and off ten times a second, its frequency would be 10 Hertz, and a switch would have a period of one Hertz of 0.1 seconds.

If the light is on 60 per cent of the time, off 40 per cent of the time, or 0.06 seconds, and on and off for 0.04 seconds, and so on, the light is 60 per cent of its maximum brightness. Similarly, if a cycle is on for 20% of the time and off for 80% of the time, that is, on for 0.02 seconds and off for 0.08 seconds, the same cycle is repeated, and the light is 20% brighter. So PWM dimming has a very big drawback "eye damage". Because the PWL uses an on-off control mode, a stroboscope is produced.

If the PWM frequency. At very low levels, the stroboscopic interval is easily picked up by the human eye. If you are in a stroboscopic environment all the time, your eyes will get tired. But if the PWM frequency is very high and the stroboscopic interval is very short, your naked eye can't catch it, and the damage to the human eye is much less.

Due to the characteristics of OLED screen, if DC dimming is used, when the brightness is too low, the voltage is too low, the screen will appear as uneven as the rag, which has a great impact on the display screen. OLED screens can't use DC dimming, only PWM. Because OLED organic materials are easy to age, OLED screens can't use high-frequency PWM dimming, so only low-frequency PWM can be used.

The use of low frequency PWM dimming OLED although eye damage, but does not prove that the LCD screen is not eye damaged. Several high-brightness LED beads backlight traditional LCD screens. These LED emit a significant portion of their high-energy blue light in this region. At the same time, OLED screens modify the organic light-emitting layer to significantly reduce the energy of blue light and reduce the damage OLED screens to the eyes.

LCD blue light is stronger than OLED, and OLED using PWM dimming also hurt eyes.


The third disadvantage:
Theoretical resolution falls short of theoretical clarity. Resolution is the number of pixels on the screen. With the same screen size, the higher the resolution, the greater the number of pixels on the screen, and the higher the density of pixels, the higher the sharpness. Screen sharpness is measured in PPI. PPI is calculated as the square of the horizontal resolution plus the square of the vertical resolution, and then divided by the screen size. PPI up to 300 is enough, 360 is clear, 420 is sharp.

This formula can only be applied to RGC aligned LCD screens, not OLED screens. The traditional LCD screen adopts RGD standard arrangement, and the three sub-pixels of RDB are the same in size and number. For the AMOLED screen, Pentilie arrangement is adopted. The number of green pixels is twice that of red and blue pixels, that is, RGCG arrangement. The area of single red and blue pixels is larger than that of green pixels.

The red and blue pixels arranged by Pentitle are arranged at a 45° right Angle, rather than horizontally and vertically. When the screen pixel PPI is not high, there will be obvious jagged feeling when displaying horizontal and vertical fonts or pictures. OLED screen theory, PPI finally fully apply the above formula, but also need to multiply by the square root of 2/3, which is about 81.65%. In other words, at the same resolution, a Pentitle screen has 81.65% of the resolution of an RGD screen. Pentitle arrangement can reduce the planting cost of OLED pixels and solve the aging problem of OLED screen.


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