How LCD and OLED work


1, LCD working principle

LCD profile as shown below, there are 7 main layers, from bottom to top is the backlight layer, vertical polarizer, positive circuit, liquid crystal layer, negative circuit, horizontal polarizer and color filter.

The role of the bottom backlight layer is to emit white light, and after the white light penetrates the color filter with color, it can become the corresponding color of light.
Note: The backlight layer is not a separate LED light bead under the pixel, but a large backlight layer shared by all pixels throughout the screen.

When we apply a voltage to the positive circuit, it will penetrate the liquid crystal layer, connect the negative circuit to form a loop, and this voltage will drive the liquid crystal layer to deflect, and the liquid crystal layer will block part of the light like a blind. At this time, we only need to control the size of the voltage, which can control the deflection Angle of the liquid crystal molecules, and then control the brightness of the red, green and blue sub-pixels. By changing the brightness ratio, you can mix all the colors you want.

2, OLED working principle

OLED stands for organic Light-Emitting Diode (Organic light-emitting Diode), which, like LED screens, also has three sub-pixels.
The difference is that OLED has no liquid crystal layer and backlight layer, and is itself a specially designed self-luminous diode. By controlling the voltage, and thus the current flowing through the diode, to change the brightness, you can control the color ratio of each sub-pixel, and finally mix the color you want.

OLED has no backlight layer, and each pixel can be independently controlled on and off, so there is no need to light up the entire backlight layer when it is turned on like an LCD screen. Each pixel of the OLED screen can be understood as an independently controlled lamp bead, which can be used to remind the screen, and some pixels can be lit separately when the phone locks the screen, and some important information such as time and notification can be displayed with low brightness and low refresh rate.


The pros and cons of LCD and OLED
1, OLED power consumption is lower

As long as the LCD screen is turned on, the whole screen is bright, and it will always consume power.
The OLED screen only opens the display of pure white screen, the power consumption will be higher than LCD, but each pixel of the OLED screen is independently controlled brightness and switch, so that you can reduce the brightness respectively, or even turn off. Therefore, unless you stay in the white picture for a long time, the OLED will last longer.
2, OLED contrast is higher

Contrast refers to the brightness ratio of black and white to dark; The higher the contrast, the brighter the picture and the more intense the colors.
If the LCD wants to display pure black, the ideal state is that the liquid crystal molecules are completely closed, completely blocking the emitted backlight, but the deflection of the liquid crystal molecules cannot be completely closed. Therefore, when displaying black, there will be a small amount of white light shining out, so what you see on the screen is not pure black, but gray with a large decrease in brightness, which determines that the LCD cannot display black in the true sense of pure black.

The OLED itself does not have a backlight, and each pixel is independently controlled, so it can directly disconnect the power supply of the display black pixels, so that it does not emit light at all, and achieve pure black in the true sense. As a result, OLED screens have higher contrast.
3, OLED response time is faster, dynamic display has more advantages

The color switch of the LCD screen is accomplished by controlling the deflection of the liquid crystal molecules. Therefore, the speed of the liquid crystal layer deflection directly determines the gray response time of the LCD. The deflection of the liquid crystal layer is related to temperature, and the lower the temperature, the slower the deflection speed, resulting in a very obvious drag phenomenon on the LCD screen at low temperatures.
OLED screens have no liquid crystal layer and are naturally not limited by the temperature of the liquid crystal layer. However, the OLED screen is not without response time, the OLED screen switches between colors, the response time is very short, and there is almost no dragging phenomenon. But the time it takes to switch from pure black to pure white, or from pure white to pure black, is much longer than other colors, but the time it takes to switch between black and white colors is also shorter than most LCDS.
Therefore, OLED has a natural advantage in the display of dynamic screen.
4, OLED can be thinner and easier to fold

The OLED screen has no backlight layer and liquid crystal layer, so it is very easy to make thin, and can be bent as much as paper. The ability to bend means that the curved screen can be easily achieved, which can significantly improve the texture of the phone itself.
LCD removes the backlight layer and the liquid crystal layer, and there are some partial positive films in the middle, so it is much thicker than OLED. The backlight layer and the liquid crystal layer are rigid substrates that can only bend very small amounts and can only be seen on large panels such as desktop monitors.
5, OLED life is shorter

LCD is used to light the backlight layer, the liquid crystal layer is only responsible for deflection control of the amount of light, the filter layer is only responsible for changing the color of the light, does not participate in the light, the voltage is applied to the liquid crystal layer that does not participate in the light.
The OLED screen voltage is directly applied to the self-luminous diode, resulting in frequent electron migration in the OLED light-emitting layer. In addition, the luminous layer itself is organic matter, organic matter is very easy to age compared to inorganic matter, so organic matter + frequent electron migration + self-luminescence, directly resulting in the OLED screen life is significantly shorter than the LCD screen.
6, LCD has the disadvantage of light leakage

Since the LCD screen has a backlight layer, and the screen panel is finally installed in the mobile phone or display, the backlight of the backlight layer is easy to leak out, when the display of pure black picture, there will be a large area of halo phenomenon at the border, which is called light leakage.
7. OLED is prone to screen burning

LCD screen backlight is a whole block of all pixels will age together. Each pixel of the OLED screen is independently illuminated, which means that different areas of the screen age at different rates according to the degree of use.

For example, if the A region displays blue for a long time, the blue pixels decay faster. The next time a solid color is displayed, the blue in that area will be slightly darker, resulting in residual shadows, as if the picture is burned on the screen, a phenomenon known as screen burning. It is not that the physical layer of the screen is burned, but the screen color difference caused by uneven aging of pixels.
8, LCD and OLED will hurt the eye, but different

The most harmful part of the light emitted by the screen to the human eye is 420-440 wavelength blue light, these high energy light will cause irreversible damage to the human retina, the traditional LCD screen backlight is completed with several bright LED light beads plus flat light plate, the light emitted by these LED light beads, the high energy blue light has a considerable part in this interval.
LCD blue light strong damage eyes, OLED using PWM dimming also damage eyes. In order to save costs, some low-end screens also use PWM dimming on the LCD. It's hard to say which will hurt more.