LCD screens are generally divided into two types: normally white and normally black. Normally black includes LG's IPS screen, Samsung's SPVA screen, and Taiwan AUO's MVA screen, etc.

And their basic principles are the same:

1. Backlight it is a point light source emitted - a kind of light that shoots "in all directions"

2. After passing through the light guide plate, the backlight will become very uniform

3. When the "all directions" light passes through the polarizer, it will be screened out, and the polarizer will only allow "horizontal" to pass through the polarizer, while those "vertical" cannot pass through.

4. The liquid crystal material is optically active, it will slowly rotate the "horizontal" light

5. When it rotates to 90°, 270°, etc., it can be completely "vertical", and then passes through the filter plate (this TN type liquid crystal matching method, the filter plate and the polarizer are at 90°), its brightness 100%; and when the rotation becomes 180°, 360°, etc., it will not be able to pass through its filter plate at all, and its brightness will be 0%; the grayscale of other angles will be between 0-100%.

6. At this time, there are only brightness colors between black and white, so each group of 3 liquid crystals will pass through the red, green and blue color filter glasses to form three primary colors

7. Such a group of 3 liquid crystals, 3 colors (the ratio is determined by the angle of rotation of the liquid crystals) are superimposed to form a required color, such a group of liquid crystals becomes a pixel.

By controlling the voltage of the liquid crystal screen, the twisting angle of the liquid crystal can be controlled, that is, the brightness obtained through the liquid crystal can be controlled.

If you can understand the above principles, the following conclusions can be easily understood.

Why do the black and white LCDs we see mostly display black?

Mainly due to cost, necessity and longevity:

1. There is no need to use colored ones, which can save the cost of color filter glass.

2. There is no need to combine the three primary colors into one, the same panel, the pixels are three times the original.

3. It is only suitable for two colors of black and white, instead of the uniform gray scale of black and white, which can reduce the cost of its control circuit.

4. The voltage control can also be very random. It does not need to be very black or white, it is almost enough, and the accuracy of the circuit design is also reduced a lot.

5. The use of circuits and components is less, the life and stability will be higher, and the cost will be lower.

6. For some products that use ambient light as a light source without a backlight, the backlight is omitted, and the power supply and energy consumption of the backlight are also omitted.

For the question of why the background black is not used to display white, the answer is: the more commonly used display panels such as glass and plastic are transparent, and the cost of blackening glass and plastic will increase

Supplement: When the LCD is turned off, it is also black. That is because most LCDs use the display method as described above. When the power is turned off, the liquid crystal does not rotate, the backlight is not turned on, and the upper and lower polarizers interact with each other. Vertically, all light is blocked, so it's black.