From bulky bulky monitors, plasma TVS, and then to the current rise of laser TVS, the members of the display family can really be a lot. Many of these display technologies have been gradually replaced with the development of The Times, but one member has been “glowing” since the birth of the 1980s, it is the liquid crystal display (LCD).
It was around the same time as plasma TVS, but was significantly better known and more widely used. So the affordable and friendly LCD display, what technology is used, and what are the characteristics?
Both solid and liquid properties – the special structure of liquid crystals
Liquid crystal display is very common in our life, TV, computer, a variety of small household appliances display, almost all use liquid crystal display. To understand liquid crystal displays, we must first understand what is a “liquid crystal” material.
In fact, from its name, you can see its characteristics: “liquid” means liquid, “crystal” means crystal – that is, a solid substance, indicating that liquid crystals have some properties of both solid and liquid.
We all know that water has three states: solid (ice), liquid (water), and gas (steam), and most substances have all three states. A liquid substance is isotropic from the point of view of its molecular distribution, meaning that its physical properties are consistent in all directions, which is the result of the constant random motion of the molecules. The solid state is a crystal, which is anisotropic in terms of molecular distribution and arranged differently in different directions. This also results in physical properties such as refractive index, polarization properties, thermal conductivity and electrical conductivity of the crystal in different directions.
In the past, people thought that solid and liquid were clear, but in 1888, the botanist Friedrich Reinitzer (Friedrich Reinitzer) extracted a compound called spiral toluate from plants, which shattered this perception. It has two melting points at different temperatures, and after melting or being dissolved by a solvent, it loses the rigidity of a solid substance and forms an intermediate state with some properties of a crystal and a liquid. Because of the unique state of this kind of substance, people named it “Liquid Crystal”, that is, liquid crystal.
Liquid crystals can be divided into three types according to their anisotropic arrangement, namely nematic phase, smectic phase and cholesteric phase.
Nematic liquid crystal molecules are elongated, they will flow under the action of external forces, so the long axis direction of nematic liquid crystal molecules are generally along the direction of flow, that is, roughly located in the same direction, and can pass through each other.
In the smectic phase, the liquid crystal molecules are layered. The long axes of the molecules in the layer are parallel to each other and perpendicular to the layer. However, the position of the molecules does not have certain rules, and this arrangement is called the direction order and the position disorder. Smectic liquid crystal molecules can only move in this layer, and the layers can slide between each other.
The liquid crystal molecules of the cholesteric phase are also layered, with each layer ordered in the same direction. But as a whole, the layers are stacked in a spiral pattern, with each layer rotating at a slight Angle to the layers above and below it.
Structure determines properties – the many effects of liquid crystals
The strange regular arrangement of liquid crystal molecules also gives it different properties, such as the twisted nematic effect (TN).
The nematic liquid crystal material is sandwiched between two glass substrates, like a sandwich. There are very fine parallel grooves on the glass surface, which are called directional films. The liquid crystals near the upper glass are arranged in the direction of the upper matching film groove, while the liquid crystals near the lower glass are arranged in the direction of the lower matching film groove. If the two grooves are crisscross, the liquid crystal between the upper and lower glass will be layered and evenly distorted. From the top view, the upper liquid crystal molecules are horizontally arranged, the lower liquid crystal molecules are vertically arranged, and the whole is twisted along different levels like a spiral, which can affect the polarization direction of the light.
So what is polarization? We know that light is an electromagnetic wave, but there are actually two types of waves, called shear waves and longitudinal waves. Hold one end of a rope in place, pull the other end of the horizontal rope tight and vibrate up and down, you see the motion of the rope is shear wave. Shear wave is characterized by the particle vibration direction and wave propagation direction perpendicular to each other, electromagnetic wave is a typical shear wave. The particle vibration direction of P-wave is parallel to the propagation direction, and seismic wave belongs to P-wave.
A light wave is a transverse wave that consists of an electric field (z direction) and a magnetic field (x direction) that are perpendicular to each other, and the direction of light vibration is always in the X-Z plane, perpendicular to the direction of light propagation (y direction).
These two vibration vectors in different directions superposition each other, from the x-z plane, just like a particle in the regular movement, according to the amplitude and phase of the vibration is different, its movement can be presented as circular, elliptical or linear, respectively called circular polarized light, elliptic polarized light and linear polarized light.
If we place a slit, because the light wave has a vibration direction, it will be limited by the slit, and if the slit direction is the same as the vibration direction, the wave will pass through the slit smoothly. If the direction of the slit is perpendicular to the direction of the vibration, the wave is blocked and cannot continue to travel. If there is an Angle between the slit direction and the vibration direction, the wave can pass through the slit but the light intensity is weakened. This asymmetry between the direction of vibration and the direction of propagation is called the polarization of the light wave, and the above slit is the principle of the polarizer.
The twisted nematic effect uses this property of polarized light to change the polarization Angle of the incident light by twisting liquid crystal molecules. So, how can this effect be applied to liquid crystal displays to make them image?
There are two polarizers in a liquid crystal display. These two membranes can only let polarized light through at a fixed Angle. When we align the two membranes at 90 degrees each other, no light can pass through. However, due to the existence of the twisted liquid crystal layer, the light from the upper polarizer penetrates down, and is rotated 90° after twisting the path of the liquid crystal molecules, so that the light can just pass through the lower polarizer, forming a complete propagation path.
When a voltage is gradually applied to the liquid crystal layer, the liquid crystal molecules will gradually rise vertically as the voltage changes, and the light passing through will gradually weaken. When the voltage is maximum, the liquid crystal molecules become vertical, and all light cannot pass through the lower polarizer.
Knowing how the liquid crystal layer transmits light, let’s take a look at its imaging process. The liquid crystal layer actually contains very small cell structures, each one or more cells making up a pixel on the screen. By controlling the light intensity of different pixels through a circuit, a monochromatic image is formed.
The principle of color LCD and monochrome display is basically the same, except that each pixel of it is composed of three LCD cells, each cell has a red, green and blue filter in front of each cell. The light is processed by the filter plate, and the spatial color mixing method is used to combine rich colors.
This kind of LCD screen based on the twisted nematic effect (TN) is now called the TN screen, which is also the oldest level of LCD technology. Its advantage is short response time, but it also has disadvantages. TN screen contrast is low, color reduction is poor, the viewing Angle is narrow, the user can only get the best viewing effect when facing the screen. Because the TN screen can not provide people with a good enough use experience, it is currently in a trend to gradually withdraw from the mainstream market.
According to different LCD arrangements, TN screens were optimized, and VA screens (Vertical Alignment) and IPS screens (In-Plane Switching) were invented.
The VA screen does not use the twisted nematic effect to make the liquid crystal appear spiral, but makes the liquid crystal molecules all vertically arranged.
At the beginning, the liquid crystal molecules are arranged perpendicular to the upper and lower substrate, and the polarized plates are attached to both sides of the liquid crystal box. When no voltage is added, the linear polarized light propagating through the lower polarizer is parallel to the long axis of the liquid crystal molecule, and the polarization state does not change, and it cannot pass through the upper polarizer, and the panel is dark. When the voltage is applied, the liquid crystal molecules rotate under the action of electric field. Finally, the long axis direction of the liquid crystal molecules will be aligned perpendicular to the electric field direction. The linearly polarized light passing through the lower polarizer will produce phase delay in the liquid crystal layer, and the polarization state of the light will change until the polarization direction of the linearly polarized light rotates 90° in the liquid crystal layer, at which time the polarization direction is parallel to the direction of the optical transmission axis of the upper polarizer. The panel is on.
Because the liquid crystal molecules of the VA screen are vertically arranged, when the external action is encountered, the screen will be greatly infected, forming a pattern like water ripples, so it is also known as a soft screen. The VA screen has a larger viewing Angle than the TN screen, but its rotation Angle is larger, resulting in a longer response time and serious residual image.
The principle of IPS display technology is shown in the figure below. The electrodes are distributed on one side of the lower substrate, and the liquid crystal molecules are arranged parallel to the substrate and at a certain Angle with the electrode direction. Orthogonal polarizers are attached to both sides of the liquid crystal box, and the light transmission direction of the polarizer on the lower substrate is parallel to the direction of the liquid crystal molecules. When no voltage is added, the linearly polarized light passing through the lower polarizer is parallel to the long axis of the liquid crystal molecule, and the polarization state does not change, and it cannot pass through the upper polarizer, and the panel is dark. When the voltage is applied, the liquid crystal molecule rotates in the plane under the action of the electric field, the long axis direction of the liquid crystal molecule is orthogonal to the polarization direction of the linearly polarized light passing through the lower polarizer, the birefringence phenomenon occurs in the liquid crystal layer, the polarization state of the light changes, the polarization direction of the linearly polarized light rotates 90° in the liquid crystal layer, at this time the polarization direction is parallel to the direction of the upper polarizer, and the panel is bright.
Compared with the VA screen, the arrangement order of the liquid crystal molecules of the IPS screen is horizontal, so it can withstand greater pressure and will not affect the image imaging, so IPS is also called hard screen.
Since the IPS technology changes from the dark state to the bright state, the liquid crystal molecules are rotated on the plane parallel to the substrate, so the display effect is similar from all angles of the liquid crystal panel. Therefore, the IPS screen can solve the problem of the limited viewing Angle of the TN screen and provide people with a wider viewing Angle.
Can not self-luminous – liquid crystal display light source
You may have found a question, where does the liquid crystal light source come from?
It is true that the liquid crystal is not a display that can self-light, and it must use external light sources to shine.
There are two main sources of liquid crystal display light, one is similar to the classroom lighting used by the long strip of fluorescent lamps, they are mainly distributed on both sides of the display or the lower end. The other is the currently widely used light-emitting diode (LED) light source, the market often said LED liquid crystal display is derived from this. Because leds are so small, using leds as light sources can
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In fact, the LCD display based on LED light is not universal, and its color performance ability is not very good. However, due to its relatively low price, low modulation voltage and other advantages, it has been active in our lives since its birth in the 1980s. In this way, the LCD can be described as the evergreen tree of the display family!