Principle Of Liquid Crystal Display

Principle Of Liquid Crystal Display

Dec 16, 2017

Compared with the characteristics of various displays, the liquid crystal has the following characteristics:

(1) low voltage and micro power. Very low working voltage, as long as the 2~3V working current is only a few Ma, power consumption is only 10-6 ~ 10-5W/cm2, which is not available to other displays.

(2) plate structure. The basic structure of the liquid crystal display is two pieces of conductive glass, thin liquid crystal box with middle irrigation, the advantages of this structure are: the opening rate is high, the most conducive to the display window; the display area is big and the small easier; automatic production, the production cost is low; the device is very thin, only a few millimeters thick.

(3) passive display type. The liquid crystal itself does not light up, by modulating the external light to achieve the display purpose, that is to rely on the reflection and transmission of the external light to form a different contrast to achieve the purpose of display.

(4) a large amount of information is displayed. In liquid crystal display, there are no isolation measures or reserved isolation areas among pixels, so it can accommodate more pixels in the same display area, and is conducive to making high-definition TV.

(5) it is easy to color. Generally, the liquid crystal is colorless, so it is easy to use the color filter film to achieve color.

(6) long life. As long as the matching parts of the liquid crystal are not damaged, the liquid crystal itself has a long life because of low voltage and small working current.

(7) no radiation and no pollution. There are X - ray radiation in CRT, and high frequency electromagnetic radiation in PDP, while liquid crystal displays do not have such problems.

Liquid crystal displays have the following shortcomings:

(1) the visual angle of display is small. Due to the anisotropic liquid crystal display is most rely on liquid crystal molecules, the incident light in different directions, the reflectivity is not the same, so from the perspective of small, although has developed a series of process, can improve the liquid crystal display angle, but will decrease the contrast in audio equipment, compared with other equipment, the speaker system is the weakest link. In spite of this, some of the problems have been ignored. The following questions are the case.

1. airtight box does not open air hole

It is argued that an airtight box should be opened in an airtight box (the diameter of the aperture is below 2 mm). The ideal sealed box should be airtight and airtight, so that when the temperature of the box changes or the air pressure changes, the air inside and outside the box will have a pressure difference. Under the differential pressure angle, the loudspeaker diaphragm will deviate from the normal position, which is easy to cause the truth.

In practical applications, most closed boxes are unlikely to achieve theoretical airtight airtight, which should be made in this direction.

Therefore, it is not too big to open the air hole. In order to rest assured, or to open a small hole as the best policy. In addition, the opening of the hole will affect the Q value of the loudspeaker system. In fact, the aperture of a small hole is very small, and the effect on the Q value is negligible, and the same is true of the measured results.

2. loudspeaker aging problems

This is a very serious but easy to ignore problem. A few days ago, I tested a loudspeaker that had been used for several years. The result is not unpredictable, one measure frightens a jump. The results of the yellowing found that the 44Hz has risen from the new purchase to today's 58Hz, and the Q value rose from 0.4 to 0.8. No wonder the effect is not as good as it used to be (according to the information, the loudspeaker lives about 5 years or so). Therefore, I suggest that the conditional friends should pay attention to the aging of the loudspeaker. It may be tested once a year.the way of contact between loudspeaker and loudspeaker panel. Many articles say that a 3~5mm thick rubber pad should be covered between loudspeaker and panel, so as to weaken the influence of box vibration on loudspeaker. I think this practice is not scientific.

The following qualitative analysis is carried out. The equivalent structure of the loudspeaker is shown in the accompanying drawings.


In order to make the diagram closer to the object, two springs and a tray frame are drawn, but a spring should be pressed with a spring.

And the frame to understand. It is a relatively independent mechanical system, when the voice coil wire into the current, the diaphragm will move correspondingly. Obviously, the dynamic interaction of diaphragm motion from the magnetic field. Now it is assumed that the diaphragm moves to the right. According to the knowledge of physics, when the diaphragm moves to the right, the disk holder must be subjected to the opposite direction (to the left) acting force (called the reaction force).

The greater the mass of the disc, the smaller the velocity of the backlash, the greater the velocity of the vibration film, the greater the amplitude of the corresponding amplitude, the louder the sound.

Therefore, the larger the mass of the disc is, the greater the equivalent vibration force of the vibration film is.

The better the total effect. When the loudspeaker is mounted on the sound box, if the disk is used

The box is connected with a rigid body so that the frame and the box are connected as a whole, etc.

The quality of the tray is greatly increased. So the impact of the backlash can be

It is greatly reduced. After padding a rubber pad between the box and the tray frame,

The glue is equivalent to one yellowing, and the spring can be compressed and stretched.

Such a frame can "move freely". Thus it can be seen,

The glue mat doesn't have to be scientific.

3. Optical properties of liquid crystals

The liquid crystal displays the birefringence due to the anisotropy of the refractive index, which has the following optical properties:

(1) the direction of the incident light can be deflected in the direction of the long axis of the liquid crystal molecule, that is, the direction of the direction of the vector n.

(2) the polarization state of the incident light (line polarization, circular polarization, elliptical polarization) or polarization direction can be changed.

(3) it can reflect or transmit the incident polarized light corresponding to the left or right rotation.

What is the phenomenon of spinning light?

When linearly polarized light through some transparent material, the surface vibration is rotated, transmission axis is called optical phenomenon, if the face light observation, the vibration for right rotation is called dextrorotatory substance, the vibration for left rotation material called left-handed material. When the liquid crystal molecules are arranged as twisted molecules (the initial state without electricity), the transmitting axis of the incident light rotates, which shows some time characteristics.

The following two kinds of cases (two optical phenomena commonly used in liquid crystal display) are analyzed.

1. when the polarized light (vibration parallel to the paper) is incident at an angle with the long axis of the liquid crystal molecule, the Marius law can be obtained.


Special case, when theta =90 degree (at this time corresponds to the case of the liquid crystal box adding), Io=I Ie=Icos2.

The above indicates that the light intensity of the ordinary light (o light) reaches the maximum, and the light intensity of the very light is zero, as shown in Figure 1.


As the speed of the ordinary light (o light) is U11, its direction is parallel to the optical axis of the crystal, which is the direction of the propagation of polarized light in the crystal. In addition, because the polarization direction of the o light is perpendicular to the optical axis, the polarization of the light is also unchanged when the polarized light incident to the liquid crystal box. If the detector is perpendicular to the polarizer (the vibratory axis is perpendicular to each other), as shown in Figure 2, the light intensity of the ejection light is zero, thus achieving the purpose of light shading.


2.The propagation of linear polarized light in a twisted nematic liquid crystal (when the liquid crystal box is not charged).

Adding a small amount of optically active substances in nematic liquid crystals, or the two surface of the liquid crystal box for molecular arrangement and distorted the director to make the vibration direction of linearly polarized light and liquid crystal molecules in the same plane and parallel to each other, so we can get the lambda = P (spiral) situation, as shown in Figure 3 shown.


When the linearly polarized light is perpendicular to the incident direction, if the polarization direction is the same as the molecular orientation on the upper surface, the linearly polarized light will rotate along the long axis of the molecule and parallel to the polarized light of the liquid crystal molecular axis at the outlet. A special case is that if the long axis of the liquid crystal molecules turn 90 degrees (this corresponds to the case of TN liquid crystal box without electricity), the direction of the electric vector vibration of the incident light also rotates 90 degrees, but the direction of light propagation is constant, so that the polarization axis of the incoming ray polarized light rotates 90 degrees. If the detector is perpendicular to the polarizer (the vibratory axis is perpendicular to each other), as shown in Figure 4. The output of light and light is maximum, thus the purpose of light transmission is achieved.


The above two cases are the optical performance of TN LCD under the two conditions of adding electricity and without electricity. The purpose of image display is to control the shading and light transmission of incident light, which is the principle of optical display of liquid crystal devices.