Basic principle of polarized light and polarized film

发布时间：2020-06-03 发布人：

Most people are still unfamiliar with the term polarizing film and are not very clear. Therefore, the phenomenon and basic principle of polarized light will be explained a bit here.

Polarized light

Human understanding of light can be divided into the following four important stages in order:

1. In the seventeenth century, Newton first began to do a systematic study of light. He discovered that the so-called white light (White Light) is a mixture of all colored lights. To explain this phenomenon, many different theories have been derived.

2. At the beginning of the nineteenth century, Thomas Young used wave theory to successfully explain most of the optical phenomena such as reflection, refraction, and diffraction.

In 3.1873, Maxwell discovered that the light wave is an electromagnetic wave, in which its electric and magnetic waves are interdependent and cannot be separated, and the electric field (E), the magnetic field (H) and the direction of the electromagnetic wave (k) are mutually perpendicular relationship.

 

figure 2

 

4. At the beginning of the twentieth century, Einstein discovered that the energy of light can only be explained by the particle theory, thus deriving quantum theory. In other words, light has both wave and particle characteristics. Because the theory of polarized light is explained by wave theory, all previous discussions have considered light as an electromagnetic wave, and in order to simplify and understand, we only consider the electric field vector E. The E of non-polarized light can be represented by Figure 2. Many symmetrical and equal-length radiation lines in Figure 2 indicate that E vibrates on the plane composed of E and H, and the chances of vibrating in all directions are equal. When the distribution of E is uneven, it is called polarization (Polarization), as shown in Figure 3 is partial polarization, when E only vibrates in one direction, it is called linear polarization (Figure 4). From a vector point of view, when the vectors in each direction in Figure 2 are projected on two mutually perpendicular coordinate axes, X and Y, unpolarized light can be decomposed into two linear polarized lights that are perpendicular (Figure 5).

 

Figure 2: Non-polarized aurora

  

 

Figure 3: Partially polarized light

 

 

 

Figure 4: Linear polarized light

 

 

Figure 5: Linear polarized lights perpendicular to each other

 

 

Manufacturing of polarized light

In general, the method of manufacturing polarized light consists of the following three steps:

1. Manufacture ordinary non-polarized light (Figure 2).

2. Decompose this non-polarized light into two linear polarized lights that are perpendicular to each other (Figure 5).

3. Abandon one polarized light and apply another polarized light (Figure 4).

An instrument that can decompose unpolarized light into two polarized lights is called a polarizer. Polarizers can use polarized light by using optical effects such as absorption, reflection, refraction, and diffraction.

There are several types of commonly used polarizers:

(1) Reflective

When the light enters the glass surface obliquely, its reflected light will be partially polarized. The continuous reflection effect of multiple layers of glass can convert non-polarized light into linear polarized light.

(2) Complex bending type

When two calcite crystals are joined, the incident light will be decomposed into two polarized lights, called ordinary light and extraordinary light.

(3) Dichroic microcrystalline

The dichromatic tiny crystals are regularly arranged on a transparent sheet, which is the first artificially made polarizing film.

(4) Polymer dichroic type

The use of a polymer film with good light transmittance to orient the molecules in the film and then adsorb the substance with dichroism is the most important method for producing polarizing films. These absorption polarizers are all in the form of film or plate or sheet, so they are usually called polarizing film or polarizing plate or sheet. Another more popular name in English is Polarizing Filter.

The origin of polarizing film

The polarizing film was invented by Polaroid founder Edwin H. Land in 1938. Sixty years later, although the polarizing film has many improvements in production techniques and equipment, the basic principles and materials used in the process are still the same as they were sixty years ago. Therefore, before explaining the principle of the polarizing film process, first briefly describe the circumstances under which RAND was inspired at the time. I believe this helps to fully understand the polarizing film process.

 

During his studies at Harvard University in 1926, Rand read a paper published in 1852 by a British doctor Dr. Herapath in 1852, which mentioned that a student of Dr. Herapath, Mr. Phelps, accidentally dropped iodine into The solution disulfate of quinine, he found that many small green crystals were produced immediately, Dr. Herapath put these crystals under the microscope and found the following picture: When the two crystals overlap, the light transmission The transition will change with the angle at which the crystals intersect. When they are perpendicular to each other, the light is completely absorbed (Figure 6); when they are parallel to each other, the light can be completely transmitted (Figure 7).

 

Figure 6: Light is completely absorbed

 

Figure 7: Light can be fully transmitted

The crystals of these iodine compounds are very small, so there are great limitations in practical application. Dr. Herapath spent nearly ten years studying how to make larger polarized crystals, but he did not succeed. Therefore, Land thought that this path might not be feasible, so he adopted the following approach:

 

● Rand grinds large particle crystals into tiny crystals and suspends these small crystals in the liquid.

● Put a plastic sheet into the above-mentioned suspension, and then put it into a magnetic or electric field to orient it.

● Remove the plastic sheet from the suspension, and the polarized crystal will be attached to the surface of the plastic sheet.

● Leave this plastic sheet in a magnetic or electric field, and it will become a polarizing film after drying.

Lanter's method is to arrange many small polarized crystals in a regular manner, which is equivalent to a large polarized crystal. Using the above method, he successfully made the earliest polarizing film and J film in 1928. The disadvantages of this method are time-consuming, high cost and blurry opacity. But Rand has discovered several important factors for making polarizing films: (1) Iodine (2) Polymer (3) Orientation. After continuous research and improvement, Rand finally invented the manufacturing method still in use in 1938, and the basic principles will be discussed in the next section.

The working principle of polarizing film

The most common polarizing film nowadays is the H film invented by Rand in 1938. Its production method is as follows: First, a soft chemically active transparent plastic plate (usually PVA) is immersed in an aqueous solution of I2 / KI Within a few seconds, many iodide ions diffuse into the inner PVA. After slight heating, they are manually or mechanically stretched until several times the length. The PVA board becomes longer and narrower and thinner at the same time. Distributed, after being stretched, it gradually deflects in the direction of the force, and the iodide ions attached to the PVA also follow the directionality, forming a long chain of iodide ions. Because the iodide ion has good polarizability, it can absorb the electric field component of the beam parallel to its arrangement direction, and only let the electric field component of the beam in the vertical direction pass. Using this principle, a polarizing film can be manufactured (see FIG. 8).

 

Picture 8

 

 

Types and development of polarizing film

Types of polarizing films used today

 

The application range of the polarizing film is very wide. It can be used not only as a polarizing material in the LCD, but also in sunglasses, anti-glare goggles, filters for photographic equipment, anti-glare treatment for car headlights, and light quantity adjusters. Polarized microscope and special medical glasses. In order to meet the requirements of light weight and easy use, the choice of polarizing film is mainly based on polymer dichroic type. There are four types of polarizing materials:

(1) Metal polarizing film

The metal salts such as gold, silver, and iron are adsorbed on the polymer film, and then reduced, so that the rod-shaped metal has the ability to polarize, and it has not been produced by this method.

(2) Iodine-based polarizing film

The composition of PVA and iodine molecule is the most important method for producing polarizing film.

(3) Dye-based polarizing film

The organic dye with dichroism is absorbed on the PVA, and extended and oriented to make it have polarized light performance.

(4) Polyethylene polarizing film

Using acid as a catalyst, the PVA is dehydrated, so that the PVA molecule contains a certain amount of ethylene structure, and then it is extended and oriented to make it have polarizing performance.

Polarizing film structure

After the polymer film is stretched, the mechanical properties are usually reduced and become brittle. Therefore, after the polarizing substrate (PVA) has been extended, a transparent substrate composed of triacetate fiber (TAC) must be attached on both sides to protect it on the one hand and prevent the film from shrinking on the other. In addition, a layer of release film and protective film can be added to the outer layer of the substrate to facilitate bonding with the liquid crystal tank (see Figure 13).

 

 

Figure 13: The structure diagram of polarizing film

 

Quality characteristics of polarizing film for LCD

Since the display of the LCD is non-luminous, in order to meet the requirements of the display to be bright and easy to recognize, the polarizing film must have clearness, high transmission, and high polarization rotation. Recently, LCDs have been used more and more widely, such as people's livelihood, military, and high-tech. In response to the diversification of LCDs and the improvement of durability, the durability and polarization resistance of polarizing films must be strengthened. In addition, in terms of appearance characteristics, with the improvement of LCD pixels, the surface of the polarizing film must be smooth and high-definition; if it is used for a long time under a high temperature and high humidity environment, the polarizing performance must also be maintained, and the adhesive used Its stability is also one of the main points required. Usually in the manufacturing process of polarizing film, it is carried out in a clean room:

1. Since the material of the polarizing film is PVA and TAC, there should be no foreign matter and undissolved resin on it.

2. During the laminating process of the polarizing film, no foreign matter should be mixed in during gumming, laminating and processing.

3. Materials such as protective film or release film must not have any defects.

4. There should be no foreign matter adhering to the surface and cut surface of the finished product or the packaging bag.

If the above conditions cannot be satisfied, a high-resolution, large-size, and high-definition polarizing film cannot be produced.

Development of polarizing film for LCD

(1) Iodine-based polarizing film

The polarizing film composed of PVA and iodine has long occupied a considerable proportion in the LCD market. Nowadays, with the continuous improvement of materials and extension technologies, the polarization degree and transmittance are very close to the theoretical value (polarization degree 100%; transmission rate 50%).

(2) Durable polarizing film

The use of dye formulations makes polarizing films resistant to high temperature, high humidity, and light, and is mostly used on LCDs for cars, ships, or airplanes. However, its polarization rate is not as good as the iodine system and its price is its disadvantage. Today's development is through the extension of PVA alignment and the development of dye molecules with high polarization properties that have uniform absorption in the visible region. The polarization performance is already comparable to iodine-based polarizing films, but the price is still higher than iodine-based polarizing films.

(3) Optical compensation film

As the technology of LCD products is more and more advanced, the requirements for the coloring, viewing angle, light leakage, etc. of the polarizing film are relatively increased, so various optical compensation films are needed to compensate. For example (STN-LCD), the polarizing film using linear polarized light may have coloring phenomenon due to the twist of liquid crystal molecules exceeding 90 degrees. The solution is to add a phase difference film.

Surface treatment

Surface processing can increase the optical and mechanical properties of the polarizing film. In order to meet the diversified requirements of LCDs nowadays, polarizing films with composite functions have been sold on the market.

1. Anti-reflection (AR) processing

When the light passes through the surface of the polarizing film, there will be a reflection loss of about 5%. Due to the loss of light and the reflected light, the visibility of the LCD will be reduced. The improved method is to deposit a layer of metal film on the surface of the polarizing film, use the principle of light interference to reduce the reflection value, and reduce the reflectivity to less than 1%.

(2) Anti-glare (AG) treatment

In order to avoid excessive concentration of light, the surface of the polarizing film is processed into a concave-convex shape to evenly distribute the light, which can achieve the anti-glare effect. After being processed by AG, its surface can reach a pencil hardness of 3H, which is more scratch-resistant, and has a high haze. It can be applied to large-size products (greater than 12.1 ”), mainly due to the backlight strength of the LCD. In addition, with the resolution of the LCD Increased requirements such as UXGA level (1600 x 1200) require more detailed treatment of AG. Currently polarizer manufacturers are beginning to notice this aspect. I believe there will be corresponding products for market evaluation recently