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Optical film

What is optical film?

Optical film, also referred to as optical sheet, is a polymer film that produces certain effects by transmitting, reflecting, or absorbing light.
Polymers are long-chain molecules whose properties may change with directions. By making films with polymers, when the films are stretched in one direction, it is easy to align the molecules in the same direction, thus achieving a phase difference by the refractive index between the stretched direction and the direction orthogonally intersecting thereto. They can also become polarizing plates by adding and stretching dyes such as iodine. 
Here we will explain the principal optical films.

➀Optical film for flat display panels
Optical film for flat display panels is mainly used in liquid crystal panels and organic EL displays. The demand for larger screens and more precise and easy-to-watch screens are constantly increasing, as is the demand for lighter films with excellent optical properties.

②Antireflective film
Also called antiglare film, it is placed on the outside of the screen for scratch and dust prevention. It can also prevent the reflection of light from the outside. It is mainly made by coating multilayers of anti-reflection materials with different refractive indexes such as fluorine monomer, etc. on the PET or TAC film with a thickness of 100 μm. The film is not only used in displays such as liquid crystal panels and plasma displays, but also in automotive instrument panels, homes, and buildings.

③Alignment film
It is in direct contact with the liquid crystal part of the LCD.

④Polarizing film
Inside the LCD, the liquid crystal layer is sandwiched between two polarizing films.

⑤Protective film for polarizing films
Polarizing films are weak in strength. This film is used to protect polarizing layers. 
Please refer to the following explanation about alignment films and polarizing films.

【Supplementary explanations】 ~Mechanism of liquid crystals~
Liquid crystals are composed of liquid crystal molecules, which are elongated molecules, and in their natural state, have the property of regularly aligning in the direction of the long axis. When applied with voltage, they can line up straight from the high-voltage side to the low-voltage side. 
When liquid crystal molecules are brought into contact with a thin plate (in this case, an oriented film) that is cut parallelly with thin grooves, the molecules will line up regularly along the grooves. When the liquid crystal is sandwiched between two alignment films whose grooves are twisted orthogonally, liquid crystal molecules will gradually change the direction of the liquid crystal layer and ultimately become twisted at 90 degrees. This is the core concept of the LCD panel. 
Furthermore, this is also sandwiched from the outside by two orthogonal polarizing films. Polarizing film is a film that only allows light waves from a specific direction to pass. If nothing is in between, the orthogonal polarizing films will not allow the light to pass.
However, if the alignment films and the polarizing films are in the same direction, the gap between the obliquely twisted liquid crystal molecules will become the path of light, allowing the light to pass through. When a voltage is applied here, the orientation of the liquid crystal molecules arranged in a twisted state will be changed along the high-voltage side to the low-voltage side.
In other words, since the path of light formed by the twist is eliminated, the two polarizing films block the light from passing through. 
In this way, the light can be blocked and unblocked by turning the power on and off. You can also adjust the amount of light passing through by changing the voltage. By aligning liquid crystal molecules in this way, when light is applied from behind, images can be displayed. This is the working mechanism for liquid crystal panels. 

【Supplementary explanations】 ~Polarization~
Like waves, light travels while vibrating in a specific direction. Ordinary light like sunlight vibrates in all directions, but laser light and the light passing through what is called a polarizing plate only vibrates in a specific direction. This is called polarization. Light that vibrates in a single direction is called a linearly polarized light, and the light emitted from the LCD monitor is such a light.

⑥Diffusion film
Also known as a light diffusion sheet, this film diffuses the light from the light source. For more details, please refer to relevant explanations. 

⑦Reflective film
This film efficiently reflects light spilled on the back of the light source to the front. In addition to its optical effect, it can also protect the LED light source from scratches and prevent uneven light and the back frame from being seen.

⑧Non-polarizing film
The backlight unit includes a reflective polarizing film to increase the brightness of the LCD and make it thinner. Non-polarizing film is a part of the structure. 
Applying a reflective polarizing film alone has several disadvantages, such as the interference pattern of iridescent light will become visible, or the film will be easily scratched when coming in contact with other adjacent optical films, or will become soft and unmanageable. To compensate for such defects, non-polarizing films are mainly used by laminating with other films.

⑨Retardation film
This film is used in LCD. It can prevent modulation such as optical distortion that arises from light refraction and color difference caused by viewing angles.

⑩Wide view film
Film with a wide view function widens the viewing angle by keeping refraction constant, controlling leakage of light, and enhancing contrast in order to enable black contrast to be seen clearly from any angle. It is indispensable for supporting the ever-increasing size of LCD displays.

⑪Brightness Enhancement Film
This function accumulates light from backlit panels to enhance the brightness of the LCD. It is made of two layers with different refractive indexes. The light with the matching wavelength passes through, while the reflected light bounces off the mirror layer and illuminates the brightness enhancement film again. By repeating this step, the loss of light is reduced, and the amount of available light is increased. 

⑫Electromagnetic shielding film
This film transmits visible light and blocks the electromagnetic wave emitted by the plasma display panel.

⑬Films shielding selective and specific frequencies
The film shields and absorbs radio waves of a specific frequency while maintaining transparency.

⑭Optical low pass filter film
The film can be attached to shooting equipment like CCD cameras to prevent moire patterns. In general, crystals with a thickness of 1 mm or more are used, but special films have been developed for compact shooting devices, such as mobile phone cameras.

Other applications of optical films

Optical films are not only used for realizing light diffusion and retardation functions in the LCD and plasma displays described so far, but also widely applied in other products, such as the following: 

➀Polarized sunglasses, polarizing film for 3D glasses
The film works to prevent deformation, discoloration, and degradation of glasses used outdoors and plays a fundamental role in ensuring the transparency of the glasses and controlling the phase difference.

②Diffusion film for LED lighting
With LED light sources, the film ensures high brightness and low power consumption, as well as light diffusion function. 

③Various sensors
It is used in illuminance sensors, etc., to make the screen darker when the surroundings are dark and making the screen brighter when the surroundings are bright.

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