What is a diffusion film?
A typical application of a diffusion film is the liquid crystal display. We often hear the words ""liquid crystal,"" which is used in display screens such as TVs, laptops, tablets, and smartphones, as well as many aspects of our lives, like a smartphone terminal. We use terminals for smartphones or tablets as examples to provide a brief overview of diffusion films.
First of all, the transparent part on the front is called the panel with a touch sensor attached to it. When we touch the panel with a finger, the touched part will respond.
Second, the next black panel is called the liquid crystal panel.
Below the LCD panel is the light source of the backlight unit, for which the diffusion film is used as the optical film. A diffusion film is laid on the outermost surface of the backlight unit and is called the upper diffusion film. A glaring sheet called the prism sheet lies below the upper diffusion film, then another layer of diffusion film, which is called the lower diffusion film, is placed below the sheet.
All these sheets are placed on a transparent plate called the light guide plate. Many light-emitting μm LEDs lie at the end (side) of the light guide plate. The light from the LED travels through the light guide plate, the diffusion films, and the prism sheet to ensure the uniform dispersion of light across the whole screen.
With the power on, the light comes out straight from the LED. Through the patterns (dot patterns) attached to the light guild plate, the entire image can be lit up. In this way, the entire screen will appear bright, not just the part with the LED light. However, the patterns (dot patterns) attached to the light guide plate will be visible, and the display screen remains uneven and hard to see.
What is needed now is the diffusion film. First, the lower diffusion film hides the uneven patterns on the light guide plate by diffusing the light. It looks white because the surface is coated with special resins with fine irregularities. It scatters straight light evenly in many directions. This is the diffusion of light.
However, only the light is scattered, and the screen will appear dim. Here, the jaggedness of the prism sheet raises the diffused light in the front direction (the direction you are looking), increasing the brightness of the screen. However, the prism sheet will glare and be hard on the eyes.
The upper diffusion film is used here to eliminate the glaring effect. By putting it on the prism sheet, the state of glaring disappears, and the screen looks even.
In this sense, diffusion films are used to diffuse the light in the liquid crystal display and to make the screen appear bright, uniform, and beautiful.
Furthermore, by brightening the whole screen with limited light source, the diffusion film helps to save power. Of course, the battery life is primarily extended by the improved performance of the battery itself, but the role played by these diffusion films is also significant.
Structure of the diffusion film
To make the light diffusion film, a transparent binding resin is used to coat and adhere transparent organic fine particles (beads), which is made of acrylic resin etc., onto the surface of PET film for optical usage (PET is a kind of plastic called polyethylene terephthalate used for making plastic bottles, etc.).
The light from the light source is repeatedly reflected and refracted when passing through the layer of transparent organic fine articles. By doing so, the light will be uniform and concentrated at the necessary angle for viewing, thus ensuring the brightness and evenness of the image.
On the other hand, organic particles are also sparsely adhered onto the other side of this layer of transparent organic particles. As a result, even if another film with a different function is placed below the light diffusion film, these two films will not be adhered to each other, the occurrence of which would be risky and lead to various adverse effects. It is common for liquid crystal panels to be composed of many layers of films with different optical functions. Therefore, one important performance of the light diffusion film is to avoid influencing adjacent members.
Excellent features of diffusion film
■Smartphones, tablet terminals, laptop computers
As mentioned above, it can be incorporated into LCD, making the screen easy-to-watch, lightweight, and small.
The LCD market has continued to expand since the 1990s. Along with the market expansion, demands for better basic performance have also increased, one of which was improved brightness. In other words, it is necessary to improve the brightness of the backlight, that is, the light source for liquid crystal displays. Therefore, the demand is sophisticated in the way that you need to diffuse light without reducing the brightness of the light source.
■TV monitors and large-size monitors
The recent trend is to make the entire outer edge of the display as narrow as possible. Though a requirement for smartphones and tablets, it also raises demands for higher performance of diffusion films.
■Lighting equipment and signboards
As mentioned earlier in the LED examples, diffusion film is used in various facilities where LED is used. diffusion film is also widely used in street lights, tunnel lighting, a variety of signboards, etc.
What we are familiar with are room lamps fitted in cars where the diffusion film is also used. In addition, the diffusion film is indispensable for the light source for HUD. HUD is short for Head-Up Display, which shows navigation information in front of the driver.
■Inspection devices and various sensors
diffusion films with light evenness can play an important role in inspection devices that use light, such as glass inspection equipment and sensors.