Brightness Enhancement Film (BEF) is a new type of high-performance optical film, also widely known as a prismatic film due to the characteristic of its microscopic prismatic array structure on the outer surface. It is a film or sheet that can improve the luminous efficiency of the entire backlight system in TFT-LCD backlight modules, and was first invented and used by 3M in the United States. By using the special prism structure of the brightness enhancement film, the optical principles of refraction, total reflection, and light accumulation can make the light in all directions focus on the central viewpoint, thus enhancing the brightness of the LCD panel and controlling the viewing angle, thus achieving energy saving.

1. Anecdotes about the discovery of prismatic membranes

In a basement in Quebec, Canada, one winter more than two decades ago, a 3M researcher was doing experiments. Located in the northern hemisphere at high latitudes, the winter sun hangs low above the horizon all day, so he invented a glass conduit with a prism, oblique sunlight into one end of the conduit, will be propagated along the conduit wall, and the whole tube like a lamp through the body glowing, so that the basement suddenly bright a lot.

After this, 3M used thin-film technology to produce this light tube, but for many years, the application of this prismatic conduit has been limited to the lighting or decoration of buildings, with only a small number of sales each year. In the 1990s, liquid crystal display technology began to develop in leaps and bounds with the popularity of laptop computers. Due to the characteristics and structure of the liquid crystal plate*, the utilization of light is very low, and how to increase the brightness of the liquid crystal display has been a difficult problem for researchers.

A chance whim led 3M scientists to try cutting this prismatic conduit and laying it flat on the LCD backlight. The unexpected happened, and due to the light-gathering effect of the prism, this novel attempted method made the brightness of the LCD positively greatly improved. Previously, 3M scientists had been inspired by the imagination of butterfly wings due to the physical structure of the scales on the refraction and reflection of light to produce different spots, using an advanced computer simulation control system in the polymer industry, successfully invented 3M™ Multilayer Optical Film (Multilayer Optical Film ) technology, by changing the structure of the film to control the light emission.

From this, 3M scientists thought of combining these two special discoveries into one. After a period of research and development, 3M combined micro-replication technology and film technology to further optimize the light-gathering function of the prismatic conduit, thus brightening it*, and named it Brightening Film BEF.

To make this product well accepted by customers, 3M engineers purchased two comparison test laptops and added one of them with two pieces of brightening film with the prisms oriented perpendicular to each other. With this inconspicuous film, the brightness of the computer screen more than doubled! When the two computers were presented to their manufacturers, they were quickly convinced.

2. Prism film brightening principle

Brightening film is usually made by using UV-curable glue to micro-replicate the prismatic structure on the surface of PET to correct the light by the phenomenon of refraction and reflection of light. It is possible to increase the overall brightness by increasing the brightness at the center of the axis by 110% by concentrating the original scattered light at an angle of about 70 degrees and recycling the unused light outside the viewing angle, using the reflection of light to reduce the loss. Typically a single sheet of the brightening film can increase brightness by about 40-60%. Even higher brightness can be achieved by using two 90-degree vertical brightening films. Brightening films / prismatic films redistribute light energy by refracting and reflecting the fine stripes formed on the transparent optical film, increasing transmittance, brightness, and viewing angle due to the prismatic and conical structure that uniformly covers the surface.

3. Production process of the prism sheet

Brightening film is mainly made of PET or PC as raw material, using injection molding or pasting acrylic resin, and then using pre-cast microstructure roller transfer, with high energy UV light to harden the micro prism structure, so as to create a serrated surface, from the upper and lower pieces, vertically staggered in the middle of the two diffusion pieces, so that the original diffused light can be concentrated again, reducing the light This allows the light that has been diffused to be concentrated again and reduces the rate of light loss in order to increase brightness.

The production process of the brightening film includes optical design, precision mold, chemical formulation, and coating. The method used in advanced countries is to realize the molding process of the micro-optical structure by light-curing UV adhesive molding technology on the processed die roll.

The most critical technology for brightening film is the technology of engraving prismatic patterns on the rollers. In lens extrusion production, the lens extrusion roller processing technology directly affects the quality of the prismatic lens molding of the product. At present, the world's common practice is in the processed steel roller coated with a layer of lower hardness metal such as copper, etc., the use of diamond tools to process the required micro-fine prismatic structure surface.

This layer of the surface is unable to reach the extrusion production state, must be processed in the microfine structure of the surface layer, and then plated with a layer of harder nickel or chromium, the most advanced process is plated with a layer of nickel-phosphorus alloy, in order to meet the diamond tool carving prismatic lens process requirements. At the same time, this roll-making technology is still a worldwide challenge due to the very high surface accuracy and surface finish required for the lenses.

4. Prism film development trend

With the development trend of thin and high-definition LCD devices, the brightening film will be ultra-thin and composite in the future. The advantage of the composite film is that the thickness is thinned and two films are replaced by the function of one film, and the selling price of one film has a chance to be lower than the sum of the prices of two films, which can reduce the cost, and the thermal stability of one film is also better than two. The main brightening laminate film technologies that are currently available on the market include the following.

One-piece Prism

One-piece molding prism mainly through the raw material extrusion molding, and then through the roller transfer technology to achieve the top special angle design and prism height change to achieve the balance of enhanced light field angle and light intensity; at the same time, the bottom increase does not affect the light characteristics of the anti-scratch design, good scratch resistance.

The composite brightening film, also known as POP (Prism on Prism), is a combination of two brightening films glued together into one composite brightening film through bonding. Its advantage is that the composite brightening film is 45% thinner than the thickness of two brightening films, and the brightness is 6% less than two brightening films, but 8% higher than one, improving assembly efficiency. The disadvantage is that the two films are easily delaminated. The optical brightness decreases.

A Japanese company makes a reverse brightening film, the microstructure is made in the substrate into the light surface, but it needs to be paired with its corresponding design of LGP, LGP upper and lower surfaces into different microstructures, the backlight module center brightness can be increased by 14%. This technology has been available for some time, but it has not been applied on a large scale due to the low yield of LGP microstructure production and high requirements for assembly precision.

A new type of integrated light guide plate, whose upper and lower surfaces are directly fused with aspheric semi-columnar microstructure arrays and micro prismatic structure arrays, respectively. This integrated light guide plate achieves the function of the four-layer complex film system of two orthogonal brightening films-diffusion films-light guide plates commonly used in current backlight modules, which verifies the feasibility of integrated light guide plate design.

5. Prism sheet selection principles

 

1. If our products are positioned as small and medium sizes, we should choose the medium thickness of the diaphragm, too thick then occupy our product space; if positioned as large size products, we should choose a thicker diaphragm to prevent too thin diaphragm is easy to warp bad assembly and environmental class experiments waving problems.

2. According to customer demand for brightness when we choose to use a BEF diaphragm or two BEF diaphragms, or by adding LED lights or power consumption to enhance the brightness; first of all, to see if there is space to put down two diaphragms, followed by the cost, which program is better; weigh the pros and cons, make a choice; the main purpose, or to make customers satisfied.

3. brightening effect of a good diaphragm, masking effect is poor, easy to see in a particular viewpoint bright lines and other undesirable; is the focus on the appearance or focus on brightness, but also according to the actual situation to make a choice, or through other components to make up for the lack of here.