Bightness enhancing films- BEFs for high bright LCD display
Passive brightness is the technique of capturing and focusing light within the backlight, out to the front of the display. Passive brightness films have very unique optical properties which focuses light already in the backlight system which otherwise would be lost or absorbed inside the display. Since these are films, there is no power draw to the system. OEM displays have some of these films included from the factory but the displays are often not tuned to maximum brightness efficiency due to costs or other factors. GDS optical engineers will study any display and compare the internal films from the OEM and make recommendations to increase the brightness performance of the display.
Reflective Polarizer- DBEF
Reflective Polarizer (RP) or DBEF is a "light recycling" film. All TFT LCDs have a rear linear polarizer. This polarizer allows 100% of light to pass in alignment to its polarization axis; 0% of the light to transfer if it is in 90 degree opposition to its axis and all other degrees to pass a percentage of the light between 0% and 100%. This means that the rear polarizer is absorbing a total of 50% of the light from the backlight. RP is designed to prevent the absorption of the non-aligned light. If a light wave is not at the polarization angle of the polarizer, the light is "reflected" backwards, compensated (turned slightly). This light reflects off the back wall of the display and attempts to pass through the polarizer again. This process is repeated until the light wave has been compensated (turned) enough to be in alignment with the polarizer, and allowed to pass. An RP film collects 90% of the absorbed light so there is a 40-45% gain in brightness.
The function of RP is well known by OEM display manufacturers and is used in nearly all displays. RP has an issue. The film expands when heated beyond 70 degrees C. If the film expands, and is trapped within the display, the film can wrinkle. Wrinkled DBEF or RP is very noticeable and once wrinkled, it will NOT go back to it original shape. GDS is trained in installing temperature resistant forms of RP. By installing high temp RP, the display can take heat beyond 80 degrees C without issues of wrinkling.
Rear polarizer absorbs light
RP/DBEF recycles absorbed light
BEF Film
BEF film is a light collimator film. It is a lens who's prisms are configured in parallel line. The film will focus the light straight forward, towards the viewer instead of in the more oblique directions. The placement of the film will orient the lenses either in the horizontal or vertical alignment. The horizontal alignment of the prisms means that light going up or down will strike the prisms and be bent forward, out the display. The vertical alignment orientation means that light going left or right will be bent and focused forward, out the display. Additionally, the BEF films will also make the light from the backlight focus out the display more efficiency.
The display OEMs will ship a display with the horizontal BEF installed. This means that the display will have light going up or down out of the display. Display OEMs want as much light forward but feel that most applications require brightness uniformity as one goes from left or right. The Horizontal BEF, taking light from the top and bottom, will not effect the light side to side. If there is a requirement to have the maximum light output without a power increase, GDS can install the other BEF orientation, or Cross BEF. Adding the vertically align BEF, light will be focused away from the right and left of the display and will be channeled forward. This is increase the light output by 15%.
BEF films focus light forward
Transflective Film- ESR
Often referred to as the "Transflective" film enhancement, ESR film is essentially a nonmetallic mirror film. When ESR film is placed into a display that has been indexed matched either through optical bonding or AR film, this display will perform in bright direct sunlight. When the display is in direct sunlight, the light will eventually hit the ESR film in the rear of backlight. The transflective film will reflect back out of the display nearly 99.2% of the light striking it. This light will reflected back out of the display making the display viewable in direct sunlight. It should be noted that TFTs absorb nearly 97% of the incident light. When a display is facing the sun, the remaining 3% of the light is enough to reflect back out the display. With the AR coatings from the contrast enhancement, the display can be viewable even without a backlight on.
In indirect lighting conditions, the ESR/tranflective film is still valuable. Rather than reflecting external light, when there is minimal ambient light, the film is highly efficient in reflecting internal light from the backlight. Coupled with BEFs and DBEFs, a LCD with ESR/transflective film will perform at incredible brightness without increasing the power draw of the system.
