What is Brightness Enhancement Film (BEF) and Dual Brightness Enhancement Film (DBEF) , and how to simulate and modeling it? Here it is.
About Brightness Enhancement Films
Brightness Enhancement Films (BEF) manage the angular output of light from liquid crystal display backlights. BEF films use a prismatic structure to focus light towards on-axis viewers of the display. BEF films can be used alone or two films can be crossed at 90 degrees to each other. A single sheet provides up to 60% increase in brightness and two sheets crossed at 90° can provide up to 120% brightness increase. This increased brightness can be translated into power savings.
How it works
BEF utilizes refraction and reflection to increase the efficiency of your backlight. BEF refracts light within the viewing cone (up to 35° off the perpendicular) toward the viewer. Light outside this angle is reflected back and recycled until it exits at the proper angle. BEF also minimizes coupling to adjacent surfaces
About Dual Brightness Enhancement Film
Dual Brightness Enhancement Film (DBEF) is a thin film reflective polarizer that increases brightness over the entire LCD viewing range. It can increase on-axis illuminance up to 60% in slab light guide displays and up to 97% in wedge light guide displays. Featuring a multi-layer, polymeric optical film that manages the polarization of light, DBEF captures and ulitilizes light normally lost to absorption in the bottom LCD polarizer. It is able to provide an on-axis brightness increase of 165% in slab light guides, and 277% in wedge light guide displays, when combined with other Brightness Enhancement Films. This increased brightness can be converted into power savings. It also provides linear light – no need for 1/4 wave film.
How it works
DBEF works through polarization recycling. The diagram shows a conventional backlit system emitting P1 and P2 light. A typical polarizer absorbs P2, but DBEF reflects P2 into the backlight, where it is recycled into P1 and P2 light. With DBEF, more P1 light is available to be transmitted through the LCD, increasing on-axis illuminance by up to 60% for a slab backlight and up to 97% for a wedge light guide. DBEF achieves additional on-axis illumination performance in a wedge backlight display by redirecting available light to the viewer.
An typical configuration in the LCD
How to simulate and model the Dual BEF film?
The Dual BEF Surface object is a rectangular surface that splits rays into transmitted and reflected components based on their polarization. In the object’s parameters, we can set the fraction of transmitted and reflected x- and y-polarized light. For this example, we will make an ideal DBEF with 100% transmission of y-polarized light and 100% reflection of x-polarized light.
In Zemax, the surface is always a rectangular plane, and may not have any material, coatings, or scattering function applied. The surface may be embedded in any isotropic, homogeneous medium. If either ray splitting or polarization is off, the surface has no optical effect. Otherwise, any incident ray is split into a transmitted ray and a reflected ray. The transmission and reflection intensity coefficients are defined as parameters 3-6, and are different for X and Y polarized light, where the X and Y directions are defined in the surface coordinate system.The transmission and reflection for X polarized light may not exceed 1, and the same limit applies to Y polarized light.
To analyze the performance of our DBEF, we construct a mock system consisting of a source, a diffusive surface, a polarizer, a reflective enclosure to prevent rays from escaping, and a detector to analyze the output power. The system is available as below:
To compare the system with and without the DBEF we will run a ray trace twice: once while always ignoring the Dual BEF Surface object and once while never ignoring it.
Never ignore:
Ignore it
The output power of the system without the DBEF returns approximately 50% which is as we expect on average for a randomly polarized source. Comparatively, the system with the DBEF yields a total power of about 82% which shows an increase in the output brightness of the system. This corresponds to an average 64% increase in output power which is equivalent to similar products on the market.
