About Brightness Enhancement Film (BEF)
Brightness Enhancement Films (BEF) are optical films used in LCD backlight modules to control and improve the angular distribution of light. BEF uses a micro-prismatic surface structure to redirect light toward on-axis viewers, increasing perceived display brightness without increasing electrical power.
BEF films can be used:
- As a single sheet, or
- As two sheets crossed at 90°, often referred to as crossed BEF
Typical performance:
- One BEF sheet: up to ~60% brightness increase
- Two crossed BEF sheets: up to ~120% brightness increase
This brightness gain can be traded directly for power savings in the backlight system.
How BEF Works
BEF improves backlight efficiency through refraction and reflection:
- Light within the desired viewing cone (typically ±35° from normal) is refracted toward the viewer
- Light outside this cone is reflected back into the backlight cavity
- Reflected light is recycled by the light-guide plate, reflector, and diffuser until it exits at a useful angle
BEF also reduces coupling of light into adjacent optical films, improving overall system efficiency.
About Dual Brightness Enhancement Film (DBEF)
Dual Brightness Enhancement Film (DBEF) is a reflective polarizer designed to increase brightness by polarization recycling. Unlike BEF, which controls angular distribution, DBEF manages the polarization state of light.
Key characteristics:
- Thin, multi-layer polymer optical film
- Reflects one polarization state and transmits the orthogonal state
- Recovers light normally absorbed by the LCD’s bottom polarizer
Typical performance improvements:
- Up to ~60% on-axis gain in slab light-guide displays
- Up to ~97% on-axis gain in wedge light-guide displays
- When combined with BEF films: Up to ~165% gain (slab LGP), up to ~277% gain (wedge LGP)
An additional advantage is that DBEF outputs linearly polarized light, eliminating the need for a quarter-wave plate.
How DBEF Works
In a conventional LCD backlight:
- Randomly polarized light contains two orthogonal components (P1 and P2)
- The LCD polarizer transmits P1 and absorbs P2, losing ~50% of light
With DBEF:
- P1 polarization is transmitted toward the LCD
- P2 polarization is reflected back into the backlight
- The reflected light is depolarized and recycled
- Over multiple passes, more light is converted into P1 and transmitted
This polarization recycling significantly increases usable output power.
Typical BEF / DBEF Stack in an LCD Backlight
A common optical stack (from bottom to top) includes:
- Light Guide Plate (LGP)
- Diffuser film
- BEF (one or two crossed sheets)
- DBEF
- LCD polarizer
How to Simulate and Model DBEF
Dual BEF Surface Object (Simulation Model)
In optical simulation software, DBEF can be modeled using a Dual BEF Surface object, which is implemented as a rectangular plane that splits rays based on polarization.
Key characteristics:
- No material, coating, or scattering properties
- Embedded in an isotropic, homogeneous medium
- Operates only when ray splitting and polarization are enabled
Each incident ray is split into:
- A transmitted ray
- A reflected ray
Transmission and reflection coefficients are defined separately for:
- X-polarized light
- Y-polarized light
The X and Y directions are defined in the local surface coordinate system. For each polarization, the sum of transmission and reflection coefficients must not exceed 1.
Example: Ideal DBEF Definition
For an ideal DBEF:
- 100% transmission of Y-polarized light
- 100% reflection of X-polarized light
This setup models a perfect reflective polarizer with no absorption loss.
Performance Analysis Example
To evaluate DBEF performance, a simplified system is constructed consisting of:
- A light source
- A diffusive surface
- A linear polarizer
- A reflective enclosure (to prevent ray loss)
- A detector to measure output power
Two ray traces are performed:
- DBEF ignored
- DBEF active (never ignored)
The figure below shows DBEF active (never ignored):
The figure below shows DBEF ignored:
Results
- Without DBEF: Output power ≈ 50%, as expected for randomly polarized light
- With DBEF: Output power ≈ 82%
This represents an average ~64% increase in output brightness, consistent with commercially available DBEF products.
Conclusion
BEF and DBEF serve complementary roles in LCD backlight design:
- BEF improves angular light distribution
- DBEF improves polarization efficiency through recycling
Accurate simulation of these films allows designers to:
- Predict brightness gains
- Optimize film stacking
- Reduce power consumption without sacrificing display performance