By Richard Cordero
Liquid Crystal Displays (LCDs) have dominated the display market with their long service life, power efficiency, and low market price and are commonly used in flat-panel computer monitors, TVs, notebook computers, mobile phones, navigation systems, and many other video display consoles. In the LCD display stack, diffusers play an important role in the backlight unit, which consists of a stack of multiple optical films that deliver the source light (LEDs) towards the LCD panel.
Properties required of these diffusers include high transmission efficiency as well as the ability to homogenize a light source and preserve polarization (i.e., non-birefringent).
What Is Birefringence?
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. It can create a number of optical artifacts in a display like Newton’s rings and moiré. LCDs only transmit a specific polarization orientation when active, and discoloration can occur if there is any material in the film stack that changes the polarization of the light. An effective way to prevent this phenomenon is to use low birefringent material in the stack; however, many OEMs do not realize this until they experiment with different films and start to see visual artifacts in the display.
An easy test to see if a material has birefringence is to wear polarized sunglasses and rotate a material in front of an LCD display. If the material does not have low stress in the composition, you will see it go from light to dark with color distortion as the material rotates.