Bistable LCD Technology Introduction

Bistable LCD can retain an image without power (Zero Power). The crystals may exist in one of two stable orientations and power is only required to change the image. Bistable LCD can change colors, and keep that color even when power is cut off.

Bistable LCD Advantages

• No flicking: eyes not get tied easily.

• Low power: no power is needed to maintain the image. Made into e-book, 20,000 pages can be read with 2 AAA batteries of 3V.

• Possible high resolution which can achieve 200 dpi. Can achieve 256 gray scale.

• Possible low cost and a green display: no polarizer (for some bistable LCDs), color filter and backlight are needed.

• High contrast and wide viewing angle: The monochromatic bistable LCD’s contrast ratio is as high as 25:1, viewing angle about 80 degree in all directions.

• Sunlight readable

Bistable LCD Colors

	Reflective Color of Bi-stable LC	Color Absorbing Layer	Character Color	Background Color
Single layer	Green	Black	Green	Black
	Yellow	Blue	White	Blue
Double Layer	Yellow Blue	Black	White, Yellow, Blue, Black
Triple Layer	Red Green Blue	Black	Full Color

How Bi-stable LCD Works

 

For Cholesteric materials with positive dielectric anisotropy, the molecules in the planar structure will easily realign parallel to an applied electric field, generating the reflective layered structure. To break up the helical layered structure is to apply voltage around 6V to 30V. In order to switch from focal conic state back to planar state, even higher voltage is needed (35V). The exact voltage required for switching depends on cell gap as well as dielectric anisotropy of liquid crystals. The focal state is less stable than the planar state.

Bistable LCD limitations

• Refresh rate is slow: 2-3 seconds

• Between each screen refreshing, the screen has to be refreshed with a dark screen.

• Bistable Liquid Crystal materials require protection from UV light. A UV blocking material with a minimum 98% cut of at 380nm and lower spectral components is required.

• The finished product design should incorporate a transparent cover such as acrylic, polycarbonate etc. to protect the viewing area of the display. Place the protective cover as close to the display bistable module as possible. The protective cover should be sufficient thickness to resist bending.

• Adding an anti-glare and or anti-reflective surface film or finish to the viewing side of the protection cover may improve the optical performance.

• The power used for screen refreshing is pretty high. So, it will not worthwhile for an application if the screen has to be refreshed more than 20 to 50 times depending on the battery used. Bistable display technology has been used for shelf label and E-reader. Bi-stable LCDs requires the use of a voltage booster. Designing into some environments needs to be cautious, examples: explosive gas. Bistable LCDs are limited to one color of character and one color of background.

• Bistable LCDs are reflective and do not require a backlight. This can be a disadvantage in areas with dark ambient light. If the lighting has to be used, it can only use front light instead of backlight. It can be expensive and lose its key advantage of low power.

Orient Display Standard Bistable LCDs

• Single Layer Bistable LCD (Background: Blue; Character: White) : JAZZ-W-B

Fig.1 White Blue Bistable LCD