How to Increase LCD Contrast
The contrast ratio (CR) is a property of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing.
If the LCD contrast is too low, it is hard to read. Different applications have different contrast requirement. For normal reading, the contrast needs to be >2; for medical, the contrast needs to be >10, for welding helmet, contrast should be >1,000.
There are many ways to increase LCD contrast, some of them are without changing the LCD design.
Optimize the LCD Voltage
- The most important: the driving voltage has to be just right for LCD. This is the most cost-effective way.
- Different viewing angles need different driving voltage. It is compromising. In discussing the best contrast, we have to fix the voltage angle first.
- The voltage is too low, the display is dim; when the voltage is too high, the non-select segments shows up (ghosting). Both affect the contrast.
Changing LCD Design
- The segments or pixels should be as big as possible.
- The gap between pixels should be as small as possible. (If smaller than 0.03mm, the tooling charge and yield will be affected.)
- If possible, try to design the driving multiplex (duty ratio) as small as possible. Static drive has the best contrast.
Using Better Polarizer
- The higher the efficiency, the better of the contrast. It is especially important for negative display. Change from 98% to 99.9% polarizer, the contrast can increase from 45 to over 1000 for negative LCD, but for positive LCD, the contrast increases from 7 to 10 for positive LCD.
- With ambient light, the more reflective of the polarizer, the better the contrast.
- With backlight, the more transmission of the polarizer, the better the contrast.
Choosing Right Liquid Crystal Material
- The better K33/K11, the better steepness of V90/V10, the better the contrast.
- The higher the voltage, the better K33/K11, the better the contrast (when V10≤2 volts)
Applying Premium Quality Orientation Layer (Polyimide)
- Use high quality polyimide (Nissan Chemical)
- Use high quality rubbing fiber and good quality rubbing equipment to make the liquid crystal molecular alignment uniform.
Changing Display Mode
- Positive LCD to Negative LCD (When the LCD is used indoor or dark environment, The contrast will increase a lot, but it will not display well with ambient light only, it is also more expensive)
- TN (Twisted Nematic) to VTN (Vertical Alignment)
- TN (Twisted Nematic) to STN (Super Twisted Nematic)
- STN (Super Twisted Nematic) to FSTN (Film Super Twisted Nematic)
- Blue STN (Super Twisted Nematic) to Negative FSTN (Film Super Twisted Nematic)
- Negative FSTN ( Film Super Twisted Nematic ) to FFSTN (Double Film Super Twisted Nematic)
- Negative FSTN (Film Super Twisted Nematic ) to ASTN (Automotive Super Twisted Nematic) (especially when temperature changes).
- FFSTN (Double Film Super Twisted Nematic) to TFT (Thin Film Transistor)
- TN (Twisted Nematic) TFT to IPS (In-plane Switching) TFT
Using Black Mask (BM)
- For negative display, black mask can block the light bleeding, the contrast can be improved. Black mask can be done either outside cell (low cost) and inside cell (high cost).