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.

Orient Display: Contrast ratio CR

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).