Response time is the time it takes for the display to either turn on or turn off. The turn on time is the time it takes, from when an electric field is applied to an active segment, for the contrast to reach 90 percent. The total turn on time can be broken down into two components, the delay time and the rise time, see figure 1 on the bottom of this page. Similarly the turn off time is the time it takes for the contrast to change from fully on to 10 percent. It can also be shown that there is a delay from when the field is removed to the 90 percent point. The total off time is the delay plus the fall time, see fig 2 on the bottom of this page.

For those so inclined, we can also describe the turn on and turn off times mathematically,

Where tr is the total on time, td is the total off time, d is the cell gap, Vapp is Vdd, K is the elastic constants of the liquid crystal mixture and h is the viscosity of the liquid crystal mixture. For a given liquid crystal mixture, the physical parameters are fixed. From the equations above we can see that to decrease the on time we can lower the viscosity, decrease the cell gap, or increase the drive voltage. The cell gap is fixed because of the birefringence of the mixture. This leaves only two variables left, viscosity and drive voltage. As the temperature goes down, the viscosity will go up. To compensate this we will need to either add heat to lower the viscosity or increase the drive voltage.

Increasing the drive voltage has diminishing returns. There is a point where increasing the drive voltage will exceed the input to the LCD driver and where there is minimal gain in the turn on time especially at colder temperatures. Unlike the turn on time which can be decreased by increasing the drive voltage, the off time is purely a physical phenomenon. We cannot drive the display off. We must rely on the surface energies and the molecules falling back into their twisted structure to provide a fast off time. At colder temperatures, the only way to decrease the off time is to decrease the viscosity and we can do that by adding a heater.

The immediate solution for faster displays is to add a heater to the back of the display. Another solution is to develop a fluid with an extremely low viscosity at very low temperatures. LXD is continually looking to push the limits of display performance.

Figure 1                                              Figure