The difference between the ON and OFF voltages in displays with many rows and columns can be very small. For this reason, the TN device is impractical for large information displays with conventional addressing schemes. This problem was solved in the mid 1980's with the invention of the super-twisted nematic (STN) display. In this device, the director rotates through an angle of 270 degrees, compared with the 90 degrees for the TN cell. The effect of twist angle on the electro-optical response curve is shown in the following diagram.
Note that the change in the tilt angle becomes very abrupt as the twist angle is increased. The consequence of this response curve is that the off and on voltages are much closer together, as is shown in the following figure.
Although it is desirable to obtain a sharp electro-optic transition, grayscale images require intermediate points along the curve. For this reason, many commercial STN displays use a twist angle of 210 degrees. This broadens the transition region enough for grayscale while allowing for conventional addressing.
Early displays operating in the 210 degree mode suffered from undesirable coloration resulting from a shifted transmission spectrum of the device. In the ON state, the pixels tended to be yellow, while the OFF state had a bluish-purple tint. In addition to not being popular with the consumer, full color displays using filters can only be made with black and white operation. This problem was solved by adding a second STN layer with the opposite twist sense to the cell. This type of device is known as the double super-twisted nematic display (D-STN).
In the OFF state, the phase shift resulting from the first layer is compensated by the second layer. This pixel appears black. The ON state is not affected by the second STN layer, and white light emerges. Since the two layers consist of the same liquid crystal material, the behavior is constant over the entire temperature range.