The result of the interaction of the forces involved varies depending on the matrix type, TN+Film, S-IPS, MVA or PVA, but is not really satisfactory with any of them – switching between two levels of gray takes more time than a black-white-black transition on any existing matrix! The difference varies from 2-3 times with S-IPS to a factor of ten with MVA and PVA matrixes, which are very slow on black to dark-gray transitions that can take as long as 100 milliseconds and more while the black-to-white transition takes a mere 12-14 milliseconds.
Moreover, the designers of LCD matrixes found it easier to achieve very low response times exactly when it is measured with the standard “black-white-black” method. As a result, the so-called “fast” TN+Film matrixes with a specified speed of 16, 12 and even 8 milliseconds appeared. Alas, they are not 2-3 times, but only 25-30% faster than the older 25ms matrixes because the response time on halftones has in fact remained the same. And this is also the reason why it is virtually impossible to tell a 12ms matrix from a 16ms one with just your eyes, without using any special tools.
Let’s get back to the technical aspect of the matter, though. As I said above, the main problem with the response time is the quadratic dependence of the crystals-affecting force on the voltage applied to the LCD cell (on the electric field created by this voltage, to be exact). The solution of this problem has long been known under the name of Response Time Compensation.
The solid line on the graph above shows the response of an ordinary LCD cell. The applied voltage is marked in red and the brightness of the cell is marked in blue (for the sake of simplicity we can suppose that zero voltage results in zero brightness. This actually depends on the matrix type, but we need not delve so deeply into the matter now). At some moment the monitor is to change the brightness of this cell from zero to some in-between brightness (not the maximum possible). The monitor’s electronics sends a voltage of V0 to the cell to turn the liquid crystals by the necessary angle and this voltage remains constant after that until the brightness of the cell is to be changed again. Since the applied voltage is far from the possible maximum, the crystals are turning round rather slowly and the cell will have reached the desired brightness only after some considerable time.
The same effect can be achieved in a different way which is shown with the dotted lines above. The monitor’s electronics apply such a voltage to the cell that the crystals reach the necessary orientation exactly by the beginning of the next frame. In the new frame the voltage is reduced to V0 to maintain the desired orientation of the crystals. As a result, the monitor can accomplish a transition between any mid-tones in exactly one frame. And I want you to note that the LCD matrix’s own frame rate does not necessarily depend on the frame rate set up in the computer’s graphics card, so one frame can last shorter than 16.7 milliseconds (LCD monitors’ standard refresh rate of 60Hz).
This works also for transitions from a brighter tone to a darker one, except that the “boost impulse” is going to be negative. It is the dotted line on the graph above.