In my earlier articles (first of all in the article called X-bit’s Guide: Contemporary LCD Monitor Parameters and Characteristics , which was wholly concerned with the strong and weak aspects of LCD monitors as well as with the accepted methods of measuring their parameters) I have repeatedly stated that the ISO 13406-2 method to measure the monitor’s response time as the total time necessary to change the state of a pixel from pure black to pure white and back again brings but very little information about the real performance of the monitor and easily misleads the user.
The problem is that the response time value obtained by this method is not the maximum and even not an average, but the minimal speed the monitor can have. In practice, however, the user never deals with a pure white color even when working with black text on white background unless the monitor’s contrast setting is set at the maximum, which would be too bright and uncomfortable for normal work. However, it is only the maximum possible screen brightness that is regarded by the matrix as pure white, so it is going to treat the white background as gray rather than white. Moreover, the user never sees a pure black color in games or movies, either. In other words, we usually either deal with black into light-gray (when working with text, for example) or gray-to-gray transitions and the formally measured response time of the matrix thus has a very small practical value.
As a side remark, I’d want to make it clear that when I say “gray” in the context of LCD monitors I don’t mean the gray color the user sees on the screen, but any in-between state of any sub-pixel – red, blue or green. As you probably know, LCD monitors yield the necessary color of a sub-pixel by using an ordinary color filter. The filter of course has no effect on the response time, which is absolutely the same for sub-pixels of any color. So there is no need to specify the color of the sub-pixel. It is simpler to assume that “white” means the state of the maximum brightness of the sub-pixel, “black” is the minimal brightness state and “gray” is all the intermediary states, as if the monitor were black-and-white.
So again, in practice we usually deal with either black-to-gray or gray-to-gray transitions of the pixel state. On the physical level, a transition is accomplished by turning the pixel’s liquid crystals by a certain angle with an electric field, the two extreme positions of the crystals corresponding to black and white and the remaining positions to grays. It may seem that the process of switching from black to gray should go faster than switching from black to white since the crystals are turned round by a smaller angle, but the fact is the applied electric field not only determines the orientation of the crystals, but also the speed of their turning round. The force that affects the crystals is proportional to the square of the applied electric field. So it takes a four times smaller force to turn the crystals round by a two times smaller angle. Moreover, the crystals are affected by the forces that drive them towards their natural order (when there is no electric field applied, the crystals in the matrix are ordered and yield a pure white or a pure black color depending on the type of the matrix). These forces depend on the current orientation of the crystals and are always directed opposite to the electric field force.