X-bit labs

X-bit’s Guide: Contemporary LCD Monitor Parameters and Characteristics (page 2)

Category: Other

by Oleg Artamonov

[ 10/26/2004 | 02:58 PM ]

Pixel Response Time

Response time seems to be the most popular characteristic of any LCD monitor as it is often the first parameter the customer pays attention to when evaluating a model.

As you know, the state of a pixel in the LCD panel changes when the angle of the liquid crystals is changed under the influence of the applied electric field. But liquid crystals is rather viscous stuff, so it takes them quite an amount of time to turn by the necessary degree – up to a few or more milliseconds. The following picture illustrates this process (the X axis shows time in milliseconds, and the Y axis – the brightness of the pixel; the pixel is changing its state from fully closed to fully open):

The manufacturers of matrices and monitors traditionally measure the response time as the total time it takes the pixel to change its state from black to white and to black again. More precisely, they measure the time it takes the pixel’s brightness to grow from 10% to 90% and return to 10%. This convention – measuring within the 10%-90% range – is not any trick of the manufacturers’. It’s rather a necessity since it is basically impossible to pinpoint the moment the pixel begins to shine or the moment it reaches 100% brightness due to noises present and limitations of the measuring equipment. So we should rather talk about the pixel’s brightness being in or out of a certain interval, which is defined as 10% here.

Unfortunately, this way of measurement gives rather a vague idea of how the monitor would work with dynamic graphics because the response time measured is the minimal response time of the matrix. Suppose we’re interested in black-to-dark-gray (like in many “dark” games) rather than black-to-white transitions. The crystals have to turn around by a smaller degree in this case, but the speed of turning is proportional to the intensity of the applied electric field, and it is this field that determines the angle of turning – the smaller angle we need, the weaker electric field we must apply. Thus, we’ve got two contending trends – the angle of turning is getting smaller, but the speed of turning is getting smaller, too! In practice, the turning time (i.e. the monitor’s response time) depends on the proportion of these trends. Measurements suggest that the response time is the smallest when the pixel’s state (color) is transitioning from black to white. Other transitions take longer to be made, and the exact value depends on the type of the matrix (I’ll describe this in detail later on, in the sections dedicated to each particular matrix type).