X-bit labs

Contemporary LCD Monitor Parameters: Objective and Subjective Analysis (page 2)

Category: Other

by Oleg Artamonov

[ 01/23/2007 | 02:21 PM ]

Response Time of Monitors and Eyes

You could have read in old reviews of LCD monitors (and in my reviews, too, as I have to confess) that as soon as their response time (the real response time as opposed to the specified value which, if measured according to ISO 13406-2, is not in fact indicative of the real speed) was lowered to 2-4 milliseconds, we would forget about it altogether just because its further decrease wouldn’t make anything better – we wouldn’t see the fuzziness anyway.

And finally such monitors are here. The latest models of gaming monitors on TN matrixes with response time compensation (RTC) technology have an average (GtG) response time of only a few milliseconds. I will put such things as RTC artifacts and inherent drawbacks of TN technology aside for now. What’s important is that the mentioned numbers are indeed achieved. But if such a monitor is put next to an ordinary CRT monitor, many people will say the CRT is still faster.

Strangely enough, it doesn’t mean we have to wait for LCD monitors with a response of 1 or 0.5 or less milliseconds. Well, you can wait for them, of course, but such panels won’t solve the problem. Moreover, they won’t differ much from today’s 2-4ms models because the problem is not about the panel, but about the peculiarities of the human vision.

Everyone knows about such a thing as persistence of vision. Take a look at a bright object for a couple of seconds, then close your eyes and you will be seeing a slowly fading-out “imprint” of the image of that object for a few more seconds. The “imprint” is rather vague – just the object’s contour – but it lasts as long as seconds! As a matter of fact, the retina retains a precise image of an object for 10-20 milliseconds after the object itself had disappeared. It’s only after that time that the image is fading out quickly, leaving just a contour if the object has been bright.

This effect comes in handy for CRT monitors. It’s thanks to this persistence of vision that we don’t notice the flickering of the screen. Phosphors in today’s cathode-ray tubes have an afterglow time of about 1 millisecond. The electronic beam makes its way through the whole screen in 10 milliseconds (at a scan rate of 100Hz). So, if our eye didn’t have any persistence, we’d see a light band, only one tenth of the screen in width, running from top to bottom. This can be easily demonstrated by photographing a CRT monitor at different exposure times.

At an exposure value of 1/50 seconds (or 20 milliseconds) we can see an ordinary image that occupies the entire screen.

At an exposure value of 1/200 seconds (or 5 milliseconds) there appears a wide dark band on the image. The beam has only passed through half the screen during that time (at a scan rate of 100Hz) and the phosphors have already gone out in the other half of the screen.

And finally, at an exposure value of 1/800 seconds (or 1.25 milliseconds) we see a narrow bright band running on the screen. This band is leaving a small and quickly fading trail whereas most of the screen is black. The width of the bright band is determined by the phosphor afterglow period.

On one hand, this behavior of phosphors makes us set high refresh rates on CRT monitors (at least 85Hz for today’s tubes). But on the other hand, this relatively low afterglow time of phosphors makes CRT monitors faster than even the fastest of LCD monitors.