Articles: Monitors

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Let’s take a simple case of a white square moving across a black screen like in one of the tests of the popular TFTTest program. Consider two sequential frames in which the square moves from left to right by one position:

I tried to picture four sequential “snaps”: the first and last of them are the moments the monitor displays the two sequential frames while the other two snaps illustrate the behavior of the monitor and our eyes in between the displayed frames.

In the case of a CRT monitor, the square is displayed in the first frame, but begins to fade out rapidly after 1 millisecond (the phosphor afterglow time), so it has vanished from the screen completely before the second frame arrives. But thanks to persistence of our vision we continue to see this square for about 10 milliseconds more so that it only begins to fade out noticeably by the arrival of the second frame. The moment the second frame is being drawn by the monitor, our brain is receiving two images at once: a white square in the new position and a rapidly fading image of it (left on the eye retina) in the old position.

Active-matrix LCD monitors do not flicker, as opposed to CRT monitors. They store the image through all the period between the frames. On one hand, this solves the refresh rate problem (the screen does not flicker at any frequency), but on the other hand – just take a look at the illustration above. The image was rapidly fading out on the CRT monitor between the two frames, but it remains unchanged on the LCD panel! When the second frame arrives, the monitor displays the white square in the new position and the old frame takes about 1-2 milliseconds to fade out (that’s the typical pixel fall time of modern fast TN matrixes – comparable to the phosphor afterglow time of CRT monitors). But the retina of the eye is going to retain the old image for 10 milliseconds after the real image has disappeared! And through all this time the “imprint” is combined with the new image. As a result, the brain is receiving two images at once for about 10 milliseconds after the second frame has arrived: the real picture of the second frame from the monitor’s screen plus the imprint of the first frame on the retina. That’s not unlike the well-known fuzziness effect, but the old image is now stored not in the slow monitor matrix, but in the slow retina of your own eye!

To cut it short, when the LCD matrix’s own response time is below 10 milliseconds, its further reduction brings a smaller effect than might be expected because the persistence of human vision enters the play as an important factor. Moreover, even if the LCD monitor’s response time is reduced to tiny values, it will still look slower subjectively than a CRT. The difference is about the moment the time of storing the after-image in the eye retina is counted from: with CRT monitors it’s the time the first frame arrives plus 1 millisecond. With LCD monitors, it’s the time the second frame arrives. Thus, the difference between the two types of monitors amounts to about 10 milliseconds.

The way to solve this problem seems obvious. If the CRT screen looks fast due to its being black for most time between two sequential frames, which allows the after-image to begin to fade out on the eye retina just by the moment the new frame arrives, we should deliberately insert additional black frames into the image frames on LCD monitors.

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