The magic word: "overdrive"
After neglecting it for years, manufacturers realized that monitor reactivity is the determining factor for quality. You must not think that crystal reactivity has always been the main objective for manufacturers, who have told us for many years that it was none of their clients’ concerns. Also there are still some (fewer and fewer every day, fortunately) who don’t want to consider the problem of dead pixels as being a real issue and who still don’t want to provide the least guarantee on this point to their clients. But that is not the point we wish to make today.
Basic reminder: ISO response time
Reactivity is supposed to be represented by the "response time" in the monitors’ characteristics. The response time (RT) has been developed by the worldwide normative organization, ISO. This explains that to measure RT you need to add up the rise time (time needed to change a pixel from white to black) and the fall time (i.e. the other way round from black to white).
Two liquid crystal cells.
On the left a white pixel, on the right a black pixel.We quickly realized that (from
the first LCD roundup in french) that this value had little to do with the actual monitor reactivity. For example in 2001, it was a 40 ms monitor that turned out to be the most reactive. It was however compared to 25ms monitors. The reason for that was that we don’t interact with only two tones: black and white, but with color. It is, however, important to emphasize that we see colors but the computer only work in shades of grey. Each panel cell emits a variable light level (256 shades in principle). In front of each cell is a green, blue or red filter. The juxtaposition of all three enables us to see a picture in color (see
this other article to understand how the monitors work).
First measurements between greys
The ISO response time measurement follows this principle:
RT = 255 (white) – 0 (black) – 255 (white).
Two problems arise with this principle. If we measure the RT = 255 (white) – 128 (average grey) – 255 (white), the response time is almost always strikingly different. The gap might be from 25 ms for the ISO RT to 100 ms for the white – grey – white measurement. This is why we thought that it would be interesting to measure not only the ISO RT as well as all color changes from white to any other color. We obtained this type of graph:
Response time from white to grey (ms)
This is indeed rather more accurate. With this example, we see clearly that the white to grey response time might be a lot slower that the transition to black.
Unfortunately, this is still not enough because, in certain flattering situations (when the response time measured with this method remains low) it can be highly misleading. Response times from white to grey can also vary greatly from grey to grey.
Bars only represents rise or fall timeFor example with this graphic provided by NEC:
RT ISO = (0 - 255) 18 + (255 - 0) 7 = 25 ms,
RT MAX for white to grey = 0 – 192 – 0 = (0 - 192) 38 + (192 - 0) 5 = 43 ms approximately (72 % > RT ISO),
RT MAX all grey levels = 255 – 160 – 255 = (255 - 160) 55 + (160 - 255) 36 = 91 ms approximately (264 % > RT ISO).
It was also our conclusion last September (
see this article), without carrying out so many measurements. There is a small difference with the previous graphic: bars don’t only represent rise time but the full RT from one color to another (rise + fall).
Bars represents complete response time (rise + fall)These graphs show one point: accelerating the ISO RT isn’t enough to improve reactivity. It is imperative to accelerate all color transitions.
