19'' comparative test and response time (page 5: Why this measurement fails)

Why this measurement fails

Flat screens are designed to work with a refreshing rate of 60 to 75 Hz. This frequency doesn’t indicate a sweep frequency (like with CRT screens), but indicates the picture rate sent by the graphic card.

With 60 pictures per second (60 Hz), the picture rhythm reaches 1 /60 = 17 ms.

At 75 Hz the picture rhythm reach 13 ms.

By looking at the above graphs, we can see that picture flow is often faster than pixel speed. At 60 Hz (1 image every 17 ms), liquid crystals are at the half-way point when the next image arrives.

With a flat screen, colors are rarely accurate in animate sequences. Fortunately, we don’t notice this. Images should quickly change from one to another and instead of displaying black, we see a dark grey. In practice, screen characters move slower than that. They don’t display radically different colors but close shades successively. The afterglow effect only appears when an object moves in a background of a radically different color.

For example, with the Iiyama AS4314UTG screen:

If the picture changes in T0 from white to black (in T1= 17 ms) the transistor applies maximum tension to the liquid crystals. This situation provides the fastest results.

At T1, the liquid crystals are in intermediate position, approximately half way between the beginning and the end (the 255 to 0 response time is 35 ms for this screen). The pixel displays an intermediate grey.

Still in T1 the graphic card changes the pixel color, it has to return to white. This is when the inertia problem shows up. What is the time required for liquid crystals to stop and come back to display white? How many milliseconds are required to slow down and reverse their movement?

The longer this time is, the more different the desired color will be from the one displayed. This is when a trail of light appears.

For the moment we are unable to evaluate this inertia time with the tools we possess. Until we succeed in establishing a new more accurate testing procedure, we will have to keep testing screens with a visual analysis of movies and games. This is the only true way to estimate a screen’s performance.