Understanding response time
Reading screen characteristics doesn’t take long, but as you will see below they shouldn’t be fully trusted.
Response time is the first LCD screen’s characteristic. Expressed in milliseconds, this measure represent the amount of time necessary to a pixel to turn from white to black and then going back to white. Tested screens two years ago, had a 60 to 25 milliseconds (ms) response time for the most efficient screens. Nowadays, screens performances are reaching 12 ms. The 8 ms screen displays are already expected in September.
In theory the faster this response times is, the smoother and clearer the movements are on your screen.
This theory would be verified if the response time was constant and valid for all colours change. But a 25 ms white/black/white screen might need 120 ms to process light grey/ dark grey/ light grey.
ExplanationsThe response time as all LCD screen characteristics are defined by the ISO norm 13406-2. And as we will see here and later, this norm deserve quickly a deep review!
To turn a pixel from white to black, the electrical tension goes from 0 the maximum limit. Highly stimulated, the liquid crystals quickly take their new position. The amount of time required to process white to black is called rise time.
To get back to white, the electrical tension is cut. Crystals get back to their initial position easily and quickly. This process is called fall time.
The response time is the total of rising + fall time.
First trouble induce by the ISO norm, the rise and fall time are measured on 80 % of the whole signal. The extreme 10% are cut from this measure. This measure is at some point justified but simplifies and flatters the result. So if one system takes more time to start or stabilize the pixel, it won’t be noticed in the results.
In the previous graphic, the rising time would be 28.5 – 12 = 16.5 ms. However by looking at the signal, it seem logical to consider a measure of 40 ms closer to the reality.
The following issue is the response time between colours. To turn a pixel from light grey to dark grey, the electrical tension start from a low electrical tension to a strong electrical tension. The electrical stimulation is noticeable but no as strong as when it need to rise from 0 to the maximum limit. So crystals are slower to move and in practice the rising time is longer.
To go back to light grey, the process is reversed. There again, crystals will take longer to get back to their positions if the electrical tension is completely cut off. Fall time is also longer
For now the ISO norm doesn’t compel the manufacturer to consider intermediates response time. So the result is that manufacturers communicate on the same response time for different panel technologies and different crystals movement speed.
So we are now in a situation where LCD screens equipped with a TN AU Optronic panel 16 ms are a little bit quicker than the 16 ms LG-Philips and the Samsung 12 ms. These panels are unmistakably quicker than the 16 ms IPS panel and slower than the TN Hydis 20 ms. How is it possible not to be hopelessly confused?
This norm ISO-13406-2 should really be reviewed soon! (one time)
