Ghosting describes how quickly (or slowly) a screen can get rid of images that are no longer supposed to be displayed. The stronger the ghosting, the more blurred moving objects will look. For example, on a very bad screen, if an object moves from point A to a final point D, travelling via points B and C, traces of the object will still be slightly visible at point B and highly visible at point C, even when the object has already arrived at point D.
In the past, to measure the actual accuracy of on-screen movement, we took a fast burst of photos to capture and then measure these 'ghost' images. However, we have now improved this test by using a camcorder that records at 1,000 frames per second.
We use the camcorder to film the screen playing a short video sequence of moving objects, of various colours and on various backgrounds.
From a distance, the movement looks smooth. From very close up, on a perfect screen, the camcorder would capture footage of the moving object appearing at point P, then disappearing to arrive at point P+1, then disappearing to arrive at point P+2, and so on.
Ideal next frame, at point P+1
Ideal next frame, at point P+2, etc.
That's what should happen in theory, but the perfect screen doesn't exist ... yet. In reality, when the moving object appears at point P+1, the screen hasn't yet had time to totally erase the image from point P. The sequence of frames below shows what actually happens in TVs, monitors, laptops and other screens sold on the high street:
As you can see, 'ghost' images are left trailing behind the actual image as it moves across the display. This affects the precision of the overall picture and makes moving objects look blurred. To quantify the extent to which a screen is prone to ghosting, we'll be measuring the time it takes for the 'ghost' image to disappear. The longer the ghosting time, the more blurred moving objects will look.
We've Tested the Lot: TN, PVA, MVA, IPS, Plasma, OLED!Most manufacturers quote the 'response time' of a screen. Although supposedly based on the time it takes for the screen to switch from white to black, it's actually measured using a transition from a light shade of grey to a dark shade of grey. This test gives a specific time that's often found in product tech specs: PVA screen panels generally have a response time of 5 ms, IPS panels of 6 ms, and TN panels of 2 or 5 ms.
Quite often, we find that the readings from our own tests or our actual impression of watching the screen couldn't be further removed from the values obtained using the manufacturers' standard. We've seen screens with a response time quoted at 5 ms that look more responsive to our eyes than those said to be 2 ms—sometimes by a long way! That's why we thought it was time to use a different system that takes into account what end users actually see on the screen. Manufacturers are only too aware that their test procedure is removed from reality and some have even stopped quoting official response times in their spec.
Here are the results we got trying out our 1,000 fps camcorder on a selection of popular products:
With the new ghosting time test, the results mirror what we actually see when watching the various screens in action! The fastest screens according to the camcorder were also the fastest according to our eyes.
Interestingly, the camcorder picks up differences—sometimes big differences—between certain products equipped with panels based on the same type of technology (IPS, MVA, etc.).
OLED: clearly shows promising results! The first and only TV to use this type of technology is the most responsive TV we've tested to date.
Plasma: the Panasonic plasma screens we tested were particularly responsive—more so than most LCDs. Only the very best LCDs equipped with certain PVA and ASV panels were able to do as well.
PVA: there are several types of PVA screen: some are very responsive, with a ghosting time under 10 ms, while others are slower at 15 ms and over.
MVA: none of the MVA screens we've tested gave a ghosting time of under 15 ms. That's equivalent to a middle-of-the-road PVA panel.
ASV: Sharp seems to make particularly responsive panels, currently used in Sony 3D TVs to reduce crosstalk.
IPS: we already knew these screens had made progress in the field and the test results confirm that. Some scored around 11 ms but for others the ghosting time was almost double that!
TN: is a real mixed bag. The 120 Hz 2 ms TN panel is excellent and easily as responsive as an OLED screen. Some 60 Hz 2 ms panels do a good job, with a ghosting time of around 10 ms, while others have trouble handling overdrive, leading to a reverse ghosting effect and doubling the ghosting time! The 5 ms TN panels often used in monitors or laptops came in at over 20 ms.
From now on, the ghosting time test will feature in all our monitor and TV reviews. It will also be coming soon to reviews of touchscreen tablets, laptops and mobile phones.
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