Illustrations of different motion artifacts that affect clarity of moving objects on LCD displays, using the TestUFO Ghosting Test. The UFO objects were moving horizontally at 960 pixels per second on a 60 Hz LCD, and captured using a pursuit camera.
Ghosting is a trailing motion artifact. It is caused by asymmetric pixel transitions: Transitions from between two colors can be faster in one direction than the other direction. This motion artifact appears only on trailing edges; observe the yellow dome.
Coronas / Inverse Ghosting
Coronas are trailing artifacts caused by response time acceleration (RTC, overdrive, ASUS Trace-Free, BENQ AMA). Pixels can overshoot their final color value before bouncing back, causing bright inverse ghosting. See TFTCentral Overdrive.
Motion blur is symmetric for both trailing and leading edges. It is the blurring caused by eye tracking on continuously-displayed refreshes (sample-and-hold). This is not pixel persistence; but a different cause of display motion blur. See Science & References. Higher refresh rates (120Hz) or flicker (CRT and LightBoost) reduce this type of motion blur, see 60Hz vs 120Hz vs LightBoost.
PWM artifacts look like repeated images, and can affect motion fludity. PWM represents Pulse-Width Modulation, a technique that many LCD backlights use for dimming screen brightness. Motion artifacts can appear at dim brightness settings. The above photograph was taken during Brightness setting of 0%. See TFTCentral PWM.
How Were These Images Captured?
Stationary Camera: Capture of Pixel Transitions
A stationary camera is good for photographing pixel transitions statically. However, it is not a very accurate representation of perceived display motion blur and motion artifacts:
Pursuit Camera: Accurate Capture of LCD Motion Artifacts
Pursuit camera are used by display manufacturers for testing (e.g. MotionMaster, and other MPRT pursuit cameras). This is simply a camera that follows on-screen motion. These expensive cameras are extremely accurate at measuring motion blur and other artifacts, since they simulate the eye tracking motion of moving eyes.
Blur Busters has developed an inexpensive pursuit camera method which operates in conjunction with the Blur Busters UFO Motion Tests. It makes possible accurate photography of motion artifacts. The test at www.testufo.com/ghosting was used to take the pictures on this page. Blur Busters is the world’s first blog to utilize a pursuit camera for accurate capture of motion artifacts, in WYSIWYG format, as seen by the human eye.
Can All LCD Motion Artifacts Be Eliminated?
Nearly all LCD motion artifacts can be reduced or eliminated by scanning backlights and newer strobe backlight technologies including the popular LightBoost tweak used by high-end computer enthusiasts. Such displays successfully bypass pixel persistence by turning off the backlight during LCD pixel transitions, and precisely strobing the whole backlight only on fully-refreshed frames (see high-speed video). These displays produces CRT motion quality, with all perceptible motion blur completely eliminated.
See the next article in this series, LCD Motion Artifacts: Overdrive.