Pixel switching time

Pixel switching time is the time it takes a display pixel to change from one brightness or color value to the next; for LCD panels it is usually quoted as the response time. In VR and AR, pixel switching time affects latency of the device. Reducing pixel switch time can reduce latency.

Pixel switching time can be improved by using superior display technology. When VR devices such as Oculus Rift (Platform) switched from LCD to OLED, pixel switch time was significantly reduced. Switching time is related to, but distinct from, persistence, which is how long each pixel stays lit during a frame.

Reviewed 27 September 2026. All cited sources re-opened (Carmack 2013, both Abrash posts via archive, Lagom, Kreylos 2014, Road to VR DK2 articles, Oculus Go blog, OLED-Info, US10360857B2, Valve Index specs, and the Chen, Peng, Gou and Yoon paper abstracts) and every quote, figure and date checked against them. About review dates.

Definition and measurement

A pixel does not jump instantly to its new value. Its light output moves toward the target over a period that depends on the display technology and on the two values involved. John Carmack noted in 2013 that the transition times for LCD pixels "vary based on the start and end values being transitioned between". The Lagom LCD test pages explain that older monitors defined response time as the time to switch from black to white and back to black, while newer monitors use the time to switch from one gray level to another, counted until the pixel is within 10% of the difference between the two levels. The same pages warn that such a figure covers only the first 90% of a transition, whatever time the last 10% takes, and that a manufacturer that does not explain its method may have picked the pair of gray levels where the panel is fastest.

Rise and fall are often unequal. Lagom notes that twisted-nematic LCD panels are usually fastest for gray-to-black transitions and can be much slower for black-to-gray ones. OLED pixels show a similar asymmetry on a far shorter time scale: in his analysis of the Oculus Rift DK2, Oliver Kreylos described black-to-white transitions as slower than white-to-black ones.

LCD and OLED switching times

LCD and OLED switching times differ by roughly three orders of magnitude. In a 2018 review in Light: Science & Applications, Chen, Lee, Lin, Chen and Wu describe LCD response time as "approximately 1000 × slower than that of an OLED (ms vs. μs)". Carmack gave figures in his 2013 essay on latency: early LCDs "were notorious for 'ghosting'", still showing traces of old images "many tens of milliseconds" after a change; a good panel of 2013 had "a switching time around ten milliseconds", displays optimized for active 3D and gaming could reach less than half of that, and "OLED pixels can have switching times well under a millisecond". Michael Abrash, then at Valve, wrote in 2012 that "LCD panels take several milliseconds at best to change state", whereas some scanning lasers and OLEDs display each pixel's data immediately as it arrives.

Liquid crystal pixels switch by reorienting the LC directors in an electric field and then letting them relax back when the field is removed. A Meta Platforms Technologies patent for VR LCDs states that "changing states of the LCs may take several milliseconds (ms), thereby limiting a response time of the LCD or preventing an increase in the frame rate". OLED switching is fast but not zero, which matters in a head-mounted display (see below).

Effect on latency and motion clarity

Switching time adds directly to display latency. Abrash's 2012 breakdown of motion-to-photon latency lists tracking, rendering, scan-out of the frame to the display and the time before the display starts emitting photons for each pixel; for LCDs the pixel data is applied immediately "but there is a delay before the change is visible". Carmack wrote that absolute delays below approximately 20 milliseconds are generally imperceptible, while a total system latency of 50 milliseconds "will feel responsive, but still subtly lagging".

Switching time also affects motion blur, although it is not the only factor. On a sample-and-hold display a pixel keeps its value for the whole frame, so motion blur depends on the frame time as well as on the transition. Chen et al. use the motion picture response time (MPRT) to quantify this and show that once the liquid crystal response time is much shorter than the frame time, MPRT is set mainly by the frame rate and approaches about 0.8 times the frame time. Peng and colleagues derived an analytical equation linking MPRT to LC or OLED response time and frame rate, and found that an LCD with a response time of 2 ms or less has nearly the same MPRT as an OLED, even when the OLED's response time is taken as zero. Their ultra-low-viscosity vertical-alignment LC mixture measured an average gray-to-gray response time of 0.93 ms and an MPRT of 6.88 ms at 120 Hz, against 6.67 ms for an OLED. Peng and colleagues name two ways to shorten MPRT further: a higher frame rate or a lower backlight duty ratio. Chen et al. add that reaching an MPRT below 2 ms is mostly done by lowering the duty ratio (for LCDs, a scanning or blinking backlight), at the cost of brightness.

In VR the duty ratio is reduced much further through low persistence. Abrash wrote in 2013 that full persistence, with pixels lit for the entire frame, "is the case with many OLED and LCD displays", and that persistence below roughly 2 ms should almost completely eliminate the smear component of judder. On an LCD the two interact: the short flash of light has to come after the liquid crystals have finished changing state (see below).

In VR headsets

From LCD to OLED

The first Oculus Rift development kit, the Oculus Rift DK1, used a 1280x800 LCD panel that, according to Road to VR's Paul James, "was heavily prone to smearing". The Oculus Rift DK2 replaced it with a 1920x1080 PenTile OLED panel driven in low-persistence mode. Oculus announced the DK2 at GDC on 19 March 2014 with a 5-inch 1080p OLED display at 960x1080 per eye, refresh rates of 75, 72 and 60 Hz and persistence settings of 2 ms, 3 ms or full; Road to VR's Ben Lang wrote that the image was "substantially sharper in the DK2 when moving your head, mostly thanks to low-persistence".

Black smear on OLED

The switch to OLED exposed a new artifact. Kreylos, writing about the DK2 in October 2014, explained that although OLED pixels switch "orders of magnitude faster than LCD pixels", they still cannot switch instantaneously. When the user turns their head, the vestibulo-ocular reflex keeps the eyes fixed on a virtual object, so pixels at the leading edge of a bright object that must change from dark to bright are watched closely, and because of their non-zero switching time they appear darker than the object for a moment. This is called "black smear"; the opposite edge leaves a fainter trail, "white smear". Kreylos wrote that the effect is barely visible on a normal screen and becomes obvious only when the display is strapped to the face.

The fix, which Kreylos credited to Oculus, is overdrive applied in software. When a pixel has to get brighter, the renderer asks for a value above the target so that the pixel, limited by its response time, lands on the intended brightness within the frame, and does the reverse for pixels that have to get darker. He gave the DK2's factors as 0.1 for increasing brightness and 0.05 for decreasing brightness. Overdrive fails at pure black and pure white, because a pixel cannot be driven below zero or above full output. Kreylos's own addition was to compress the brightness range of the image slightly to leave headroom; for the DK2 factors this cost 13.85% of contrast, a loss he found "hardly noticeable".

Fast-switch LCDs

Later headsets returned to LCDs designed for fast switching. Oculus announced the Oculus Go in October 2017 with what it called a "high-resolution fast-switch LCD screen". OLED-Info's Ron Mertens described the move from OLED to LCD as one of the ways Oculus lowered the headset's cost, adding that it was then too early to say how the display would compare with the OLED in the Oculus Rift.

An LCD headset can hide the remaining liquid crystal switching time by flashing the backlight. In the global illumination mode described in the Meta Platforms Technologies patent (filed 31 March 2017, granted 23 July 2019), the backlight "turns on only after a frame of data is written (data scan out and charging of the pixels) and all the LCs in the LC layer have completed state changes". The patent says the backlight is on for only a small portion of the frame time (20% is the example given), which gives low persistence, and that keeping the illumination period shorter than the frame time reduces image streaking and latency. The Valve Index uses this approach: Valve specifies dual 1440 x 1600 LCDs with "ultra-low persistence global backlight illumination (0.330ms at 144Hz)", and states that the illumination period ranges from 0.330 ms to 0.530 ms depending on frame rate, which it calls a 5x improvement over first-generation PC VR headsets.

Field-sequential displays

Abrash noted in 2012 that color-sequential LCoS displays buffer pixel data and show it a frame or more later, lighting the red components of all pixels at once and then green and blue in turn, which adds latency. Chen et al. note that field-sequential color could triple the resolution density of an LCD, but that more advanced LC mixtures and fast-response LCD modes are needed to suppress the color breakup it causes.

Research

Several papers since 2017 have proposed faster liquid crystal modes aimed at VR displays. Gou, Chen, Li, Lee and Wu reported in 2017 a vertically aligned LCD with hole-patterned fringe-field-switching pixel electrodes that reached a submillisecond gray-to-gray response time, 85% peak transmittance under crossed circular polarizers and a single thin-film transistor per pixel, which they described as "particularly attractive for the emerging virtual reality displays". In a 2018 follow-up the same authors reported that a new LCD device driven at 90 Hz with a 17% duty ratio had an MPRT of 1.5 ms, which they described as comparable with a cathode-ray tube.

Yoon and colleagues argued in 2018 that VR head-mounted displays need a resolution above 2000 ppi together with "super-fast response time" and high contrast. Their vertically aligned device showed a decay response time four times faster than conventional fringe-field switching, a contrast ratio over 5000:1 and a pixel pitch under 4 micrometres.

See also

• Latency
• Motion-to-photon latency
• Pixel persistence
• Low persistence
• Refresh rate
• Smearing
• Judder
• LCD
• OLED