Alternating positive and negative voltages
It can be used to create a strobing effect that can be visible to the human eye. If the positive and negative alternating voltage is applied faster than the eye can follow, then it essentially makes all of the images appear as one image, unless there are changes from frame to frame. This is known as pixel inversion.
The effect of pixel inversion was first seen on picture tubes but has now become common in flat-panel displays. The majority of these use liquid crystal display (LCD) technology, which was initially based on light modulators that rotated polarized light. Pixel inversion was an undesirable artifact caused by rotating polarizers' slow response time.
Nowadays, LCDs use fast switches such as twisted nematic cells or ferroelectric liquid crystals instead, which gives rise to pixel inversion due to the superimposition of two images.
Plasma and OLED
Television and video display systems that do not use pixel inversion include plasma displays and OLED. Plasma displays were originally based on a form of gas discharge whose response time is much faster than any liquid-crystal cell, and thus they did not require the application of an intermediate polarizing layer to avoid image retention; however, it was economical for manufacturers to add such a layer anyway.
This can still cause problems when devices are connected with cables or adapters; sometimes an "inverter" (which applies negative voltages at all times) must be inserted between such incompatible devices to prevent damage from occurring. Very early plasma televisions were known to use this technique, but later models introduced in the 1990s eliminated it.
Organic light-emitting diodes (OLED) are even faster than plasma displays, so pixel inversion is hardly ever needed. Also, they do not contain any polarizing layers that might cause image retention problems when voltage is applied at alternating polarity.
Strobe backlight
Pixel inversion may be done deliberately on some displays using a strobe backlight or "scanning backlight." The human eye responds more strongly to changes of brightness than steady illumination; for example, rapidly flashing a bright light can make it appear as if it is constantly lit.
Some CRT computer monitors use scanning of the backlight to make the image appear brighter with little power consumption; color monitors can use it to improve the perceived contrast ratio.
This technique is used in LCD monitors with non-inverted (i.e., "normal") pixels as well, where a rapidly alternating voltage between positive and negative polarity causes each sub-pixel of the display to act like an analog switch that is constantly either on or off.
Such displays may reduce flicker by scanning lines during horizontal periods (when they are not refreshed), although this only works if all rows are scanned before any given row is refreshed again at the end of the frame period.
For example, some computer monitors scan during horizontal retrace, revising every line once per frame, while others more typically revise every other line twice, once per frame. The former typically exhibit higher PWM flicker than the latter, but also have a much lower response time and less ghosting during fast motion due to faster pixel switching.
The simplest approach is to display an entire moving image with the alternating polarity of scanlines or individual pixels, which can be done without risk of image retention because it only affects brightness rather than the static state of the screen.
For example, in a progressive signal, each new frame begins as all black, so there is as yet no residual charge from prior lines as would happen with an interlaced video where one field precedes another for part or all of its duration. Scanning within a frame always results in undefined behavior, so for it to be done without leaving ghost images is either impossible or requires using a refreshing pattern that keeps bright objects from being displayed when the polarity of their pixels alternates.
The process used by field-sequential color CRTs where one line is scanned at normal intensity, followed by alternating lines in two primaries, with the second primary being blanked during retrace intervals between successive sets of RGB signals so its peak brightness does not add an additional charge to the already-illuminated phosphors.
The advantage over interlace is that changes in only one line are visible on the screen; however, this also applies to flicker due to PWM if switching occurs during horizontal flyback (the interval between fields).
How to reduce pixel inversion?
So now you know what is pixel inversion and how it can affect your eyes. The following tips will help you reduce the effects of pixel inversion:
- Always look for a good and well-lit place to work and study. If you need to, use a well-lit desk lamp or put some lights on top of your monitor so that it does not feel too dark to read through the text.
- When buying a new PC screen, ask for an anti-glare screen — the ones with a matte finish will always be better than glossy ones since they tend to lessen the glare from external light sources such as sunlight or bright lamps around your room.
- In addition, if possible, angle your computer display away from direct sunlight that comes into your room.
- Make sure your monitor is at a comfortable distance from your eyes so that you do not have to strain yourself too much just to see the screen clearly.
- Do not sit or read for hours in front of a computer since it will eventually damage your vision due to excessive exposure to light as well as bad posture as you tend to scrunch up your shoulders and slouch down your head instead of sitting straight.
- If possible, turn off the energy-saver mode on your monitor because this feature tends to make the screen flicker more frequently. It will also help if you can adjust the brightness settings of your display lower so that it does not hurt your eyes whenever you stare at it for long periods of time.
Pixel walk and other artifacts
It can be mistaken for other similar visual artifacts:
Overdrive
It is used to increase pixel transitions in the opposite direction. The artifacts are transient and consist of bright trails behind moving objects, red/green/blue trailing in after-image effects when high contrast edges are displayed against a dark background, or smeared/streaked colors when scrolling video.
FRC
To alternate pixel polarities at a rate beyond human's ability to perceive flicker. The advantage over field-sequential color schemes is that all of the images can be displayed simultaneously, not interlaced between primary colors for successive fields. The drawback is that it causes half the vertical resolution in each dimension, and thus suffers from an effect known as Temporal dithering.
How do you turn off pixel inversion?
It's easy to get rid of this problem by turning off pixel inversion on your monitor. You just need to go into the settings menu and look for "pixel inversion." From there, you should see some options involving inverted or noninverted images; pick whichever one works best for you! Of course, make sure that the colors on your monitor are displayed correctly before doing this!
FAQs
Q: I don't use my monitor/TV for long periods of time, so should I be worried about this problem?
A: No. If you're not exposed to light or straining your eyes, then there is nothing to worry about even if you have this problem. Also, if you can turn off energy-saving mode on your monitor, this will also help reduce the effects of pixel inversion.
Q: Can CRTs have pixel inversion?
A: Yes they can because CRTs still use pixels even though each pixel is already made up of three sub-pixels. The pixel inversion can happen on a CRT monitor when the electron beam passes through a phosphor with a different charge.
Q: Why is it called "pixel inversion," when pixels have always been made up of red, green, and blue subpixels?
A: It's called "pixel inversion" because each pixel has all three subpixels turned on, but they are turned off at every other horizontal line or field (alternating between rows), or flipped upside down.
Bottom line
Pixel inversion is a phenomenon where each pixel of an image turns off every other line of pixels, alternating from row to row. This creates a distorted pattern that can be seen on CRTs and LCD/LED monitors so it's good to turn this off if the pattern bothers you!