Definition
An IPS panel is a display technology used in monitors and smartphones that keeps liquid crystal molecules lying flat, parallel to the glass, and spins them in place to control light. Because of this, colors stay accurate and barely fade or distort when you view the screen from an angle.
The secret to a screen that looks sharp from the side
Picture the blinds on your living room window. Tilt the slats up or down, and depending on your angle, you can see outside clearly or not at all. Early LCD screens worked a lot like those blinds. They stood liquid crystal molecules up and laid them down, front to back or top to bottom, to control light. So if you looked at the screen from the side or from above instead of straight on, the colors washed out or even flipped to dark.
The IPS panel fixes this by changing how the crystals move. The molecules stay flat against the bottom of the screen, and they only rotate sideways, in place, like a record spinning on a turntable. Even the name says so: "In-Plane Switching" means the crystals change direction within the same plane.
Because the molecules always lie parallel to the glass, the shape of the crystals that backlight (the light behind the screen) passes through doesn't change much when you look at an angle. Whether you look down from above or sneak a peek from the side, you get a wide viewing angle with far less change in color and brightness than on TN or VA panels. One catch: in dark scenes, black can look slightly washed out when you view the screen from an angle.
Why tablets and pro monitors love IPS
There are several types of monitor panels. The TN panel, a favorite of early gamers for its very fast response, had such a narrow viewing angle that the screen went nearly black if you tilted your head just a little. The VA panel has great contrast and deep colors straight on, but from the side, the colors look faded and washed out.
An IPS panel, on the other hand, delivers consistent, rich color from almost anywhere in front of the screen. So when several people gather around to watch a video or check photos, everyone sees nearly the same color.
That's exactly why IPS is so common in tablets and laptops, which people often look at while lying down or leaning at odd angles. It's also why it's common in professional monitors for photographers and graphic designers, where color is everything. That said, most smartphones today use OLED screens, which make their own light.
A little more precisely: where this all-rounder falls short
Like any technology, IPS has structural limits. Because the crystals lie flat and parallel to the screen, a tiny bit of light slips through the gaps when the panel tries to block light to show pure black.
Since that leaking light keeps black from being truly inky, the contrast ratio (the difference in brightness between the brightest and darkest parts of the screen) is usually around 1000:1, lower than on VA panels.
Also, if you watch a dark movie in a pitch-black room, you may notice IPS glow, a soft, hazy shine in the corners of the screen when you view it from an angle. Its shape and strength change as you move your head. A glow that stays fixed in the same spot no matter where you look from is a different thing, called "backlight bleeding."
In the early days, the crystals took time to rotate, which could cause ghosting (trailing afterimages) in fast games. But "Fast IPS" technology has since pushed response times down to around 1 millisecond (ms). Now IPS gets the best of both worlds, great picture quality and quick response, and is very popular for high-performance gaming monitors.
Common misconceptions
An IPS panel shows perfect black in any environment.
Because the crystals lie flat, it's hard to block 100% of the backlight, so black looks slightly gray. In a dark room, you may also see IPS glow, a hazy shine in the corners when viewed at an angle. For truly perfect black, OLED is better, since its pixels can switch off completely on their own.
Where you meet it
If you put an IPS tablet down on a table and look at it from a slanted angle while lying down, the text and photo colors barely change.
When a design team crowds around one monitor to review a draft, everyone sees the same undistorted colors.
In one sentence
IPS is a display technology that rotates liquid crystals flat and sideways, parallel to the screen, so colors look almost the same even from an angle.