If you can recall your high school physics or chemistry lessons, you may remember learning about the three phases of matter: solid, liquid, and gas. But there's actually a fourth fundamental phase of matter, plasma—a word you might recognize from your TV. Though it might sound like a silly marketing term, plasma TVs actually utilize this obscure fourth phase of matter to pry out television display panels with incredible contrast and color. Panels that often out-perform their LCD or LED counterparts.
It's a Matter of Phase... or a Phase of Matter
The first thing to realize about plasma is that plasma is not really a "thing." Rather, things can be a plasma in the same way that things can be liquid or solid. Think back to the classroom and gases, liquids, and solids. Each of those phases could be easily demonstrated using water: it can exist as a solid in the form of ice, as a liquid, and as a gas in the form of steam. Simple, straightforward, yet incomplete. Unlike the previous three, the fourth fundamental phase of matter isn't really intuitive or easily demonstrated, which is probably why most people don't learn about it until well into their secondary education, if at all. But while plasma is by far the least common state of matter on earth, it's the most common state in the universe in terms of volume.
Plasma at the Molecular Level
In order to fully understand the four states of matter, we have to look closely at the behavior of the molecules in the substance at each phase. In solids, the molecules are relatively rigidly ordered and are huddled quite close together; they don't move around at all. Liquids feature molecules that are still packed closely together but, unlike in solids, they are free to move around a bit. In gases, all bets are off; the molecules are dispersed and freely move around.
In the transition from gas to plasma, the molecules of the substance actually undergo a change by being stripped of electrons. Now the space is taken up by molecule fragments: the ionized molecules and free electrons. This mixture of ionized molecules and electrons is typically extremely hot (the "cold" plasma present in televisions is still in the sub-1200°F range) and therefore extremely dangerous in large amounts.
Turning Plasma into Pictures
Luckily, the plasma in your television is extremely well contained in millions of little compartments or cells. Each of those cells is filled with noble gases (like neon) that can be turned into plasma by applying current through the cell. Now plasma by itself doesn't actually emit light, but it's capable of electronically exciting other compounds that do. Displays exploit this ability by adding trace amounts of mercury into each cell, allowing the plasma to promote mercury electrons from their rest state to an excited state.
As these excited electrons relax into the original (or ground) state, they need to shed their excess energy by releasing high energy photons, or ultraviolet light. This light emission by itself is not very useful for use in display devices (unless you are a pistol shrimp with special eyes) so it must be further down-converted. Using a similar trick as in LEDs, the invisible UV light is directed at a phosphorescent compound coating on the viewer-facing side of the cell, yielding the vivid and bright image that you are used to. While this process might seem slow and complicated when described, it takes nanoseconds from beginning to end, allowing for the high level of responsiveness plasma televisions are famous for. Since each cell can essentially be completely shut off, plasma displays can produce superior black levels that most LCD and LED TVs just can't compete with.