In discussing phases of matter, one of my students inquired about plasmas. We briefly discussed the ionized gasses and I told him that plasma TV’s actually contain such gasses. He knew that the temperatures of plasmas is very high and we both wondered if the actual temp. inside a plasma TV is on the order of 1000’s degrees Celsius. He actually wanted to know if he would burn his hand if her were to punch the screen. I told him that even if he did not get burned he would get in a lot of trouble with his dad were he to do so.
Here was the detailed response from another teacher —
While the temperature of the plasma in a neon light, fluorescent lamp, or plasma screen TV may be in the thousands of degrees, the low gas density in these evacuated tubes and screens means that the actual amount of heat energy is very slight. You won’t burn your hand.
Tesla’s invention of neon/gas discharge illumination was an answer to the inefficient hot filament lamp of Edison. The Edison lamp heated a resistance element up until it radiated visible light but most energy went into heat. Tesla figured that if you could excite gas at a low pressure you could achieve radiation without the unwanted heat. The high temperature of the gas is the result of the small electric current going through the gas but the radiation is the ionization resulting as atoms gain then lose electrons. So the actual heat energy content in the rarefied gas is quite low while the temperature extremely high. A cup of hot coffee has more heat energy than a small neon sign having a gas temperature of 2,000ºC. The glass walls of the sign will melt at a temperature less than that but the low density of the gas has not enough heat to warm the glass which conducts the heat away quickly. The gas near the tube walls ceases to glow and is much cooler.
A plasma TV does use phosphors for the color but the discharge is through a gas so the electrons hitting the phosphors will give illumination. You can put your hand on the screen and it will not even be unpleasantly warm. One neat trick is to put a neodymium magnet near a plasma screen and watch the disruption of the electron flow to the phosphors.
Florissant lamps used mercury and argon as the conducting plasma to create UV which makes the phosphors glow on the inside of the tubes.
And another wrote:
I thought Kevin’s question about plasma TVs was interesting, and it just so happens that I’m dating a TV (and all things electronic) repair man, so I forwarded Kevin’s post to him. Here’s his reply:
“Plasmas are created when enough energy is applied to a gas, ionizing it. This energy doesn’t have to be direct heat. A plasma television is made up of over 2 million little boxes called cells. Each cell (pixel) is lined with phosphor, similar to a conventional CRT, and filled with a mixture of gasses (typically neon & xenon). When excited with a high enough voltage (not heated) the gases ionize (electrons jump orbit), emitting ultraviolet light. The UV light strikes the phosphor and causes it to glow its characteristic color (either red, blue, or green). This action is similar to a fluorescent light where the ionized mercury gas emits UV light striking the phosphor lined glass tube. Most of the energy is converted into light, not “heat” (infrared) so to speak. All this is mounted to a massive heat sink which quickly wicks away any heat (conduction).
Television repair man