Facts About Plasma TVs

by Kevin Ray

n last month’s Bluffton Technical we explored what makes LCD TVs tick. If you read that article, you discovered that the technology involved in making one work is quite amazing. While plasma technology is easier to explain; it is no less amazing. In this article we will advance the understanding of how plasmas work, and myths that have been exaggerated regarding plasma technology.

What is Plasma and How Does It Work?

Big, flat plasma displays are made of a grid of tiny gas-filled chambers. They’re called “plasma” displays because in making the light that forms pictures, the gas in these tiny chambers is electrified and changed to a “plasma state.” In fact, the earliest plasma displays were referred to as “gas plasma displays.”

So what exactly is plasma? Most of us learned that there are three states of matter: solids, liquids, and gases. Scientists also study a fourth state of matter known as the plasma state. Lightning which is an electrically charged gas is a naturally occurring example of plasma. Neon signs, fluorescent lights, arc welders, and even some air purifiers all generate plasma to work their magic.

How does plasma work? We’ve all seen how neon lights’ pure, vivid colors attract our gaze. Their colors vary with the kind of phosphor that coats the inside of the glass tube. Imagine shrinking millions of red, green, and blue neon lights to the size of less than a millimeter each, and then arranging them in a grid the size of a large TV screen. Next, imagine simultaneously controlling the individual brightness of each of those lights relative to all the others precisely as directed by a video signal. If you could do this, you would be able to show on a flat, thin screen all of the color tones, brightness levels, and image details we’ve come to enjoy on smaller, thicker CRT (tube-type) screens.

Plasma Life Span

Phosphor-based displays include plasma, conventional Cathode-Ray Tube (CRT), and rear projection CRTs which appear quite bright when they are brand new. Over time their brightness decreases slightly, and at some point, they will be half as bright as when first used. The technical term for this phosphor-brightness trend is “Life to Half Brightness” or LTHB. This term is more accurate than the terms “lifespan” or “half-life” since the phosphors’ brightness potential never dims completely. The LTHB of a 2005 Fujitsu Plasmavision display is about 60,000 hours. (That’s 27 years of typical family TV use). Compare that to a typical TV in which the LTHB may be as short as 10,000-20,000 hours.

Contrast Ratio

Many factors help determine a customer’s satisfaction with a plasma display, including: picture size, peak brightness, color accuracy, signal processing, motion handling, resolution, pixel aperture ratio (fill factor), and contrast ratio. The way you perceive contrast ratio is greatly affected by the light in the room. In a totally dark room, you’ll be able to see the benefit of a higher contrast ratio. In a brighter room, a much higher contrast ratio may go unnoticed, dwarfed in importance by other factors such as peak brightness, black levels, and so on.

Contrast ratio (CR) is measured by gauging the energy coming off different parts of black and white test patterns shown on the set. If the white area is 1,000 candelas, and the black level area is 1 candela, the screen has a contrast ratio of 1,000:1. Engineers test CR in pitch-black testing rooms. Otherwise, ambient light will lower the CR measurement. In typical viewing environments such as living rooms, dens, and offices, stray light drops the actual contrast ratio from its “rated” 3,000:1 spec down to a satisfying, but numerically modest 700:1 ratio. Meanwhile, human eyes have their own limitations. Our irises dilate and constrict in response to room light changes. More often than not, we will detect contrast ratios no greater than 100:1.

Burn-in

Burn-in can be described as a permanent “ghost” of an image that has been left static or stationary on any phosphor-based display. Burn-in can happen with plasma TVs, but it is highly unlikely to occur when used as a normal TV in normal viewing applications. Techs who work on flat-panel LCD screens say these displays can also burn-in if abused.

Station logos, subtitles, and video game scores also drive burn-in worries. Plasmavision display features such as video orbiter (which moves the entire image an inch or two every hour) and an automatic white screen setting (a screen refresh feature exclusive to Plasmavision TVs which bathes all pixels equally in white for a minute or so when you turn off the source signal) eliminate these worries for home theater viewers. So, don’t worry! Large plasma TVs are excellent performers as well as a great value.

This article provided by Kevin Ray of Custom Audio Video, LLC. Member CEDIA (Custom Electronic Design & Installation Association)