LCD Or Plasma - What's Your Pleasure? Understanding Modern Flat-Panel TV Technologies

Major Disadvantages

The size of the pixels themselves is a big problem in plasma displays. It's difficult, if not impossible, to reduce the size of plasma pixels to less than 0.5 or 0.6 mm. Consequently, plasma TVs don't exist in sizes under 32" (82 cm) diagonal. To achieve a resolution that was competitive, plasma had no other choice than to increase the size of the displays, to 32 to 50 inches (82 to 127 cm).

As for image quality, certain problems still dog plasma technology. These problems are essentially due to the nature of the pixels. Since a plasma pixel needs an electrical discharge to emit light, a pixel is either lit or unlit, but has no intermediate state. Therefore manufacturers use a method called PCM (Pulse Code Modulation) to control brightness.

The method is simple. To light a pixel brightly, it's lit very frequently. To get a darker shade, it's lit less often; the user's eye calculates a kind of temporal average. This method is functional, but several problems are associated with it. Most significantly, while it's effective for medium and bright colors, darker colors suffer from the reduced quantification, making it more difficult to distinguish between two dark shades.

While the technology results in a uniform image when the viewer is far enough from the panel, it causes visual discomfort at close distances. It's generally accepted that the human eye isn't capable of distinguishing flickering if the frequency is above about 85 Hz, but that's not exactly true. In fact, the eye is perfectly capable of doing so, but the brain can't "render" the images that fast. Consequently, an image at 85 Hz can cause eye fatigue without the viewer even being aware of the flickering.

That's unfortunately the case with plasma pixels. The flickering can be a cause of discomfort if you're too close to the panel. So, the image on a plasma display is bigger, but you have to be that much farther away from it. Consequently, the immersion experience is no more intense.

Plasma pixels are also subject to burn-in. On a CRT monitor, when the same image is projected for a very long time, it becomes permanently imprinted on the phosphor. After too long an exposure, when the image changes, the preceding one remains visible, as if it were engraved into the monitor. This phenomenon is due to premature aging of the scintillators. When they're used continuously, they age and become less efficient. Since plasma displays use scintillators, they're also subject to burn-in just like CRT monitors.

Under the normal use conditions of a television set, this is not really a problem, since the image being projected changes constantly, so the pixels age uniformly. But for certain business applications this can be an issue. For example, on a screen that displays the same TV channel 24/7, the channel's logo (CNN, NBC, MTV, etc.) will get burned into the display because it stays in the same place permanently. Also, where a plasma screen is used for static advertising displays, a fixed image projected over a long period can become burned into the panel.

This phenomenon is what limits the life of plasma displays. Contrary to legend, plasma screens don't leak and need to be recharged. But the scintillators do age, and unfortunately there isn't much that can be done about it. To make matters worse, not all scintillators age in the same way; the blue channel always ages quicker than the others (though that situation has improved compared to the earliest plasma panels.)

Finally there's the economic factor: plasma displays are expensive. Not only are the panels themselves difficult to manufacture, but the panel's control electronics require specific high-performance semiconductors. That's because the control lines for the electrodes have to carry several hundred volts at high frequencies. The power consumption of plasma displays, which is always greater than that of LCD displays, is one of the direct consequences of these very high voltages. As an example, a 42" (107 cm) plasma display will consume 250W, while an LCD display with the same diagonal measurement will consume only 150W.