Geek 101: LCD and Plasma Basics

Refresh Rate

A display's refresh rate describes how quickly it can show a new video frame. The rate is usually expressed in hertz (Hz), which effectively means how frequently the screen refreshes every second (for instance, a 60Hz screen refreshes itself 60 times per second). LCDs generally come with 60Hz, 120Hz, or 240Hz refresh rates. The refresh rate is an important indicator of how well a display can handle fast motion--an important thing to consider if you watch sports or action movies. In general, the higher the refresh rate, the smoother the motion. This is more of a consideration for HDTVs, but some 120Hz computer monitors are on the market too.

The refresh rate's impact on display performance is a controversial issue in home theater circles. In PCWorld Labs tests, we found that, in general, 120Hz is currently the sweet spot for HDTVs; you'll get smoother motion than you would with a 60Hz set, and the prices for 120Hz HDTVs are coming down. The jump between 120Hz and 240Hz wasn't as drastic in our testing, so getting a 240Hz HDTV may not be worth the extra cost.

Keep in mind that a higher refresh rate will not necessarily guarantee improved motion performance. We saw some 120Hz HDTVs fall flat in our motion tests, so other factors--such as the electronics behind the screen--may be in play. See Melissa J. Perenson's story "HDTV Motion: The 120Hz Difference" for more on our testing.

Your best bet, of course, is to look at HDTVs in person before you buy one.

Contrast Ratios Are Meaningless

Many manufacturers make a big deal about contrast ratio, which is supposed to specify how wide a range of light shades and dark shades an HDTV or monitor can display. Overall, however, this is a meaningless measure of a screen's quality. Currently there's no standard way of measuring it, which explains to some extent why one company might list its products as having contrast ratios in the 20,000:1 range, while another may brag about ratios of 1,000,000:1.

Until the electronics industry settles on one way of measuring and expressing contrast ratio, you should probably ignore this metric altogether. Instead, judge a screen using your own eyes. You'll want to look for screens that have dark, inky blacks, and avoid ones whose blacks more resemble gray.

3D

3D displays generally work by showing two video streams at one time: one for the left eye, and one for the right. The frames from these video signals are interleaved, so you need to wear glasses to filter out the two signals (that is, the shutter in the left lens will filter out the image meant for the right eye, and vice versa).

Samsung's 3D glasses
Samsung's 3D glasses.

Both the display itself and the underlying electronics play a role in 3D technology. A 3D HDTV or monitor needs a fast refresh rate (120Hz or faster), but it doesn't matter whether the screen is LCD or plasma. In the case of polarized 3D displays, a film placed over the screen facilitates the 3D effect (you'll still need glasses to filter out the two video signals, though). And since a 3D TV requires two video signals for the 3D effect, it needs two HD tuners.

An upcoming 3D technology is "autostereoscopic" 3D--in other words, displays that don't require any glasses. Autostereoscopic 3D is coming in the not-too-distant future for handheld gadgets such as smartphones and the Nintendo 3DS, but it will be a while before you can get an autostereoscopic 3D TV.

For more on 3D HDTV, see our earlier Geek 101 story on the topic.

What's Next: OLED

Sony 11-inch XEL-1 OLED TV
Sony's 11-inch XEL-1 OLED TV.

Organic light-emitting diode (OLED) displays are a relatively new technology; they emerged first on mobile devices and are now making their way into PC monitors and televisions. Unlike LCD, OLED requires no backlight--the individual pixels light themselves. This allows OLED displays to be very thin.

At the moment, OLED remains impractical on larger devices, mainly due to manufacturing costs. For example, LG's upcoming 15-inch OLED TV is expected to cost 2000 euros, or roughly $2500 at current exchange rates. This makes OLED TV sets more of a proof-of-concept for the time being.

Life span has been a concern for OLEDs. Paul Semenza, an analyst with market research firm DisplaySearch, tells us that two problems exist: the degradation of the screen as a whole, and the life span of the individual color pixels. If one color pixel has a shorter life span than another, it'll result in inaccurate color as the screen ages. According to prior reports, the blue pixels on OLED displays may die out relatively quickly, though engineers have been hard at work to correct the issue. Semenza says that OLED life span has improved over the past few years, but that a lot depends on the manufacturing process.

For now, an OLED screen should work fine for a cell phone that you'll own for a couple of years and then replace, but it's hard to say how an OLED screen will hold up to ten years of use, as many monitors and TVs are expected to do.

Also, Semenza says that making bigger OLED screens in large amounts will require new factories and new manufacturing methods, so don't hold your breath waiting for a 40-inch OLED HDTV.

Despite those issues, OLED is still a technology worth following and looking forward to, even outside the living room. Last year, Sony demonstrated a flexible OLED display, which could one day be used in all sorts of gadgets.

For more technical details on flat-panel display technologies, see PCTechGuide's overview. And for more on the future of 3D TV, see David Ayala's 3D HDTV overview.

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