LED vs. LCD: Which is better?

By Ty Pendlebury and Philip Wong on 01 July 2009

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You're walking through your local electronics store looking for a new TV, and you come across a thing called an "LED TV". Which then leads you to ask: "Is that the same technology they use for the giant screens at football games?" And the answer, quite simply, is no.

While the giant Mitsubishi screen at Melbourne's MCG, for example, is made up of thousands of LEDs that are used to directly produce a picture, Samsung's "LED" TVs are actually LCDs. That's right, they're not LED TVs at all. Confused?

Samsung's televisions use a series of LEDs to light up the panel, and it's not the only company that does this. But what is backlighting anyway?

Why do LCD screens need a backlight?

As a consumer technology, LCD has been in widespread use since the early 70's where it first appeared in digital watches. As its name suggests, Liquid Crystal Display is a liquid which has been sandwiched between two plates, and it changes when a current is applied to it.

While we've had black-and-white LCDs for years, colour LCDs are a lot more recent, but the technology is the same. As we all know, you need to press a button to read a watch in the dark, and an LCD TV is no different. It needs a light behind it because it emits no light of its own.

A selection of LCD screens, including Samsung's 6 and 7 series, showing the differences between backlighting technologies. The screen on the right features a fluorescent tube backlight. (Credit: CNET Asia)

What types of backlights are there?

At present there are two main methods of backlighting in LCD flat-panels: Cold-Cathode Fluorescent Lamp (CCFL) and LED (light-emitting diode). There are several others, and this includes Sony's Hot Cathode Fluorescent Lamp (HCFL), but only one television currently uses this method.

CCFL is the most widespread method of backlighting for LCD televisions, and consists of a series of tubes laid horizontally down the screen.

A cutaway of a CCFL-backlit LCD showing the different layers of polarisers and filters, and the thin fluorescent tubes themselves (right).(Credit: Ty Pendlebury)

LED backlighting is still relatively rare, but has been in use in televisions since 2004 when it first appeared on a Sony WEGA television. Though there are several different ways of backlighting using LEDs (as we'll explain shortly), the idea is the same: a lot of LED bulbs are used to light the screen.

Direct or edge mounted?

There are two different methods of LED backlighting: direct and edge. The main advantage of direct lighting is that it can be used to increase contrast levels by turning some LEDs off — thus increasing the amount of black in parts of the picture. LG is one of the champions of direct-lighting.

In comparison, edge-lighting's main advantage is that it can be used to make screens that are incredibly thin — the LEDs are at the side and not behind the screen. Of course, you lose the ability to switch off parts of the backlighting for better contrast, and picture quality could also suffer if light isn't sufficiently well dispersed.

White or RGB light?

White LED is very similar to CCFL because it uses a blue light source that is made to look white by the presence of a sulphur coating on the bulb. As a result, the television will potentially be stronger in the green portion of the spectrum, but as some CCFL technologies enable better red and blue response, better white LEDs could also be possible. The Samsung UA40B7100 is an example of a TV that uses White LEDs.

An edge-mounted, white LED module (Credit: Ty Pendlebury)

RGB LEDs, on the other hand, are potentially capable of a broader colour range because they use three LEDs coloured red, blue and green. Its proponents argue there is less of a green "push" as a result and the colour spectrum is more evenly distributed. The Sony Bravia KDL-46XBR45 is an example of a television that uses RGB LEDs in its backlight.

An RGB backlight(Credit: Samsung)

Samsung's technology under the microscope

The Korean company's edge-lit LED-backlit unit comprises of two major components: a long LED module with a row of tiny white diodes and a thin screen-sized plastic sheet known as a light guide plate. Four LED modules are deployed along the left, right, top and bottom of the panel. The combined light output is then funnelled and redistributed evenly across the screen.

Essentially, the edge-lit LED system lacks fine backlight control compared with its predecessor. To put this into perspective, a Samsung series 9 panel can turn on selected LEDs to bring out the sparkle of stars in a galaxy, while switching off the remaining bulbs to produce deep blacks for the background. In the case of the new LED TVs, the lumens are set at screen level, so there's a contrast trade-off when rendering scenes with both bright and dark portions.

Samsung's edge-lit screen requires a light-guide, which is used to distribute light across the screen. (Credit: CNET Asia)

Is the price premium for LED worth paying?

We find it interesting that TV manufacturers are currently asking a higher asking price for LED-backlighting when many cheap devices — particularly mobile phones and netbooks — use LEDs as backlights. According to Samsung, LED backlights currently cost three times more in large sizes than the equivalent CCFL arrangement, and this is mostly due to a lower number of manufacturers. Presumably, when the technology begins to take a firmer hold the price will come down due to increased competition.

The few LED-backlit screens we've seen have been very good, and is definitely better than CCFL backlighting. In the future it will become the default method of backlighting. While we personally prefer plasma, LED's combination of thin design and sharp picture quality will find favour with many people. If you're looking for a further explanation of how LCD screens work, then you can try this video on the 3M site.

Topics: tv, samsung, led, home theatre, hdtv, backlit, led tv, sony, direct, edge

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