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Friday, May 24, 2024

LED vs. LCD: which is better?

You’re walking through your local electronics store looking for a new TV, and you come across a thing called an “LED TV”.

This isn’t the same technology they use for the giant screens at football games; in fact, the LED screens you see in shops are actually LCDs, and the term “LED” is the invention of Samsung’s marketing department.

How do they get away with this? Samsung’s televisions use a series of Light Emitting Diodes (LEDs) — like the ones used in LED torches and alarm clocks — to “backlight” the LCD 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 ’70s where it first appeared in digital watches. As its name suggests, Liquid Crystal Display is a liquid that 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.

It’s helpful to think of an LCD panel as a sandwich, consisting of different layers. On a typical TV you have a polarised filter, followed by a protective glass layer, followed by the LCD sheet, and then a light source at the back.

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 backlighting consists of a series of tubes laid horizontally behind the screen. It used to be the most common method of backlighting for LCD televisions, but it is quickly being superseded by LED.

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/CNET Australia)

LED backlighting has been in use in televisions since 2004 when it first appeared on Sony WEGA models. Though there are several different ways of backlighting using LEDs (as we’ll explain shortly), the idea is the same: a series of LED bulbs throw light from behind to illuminate the LCD panel.

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?

When using LED backlighting, there are several different coloured ones you can use, but the two main options are white and RGB.

White LED is very similar to CCFL, and is meant to simulate the white light of the sun for a more “natural” result. But the LEDs aren’t actually white; this approach uses a blue light source that is made to look white by the presence of a sulphur coating on the bulb. CCFLs work in the same way.

As a result, the television could potentially be stronger in the green portion of the spectrum, but some CCFL technologies enable better red and blue response, so 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/CNET Australia)

RGB LEDs, on the other hand, are potentially capable of a broader colour range because they use three LEDs coloured red, blue and green, which is a broadcast standard. RGB’s proponents argue that 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 used RGB LEDs in its backlight.

An RGB backlight
(Credit: Samsung)

An LED backlight under the microscope

Here we have Samsung’s edge-lit LED unit, which comprises of two major components: a long LED module of tiny white diodes and a thin screen-sized plastic sheet known as a light guide plate. Four of these LED modules are deployed along the left, right, top and bottom of the television. The combined light output is then funnelled and redistributed evenly across the screen by the light guide.

While “dynamic” edge-lit LED systems exist, they still lack the fine backlight control that direct lighting allows. 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 still asking for a higher price for LED-backlighting when many cheap devices — particularly mobile phones and netbooks — use LEDs as backlights. As of 2009, Samsung said that LED backlights cost three times more in large sizes than the equivalent CCFL arrangement, and this is mostly due to a lower number of manufacturers. Presumably, as the technology continues to take a firmer hold, the price will keep coming down.

In 2011, only the budget LCD televisions use CCFL backlighting, and all of the major manufacturers use LED lighting in their mid-range and premium models. It won’t be too long before it will become the default method of backlighting. While some people still prefer the look of a plasma, the LED’s combination of thin design and sharp picture quality will soon 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.

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