Monitor tech: LED backlighting

The ins and outs of LED-based monitors.

The HP Dreamcolor LP2480zx uses RGB LED technology to produce dreamy color. Josh P. Miller/CNET

Editors' note, November 24, 2010: The color accuracy and gamut section of this story was updated with more detail and the explanation of how LED lighting affects color was corrected.

As the computer monitor editor here at CNET, I get a lot of questions about different monitor technologies and what effect, if any, they have on the monitor's actual performance. The most frequently asked question of late would be, "What's the difference between LCD and LED?"

It's no surprise this question is so widespread given the fact that monitor vendors are continually inundating us with news of the "latest and greatest" monitor technology with acronyms being bandied about like so many iPads on talk shows .

While prevailing consensus states there is no "wrong" question, the question at hand is actually wrong. Yes, it is. LED and LCD technologies are not mutually exclusive, so to ask what their difference is, is like asking what the difference is between Vibram Five Finger shoes and rubber soles.

So let's break down what LCDs and LED backlights are, as they relate to monitors, and delve into the real advantages of LED backlighting technology.

The term LCD stands for liquid crystal display. LCDs are an upgrade (in most respects) from tube-based CRTs (cathode ray tube) from years of old, although some still prefer CRTs over LCDs. In short, LCDs use liquid crystals to express what you see on screen. The crystals act as a shutter for the backlight, and depending on the type of charge given to them by the monitor's built-in electrodes, the crystals will either allow light through to the user or shut it out. Thereby allowing the pixels to express their appropriate colors, making up what you see on the screen.

LED-based monitors are still LCDs (they still use liquid crystals to express images onscreen), but they use a different type of backlight than what is normally used. Most monitors of the last few years have used cold cathode florescent (CCFL) tubes as their backlight of choice.

Today, more monitor vendors are fully embracing LED backlight technology, with Viewsonic recently announcing that it's shifting its entire monitor line to use LED backlights.

To understand LED backlights, you must understand the three types currently being used in modern computer monitors.

Types of LED backlights
First up are edge-lit backlights using white LEDs (WLED) or EL-WLED. This is the type of LED backlight most commonly used in today's monitors. This entails white LEDs aligned along the edge(s) of the monitor matrix, right behind the liquid crystal array.

Using a special diffuser, the light is spread to cover the entire screen. Monitor vendors have recently refined the technology so that only one edge (not four) need be outfitted with LED lights.

The EL-WLED solution is the cheapest and smallest of the three technologies, which is why it's the most widely used. The advantages of its small design can be seen in displays like the superthin AOC e2243Fw and Samsung PX2370 . You'll also see these used in many notebook and Netbook screens, and recently HDTVs have began adopting the technology as well.

An example of how edge-lit white LED backlighting works. Sharp

Our next technology is RGB LED. Instead of using white LEDs on one edge of the screen like the previous technology, RGB LEDs are aligned all over the panel matrix.

Each individual light is capable of producing red-, green-, or blue-colored light. This gives the display access to a high color gamut with colors more accurate than what's capable on EL-WLEDs.

Unfortunately, you won't see this technology in use everywhere, as it's very expensive and doesn't allow for a thin design. The $3,500 HP DreamColor LP2490zx uses RGB LED backlights, and its panel measures about 2.25 inches, compared with the $300 Samsung PX2370's 0.6-inch panel depth.

The final type is WLED on a flat array, covering the entire screen. Think of it like a RGB LED only using white LEDs instead of tri-color ones. Currently, it's only used in LED-backlit HDTVs and spoken about in depth by CNET's own David Katzmaier.

This brings us to the real question you should be asking: what advantages do LED backlights provide over CCFL backlight solutions? Well obviously this depends on the type of LED backlight; for our purposes we'll discuss the two used in modern LED-based computer monitors, EL-WLEDs and RGB LEDs.

The following are the most frequently touted advantages of LED-based monitors. Let's get to the bottom of them.

LED backlit monitors have high color gamuts and provide more accurate colors
Color gamut is controlled by the monitor's color filters and the backlight's radiation spectrum. While CCFL-based monitors typically offer between 72 and 102 percent of the NTSC color space , RGB LED can cover up to 114 percent. EL-WLED does not offer high color gamuts and typically hit around 68 percent of the NTSC color space.

When it comes to color accuracy, before the backlight can be a factor in the precision of a monitor's colors, the display itself must first be capable of producing a high color bit depth. While the color gamut refers to the range of colors a monitor is capable of producing, bit depth determines how many steps of those colors are within that range. The HP DreamColor LP2480zx for example has a 10-bit panel, which allows it to theoretically produce 1.07 billion different colors. Most monitors are limited to 6-bit panels, allowing only 262,000 colors, but through a few technical tricks can "fake" up to 16.2 million colors.

The use of RGB LEDs for backlighting can provide a color spectrum that closely follows the color filtering in the LCD pixel itself. This method allows the color component to be very precise with the color it's using. Also, each individual RGB LED can be tailored to produce the most vivid colors. EL-WLED backlighting is not capable of influencing color in this way.

LED backlit monitors are always superthin
Only if they're using EL-WLED backlights, as most LED-based monitors do. It's a trade-off between cost and performance. RGB LED provides incredible color performance, but is expensive and bulky. EL-WLEDs don't really influence performance, but can produce monitors less than an inch thick.

LED backlights have lower power consumption
Yes! At least EL-WLED backlights do. Well, for the most part. The biggest factors that affect monitor power consumption are screen size and luminance. Check out our most recent monitor power consumption chart. Most of the LED-based monitors (all using EL-WLED) are rated as "Good."

LED backlights take less of a toll on the environment when it's time to dispose of them
True. CCFL technology makes liberal use of mercury, and thanks to the element's high toxicity level, can be a danger to the environment. LEDs contain no mercury and can be recycled much easier.

For most consumers (buying EL-WLED), LED-based monitors will offer low-power-consuming thin panels that are much easier disposed of than CCFL-based displays. Beyond that, unless you're willing to shell out $3,500, that's all you get. Not to shake a stick at power consumption or the environmental impact your purchases have, but you should know where you stand when browsing through the monitor aisle at Best Buy.

The real question is when will vendors began offering more than just TN-based LED monitors in great numbers. Both BenQ and NEC have made some announcements in this regard this year. Keep an eye on the monitor hub for more coverage on this.

I hope that clears up the big-ticket questions covering LED backlighting. If you have any additional questions or comments, please post them in the comments section and I'll do my best to answer them there or do a followup post.

 

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