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LED Backlight

Monitors with LED backlight, often referred to as LED monitors (which is not quite correct because true LED monitors are those huge ad billboards in which the image is indeed formed by LEDs), have been quickly gaining popularity. They were expensive, premium-class products at first, but now, for example, BenQ offers a 19-incher with LED backlight for less than $200.

But if we are going to talk about the real, not marketing, advantages, we want to know what type of LED backlight we mean. There are three of them available today:

  • A LED backlight based on RGB triads, each triad including one red, one green and one blue LED. This type of backlight ensures an excellent color gamut and very pure colors but is only used in professional monitors (NEC’s SpectraView Reference 21, Samsung’s XL20, XL24, and XL30) because it is expensive and not economical. The only attempt to develop a rather inexpensive monitor with RGB triads that I know of is ViewSonic’s VLED221wm. And it is a downright failure because the manufacturer had to save on everything else, and particularly had to install a matrix with poor viewing angles.
  • A flat backlight based on white LEDs. As there is only one third of the total amount of LEDs here, this backlight is much cheaper than the triad-based backlight, but it cannot deliver an extended color gamut. In the backlight of this type LEDs are uniformly distributed in the plane parallel to the matrix, which allows setting the backlight intensity differently in different parts of the screen when necessary. This is the further development of the dynamic contrast technology. It is currently employed in LCD TV-sets only.
  • An edge backlight with white LEDs. The LEDs are placed in a line along the edge of the matrix, and the uniform brightness of the screen is ensured by a special design of the diffuser. This backlight does not offer the option of zonal control over brightness. It does not ensure an extended color gamut, either. Instead, it is economical and compact, which makes it popular among notebook makers. It is also often installed in monitors. By the way, as opposed to cold-cathode lamps, light-emitting diodes do not contain mercury and thus can be recycled easier.

So, ordinary home monitors usually come with an edge backlight based on white LEDs. Other types of LED-based backlights may become popular in the future, but that’s what we have now.


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The manufacturers often promise that a LED backlight affects nearly all of a monitor’s characteristics including contrast ratio, color accuracy, brightness uniformity. Is it really so?

Contrast ratio is only determined by the characteristics of the LCD matrix, namely by the ratio of the transparency levels of open and closed pixels. It depends neither on the backlight nor on the type of that backlight.

Dynamic contrast. As opposed to gas-discharge lamps, LEDs can be lit up instantly or turned out completely, which leads to extremely high levels of dynamic contrast (to remind you, it is calculated as the ratio of white on an all-white screen to black on an all-black screen). But in real applications, for example when watching a movie, there are no absolutely black frames even in the credits. Most of the time there is something on the screen besides blackness and a monitor with a huge specified dynamic contrast will never have the chance to deliver it in practice. As a result, there is no practical point in increasing the dynamic contrast higher than 10,000:1 which has already become standard for any monitors, including those with a backlight based on fluorescent lamps.

Uniformity of brightness. As I’ve said above, home monitors utilize an edge backlight, i.e. a line of LEDs along the edge of the matrix. The whole screen is lit by means of a special diffuser. This is the same design as is used in notebook displays and in inexpensive models of monitors with lamp-based backlight: the lamp is also located near the edge of the screen (more expensive models have a few lamps, located behind the matrix). The uniformity of brightness depends only on the design of the diffuser and you can often see various defects like bright spots or a brighter zone at the edge of the screen where the lamp or the line of LEDs resides. Therefore, LED-based monitors can be both better and worse than lamp-based ones. It depends on the specific models.

Color gamut is due to the properties of the color filters of the LCD matrix and the backlight’s radiation spectrum. The 3-color backlight (red-green-blue) can ensure a really large color gamut with pure and saturated colors (you can refer to this review for details), but home monitors currently use white LEDs with a continuous radiation spectrum of a complex shape. As a result, the color gamut of such monitors differs but little from that of monitors with lamp-based backlight. I don’t say they are identical, but the difference can hardly be spotted with a naked eye.

Color accuracy. Obviously enough, the other aspects of color reproduction do not depend on the type of backlight at all. They are determined by how accurately the monitor is set up, by the monitor’s electronics, the characteristics of the LCD panel, etc.

So, although it would be wrong to claim that LED backlighting has no advantages, those advantages do not affect the image quality. The only indubitable high of LED-based monitors is their lower power consumption, but their image quality won’t be any better until the manufacturers switch from the white edge backlight to other technologies. And I have doubts that this will occur in near future because edge backlighting is cheap while the more advanced alternatives are far more expensive.

Summing it up, you should only deliberately look for a LED-based monitor if you care about power consumption. Otherwise, you should base your choice on other parameters, ignoring the type of backlight altogether.

 
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