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The Different Types of
LED Backlighting
In modern LCD desktop displays, there are several
different types of LED backlight units (BLU's) used.
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RGB LED Backlighting - This type of
backlighting is based on RGB triads, each including one red, one green and one
blue LED. RGB LED backlighting ensures an excellent colour gamut and very pure
colours, but is only really used in professional-grade displays such as the
Samsung XL20, XL24, XL30 screens, and modern professional models like the HP
DreamColor LP2480zx. This type of backlighting is only used in this sector due
to its high cost and it is not economical to produce at the moment.
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An edge backlight with white LEDs (W-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 like the direct lit method does (see below). It can not offer an
extended color gamut either. Instead, it is economical and compact, which
makes it popular among notebook makers and with manufacturers producing
ultra-thin displays and keen to keep costs to a minimum. This is the variation
commonly being used in desktop displays at the moment.
Modern displays and LCD TV's have progressed from having 4 sides lit by LED
backlights to only 1 side. This helps reduce cost and improve efficiency.
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A flat backlight based on white LEDs (W-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 and is also referred to as
'direct lit' W-LED.
A note about white LEDs - A white LED is
actually a blue LED with yellow phosphor to give the impression of white light.
The spectral curve has big gaps in the green and red parts.
Features - Advantages
and Disadvantages
If marketing is to be believed, LED backlighting
offers you all kinds of advantages, but it's important to understand what is
true, and what is not. We will discuss different aspects and whether they are
influenced by a different backlight source:
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Colour Gamut - this is controlled in
monitors by the properties of the colour filters of the LCD matrix, and by the
backlight's radiation spectrum. You will see
CCFL backlighting offering colour gamuts covering between 72% (commonly
referred to as "standard gamut" / sRGB) and 102% of the NTSC reference colour
space. The CCFL backlighting above 72% are commonly referred to as wide gamut,
or W-CCFL / WCG-CCFL. In LED backlighting, the RGB LED format can offer really
large colour gamuts with pure and saturated colours. These can cover typically
>114% of the NTSC colour space, and is one of the reasons they are often
employed in high end professional screens. W-LED backlighting cannot offer
these extended gamuts, and on paper actually cover slightly less of the NTSC
colour space than standard gamut CCFL (typically 68%). The difference is
hardly detectable by the naked eye however.
 
Above: colour
gamut of a typical standard gamut screen (72% NTSC) on the left,
and an RGB LED backlit display covering 114% of the NTSC colour space on the
right
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Colour Accuracy - aspects of colour
reproduction do not depend on the type of backlight at all. They are
determined by how accurately the monitor is set up in the factory, the
characteristics of the panel technology and the internal electronics to a
degree.
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Static contrast ratio - LED
backlighting models are advertised with massive contrast ratios, ranging
commonly into the millions now! Figures of up to 20 million:1 are common at
the time of writing. Be aware though that these are normally headline
dynamic contrast ratio figures, with the normal static contrast ratio
rarely even mentioned. It is important to understand that static contrast
ratio is only determined by the characteristics of the LCD matrix itself and
not by the backlighting type or nature. It is determined by the ratio of the
transparency levels of open and closed pixels.
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Dynamic contrast ratio - As opposed to
gas-discharge lamps (CCFL), LEDs can be lit up instantly or turned out
completely. This can lead to extremely high levels of dynamic contrast as we
have mentioned above. Figures in the millions are very common now. 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 real
practical point in increasing the dynamic contrast higher than about 10,000:1
which has already become standard for many monitors, including those with a
backlight based on CCFL lamps. Keep in mind that DCR figures are often
exagerated as a result, and since you will probably never get to utlise the
full figure in practice, don't be fooled into buying into the hype too much!
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Uniformity - Most desktop monitors use
edge-lit W-LED backlighting with a line of LEDs along the edge of the panel.
The whole screen is lit by means of a special diffuser, and so it is this
which really determines the brightness uniformity you experience. 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. Having an LED
backlight does not guarantee you better uniformity. In fact, good uniformity
is harder to achieve in the long term as the LEDs age, with each LED possibly
aging at a different rate. With RGB LED units, the use of three separate light
sources for red, green, and blue means that the white point / colour
temperature of the display can move as the LEDs age at different rates as
well.
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Instantly On - some manufacturers mention
that an LED can be instantly turned on, meaning there is no warm up time like
there is with a CCFL backlight. This is true, but it's debatable how important
this really is to an end user.
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Size - LED backlighting units can be very
thin, allowing manufacturers to produce ultra-thin displays with sleek and
attractive designs. You also see this technology used in laptops and LCD TV's
for the same purposes. This technology has allowed production of thinner
screens which are in high demand by consumers. Manufacturers are actively
working on reducing the size of the LEDs to be used in these modules to
improve things even more. Screens using a flat W-LED or RGB backlight behind
the panel cannot offer the same thin profile however.
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Environmental - LEDs do not contain
mercury, unlike CCFL's and so can be recycled more easily. You will see
mention of various certificates and compliance standards as well, such as
'RoHS compliance'. These can show the displays have met recylcing standards.
Certainly a benefit of LED backlighting for the environmentally concious.
 
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Power Consumption - This is perhaps one
of the key advantages of LED backlighting in modern times. The technology uses
less energy and so you can save money and energy and reduce your carbon
footprint at the same time. For example, the non-LED version of the 24" BenQ
G2420HBD consumer display has a 49W consumption compared to the 24W of the LED
version of the same display (G2420HBDL). The BenQ LED monitors are typically
marketed as offering a reduced power consumption of 36% in comparison to
traditional monitors. Other manufacturers quote similar figures, with 35 - 40%
energy saving being common. You will also see various ratings and
'certificates' applied to these screens such as Energy Star and the likes.
LED Backlighting and Panel Technologies
At the professional end of the market where RGB
LED backlighting is used, it is combined with high end
panel technologies such as AMVA (from AU Optronics) or IPS (from
LG.Display). These panel technologies are more expensive to produce than the
widely used TN Film panels in the mainstream market. When you are using an
expensive backlighting unit, it obviously should be paired with a higher-end
panel though. In fact, modern RGB LED displays such as the HP DreamColor
LP2480zx even use a one-of-a-kind true 10-bit H-IPS panel (not 8-bit
+AFRC like some of the other modern "10-bit" screens). RGB LED models are few
and far between though of course.
W-LED backlit models are becoming more and more
mainstrea. Initially the technology was combined exclusively with TN Film
panels, since low production costs (and low retail costs) were the name of the
game. There are many TN Film based models out there with LED backlighting now.
More recently, in the later half of 2010, we have seen models emerge combining
W-LED backlighting with VA and IPS matrices. AU Optronics have released
modules in several sizes which combine their AMVA panel technology with LED,
and LG.Display have begun to release a combination of IPS and LED. The
BenQ EW2420 and VW2420H were two of the first VA based screens on the market
with LED. The
NEC EA232WMi and forthcoming
models from LG will be some of the first to use IPS + LED. We expect this
trend will continue.
Further Reading
Wikipedia -
Backlighting
X-bit Labs -
LED Backlighting
AU Optronics - LED Technology
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