Advanced backlighting technology using LED-based light sources and segmented control can create a vibrant viewing experience, while also significantly reducing power consumption in LCD TVs by as much as 80%.
Solid-state backlighting using LEDs has a number of distinct advantages in LCD TV applications. Compared to the Cold Cathode Fluorescent Lamps (CCFL) and Hot Cathode Fluorescent Lamps (HCFL) that currently dominate the market in large-area LCD backlighting, LEDs can offer significantly better power efficiency.
This is not inherently a result of their optical efficiency (lumens per watt), which currently only matches that of CCFL, but is due to the fact that LEDs can be dimmed much more flexibly and efficiently to match the required picture brightness. Two-dimensional backlight dimming using an addressable array of LEDs creates a more vibrant viewing experience, with much better contrast ratios, wider color gamut and better color saturation.
Various dimming techniques have been introduced for CCFL and HCFL backlighting over the past few years. For example, in some cases the entire backlight is dimmed, matching the required picture brightness -- a technique called 0-D dimming. When dimming is performed along a single axis (for example, by controlling the intensity of a single HCFL lamp or a group of CCFL lamps in parallel) it's called 1-D dimming.
Recent cost and performance improvements in LEDs, that now make LED backlighting a much more practical proposition, open up the possibility of new and more effective backlight dimming techniques. The fact that LEDs can easily be arranged in a two-dimensional array and individually controlled now makes it possible to perform 2-D (horizontal and vertical) dimming -- something that is not possible with conventional CCFL or HCFL lamps. This allows the backlight to locally produce more light behind bright areas of the displayed picture and less light behind dark areas of the displayed picture.
In practice, an array of 10 x 18 high-efficiency white LEDs is sufficient to locally optimize the backlight intensity for typical picture content, leading to much better contrast ratios and significantly reduced average power consumption in the backlight. This localized control of backlight output based on picture content can save on average about 50 percent of power consumption for typical TV picture content.
From white to RGB
If RGB triplets of colored LEDs are used instead of white LEDs, then 2-D LED backlighting offers even greater advantages. The color gamut achievable by controlling the intensity of the red, green and blue LEDs in an RGB triplet is significantly wider than that of a conventionally backlit LCD panel. As a result, an RGB LED backlight can create brighter, deeper, more saturated colors.
Intelligent saturation control can therefore be applied to map the color space of the video content (sRGB) to the LED backlight's color space. Such mapping algorithms should leave whites, skin tones and soft colors untouched, but can expand saturated colors to vibrancy levels that can only be generated by the LEDs.
Figure 1: 2D color dimming
Arranging the RGB LEDs in a two-dimensional array and individually controlling them on a per color basis (2-D color dimming) reduces power dissipation as well as improving color gamut and contrast ratio. This is because the individual backlight segments only need to generate that part of the visible spectrum that will be transmitted by the LCD pixels in front of them -- see Figure 1.
A conventional white backlight generates a visible spectrum with a fixed color white-point, only to have much of its spectrum energy blocked and dissipated as heat in the LCD panel's color filters. Localized color control of backlight output based on picture content can save on average about 80 percent of power consumption for typical TV picture content.
Next: Backlight dimming complexities
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