The compact fluorescent tubes (CCFL) commonly used to backlight LCD panels have attracted the attention of the Restriction of Hazardous Substances (RoHS) Directive because they contain mercury.
Right now they have an exemption from the directive, but its looming presence has pushed LCD makers to look elsewhere – to LED backlights.
Being mercury-free is not the only advantages LEDs have.
CCFL tubes operate at high frequencies and voltages that require an inverter; which is expensive, prone to generate EMI, and wastes power.
The glass tubes themselves do not handle shock and vibration particularly well, and if the tubes are left vertical for any length of time, they become discoloured at the bottom as the heavy mercury sinks.
To ensure that a partial failure does not completely blank a screen, multiple tubes are generally employed, adding expense and thickness.
Designer’s nightmare
When you consider all of these points, CCFL backlights are a designer’s nightmare, particularly when applied to a mobile application such as a tablet PC, where power consumption, ruggedness and reliability are paramount, and where the CCFL inverter could interfere with built-in radio modules.
However, LEDs are not perfect. It has proved impractical to mount a large array of LEDs behind an LCD panel because of power consumption, heat dissipation and overall cost.
The alternative is the edge-lit backlight, with rows of LEDs along one, two or four edges, which requires careful design to achieve even illumination. Done well, this results in a thin, lightweight panel with lower power consumption than the CCFL alternative.
LED backlights
LED backlights are fundamentally more rugged and stable than CCFLs, and they can be mounted at any angle without long-term degradation – LED-lit landscape displays can be mounted as portrait without the yellowing likely at the bottom of vertical CCFL tubes. And white LEDs can offer a colour gamut which theoretically exceeds 100% of NTSC.
Dimming from 0-100% intensity is not an issue with LEDs, whereas if CCFL running voltage is reduced too far, particularly at low temperature, the tube goes out. It is almost impossible to dim CCFL tubes down to 10% of their optimum brightness – a real problem on a ship’s bridge, where operators have to maintain night vision.
Developments in the past few years mean power LEDs now last longer than CCFLs, and the cost of white, red, green and blue emitters has dropped dramatically.
Larger TFT panels, in particular those destined for domestic TV applications and digital signage, are starting to benefit from LED technology.
With RGB illumination, it becomes possible to sample the video signal and control the colour and intensity of the backlight, or even parts of the backlight. This technology produces not only startlingly good colour gamut, but extremely high contrast.
LED backlights are now common on panel sizes up to 10.4in, and are expected to become standard up to 19in over the next year or two.
Development is advanced on sizes up to 108in, and these panels are likely to have LED backlighting systems as the norm well before the 2012 deadline imposed by RoHS for the abolition of the CCFL.
Martin Cobb is displays product manager at Pacer International