FEATURE
The Mighty Midget
Can light-emitting diodes (LEDs) take color rendering in LCD monitors to the next level?
At the 2006 Hollywood Post Alliance technology retreat, I had the privilege of co-chairing a half-day session on trends in display technologies. Our expert presenters covered the gamut of display issues from color rendering to de-interlacing, cadence detection, motion processing, and pixel scaling.
One question that came up a few times pertained to the future of the cathode-ray tube (CRT), and what would eventually replace it. There are plenty of microdisplay “contenders” out there, and two in particular (plasma and LCD) are strong contenders for the title.
Both offer high pixel resolution, a myriad of screen sizes (although LCD has the edge in smaller screens), thin form factors, and bright images. Plasma holds an advantage over LCD in its viewing angles, black levels and color saturation, but is impractical to manufacture in sizes smaller than 37 inches.
Thus the appeal of LCD displays, which can be manufactured in almost any size imaginable. (LG Philips LCD recently showed a 100-inch 1080p LCD panel!) For now, it looks like LCD will become the display of choice for equipment racks, location shoots, and workstations.
All fine and dandy, but there’s still that issue of color to work out. Conventional LCD monitors use cold-cathode fluorescent lamps (CCFLs) and individual red, green, and blue color filters, an approach that works well for the casual viewer but is not particularly accurate when reproducing a specific gamut of colors, such as REC709 or SMPTE-C.
The limitations of CCFL backlights are well known. They emit plenty of blue-green spectral energy, but are usually deficient in red, orange, and yellows. To correct the problem, manufacturers of LCD panels are turning their attention to LEDs in a variety of backlight configurations, including:
- White LEDs with red, green, and blue filters
- Individual red, green, and blue LEDs with no filters
- White LEDs with discrete, tunable red, green, and blue elements
Because LED color is precise over specific wavelengths of light, it becomes a lot easier to achieve and even exceed electronic display color gamuts by using them as an illuminant, and not CCFLs. (The same holds true for LCD and DLP projectors, which currently use short-arc lamps containing salts of mercury.)
Figure 1. LG showed this 47-inch 1920x1080p LCD monitor with LED backlights at CES 2006.
Demonstrations of LED-backlit LCD monitors at CES 2006 claimed to exceed the SMPTE-C color gamut by 5%. Nearby exhibits of DLP and LCD rear-projection TVs, also equipped with LED light engines, made the same claims. Color saturation in all of these demos was quite impressive; in particular, reds, yellows, and greens seemed to pop off the screen.
LEDs are no Johnny-come-lately — they’ve been in use for years. Walk through Times Square or down Las Vegas Boulevard, and you’ll spot plenty of LED signs. Scoreboards in stadiums and arenas are jam-packed with LEDs in all sizes and pitches. They’re bright and last a l-o-n-g time (50,000+ hours is the going industry claim for lifespan).
There’s just one catch: LEDs are power-hungry. Not for voltage — they can operate with less than five volts. No; LEDs are gluttons for current, a problem that CCFLs don’t have. Potentially, you could be trading improved color for increased power consumption in your next LCD monitor, something that goes against the trend towards “green”, environmentally friendly displays.
The solution is to pulse the LED elements on and off rapidly, something that LEDs do quite well. The on-off pulse-width modulated cycle is fast enough so you won’t notice any flicker, but it will cut the electric bill substantially. PWM switching also presents further options for precise color shading and white balance in a professional monitor, whether white or RGB LEDs are used.
The move to LEDs is also motivated by another environmental concern: Eliminating potentially toxic metals such as mercury and lead from everyday consumer products. In fact, the European Union has enacted restrictions this year against the importation of products containing lead and mercury unless there is no viable alternative.
(That last bit of language was no doubt added to placate Philips Electronics, one of the world’s largest manufacturers of (you guessed it) projection lamps and CCFLs through its Sylvania and Osram divisions.)
Like it or not, CRTs are going away. For now, it looks like LCD technology will be the technology of choice to for editing, engineering, and production monitors, and that LEDs will eventually provide the photons to do the job. Throw in additional improvements to LCD viewing angles and black levels, and you could be pleasantly surprised in a few years…
This article appears in an upcoming issue of Studio / Monthly.
