The technology exists to recalibrate displays using either remote imaging systems which look at each pixel individually, or complex mathematical formulae. However, Visioneered Image Systems, which has a corporate office and production facility in Garden Grove, California, has developed a system that allows internal calibration of the display to maintain its original brightness.
Display manufacturers will measure and characterize their LED modules, and make adjustments so that the whole display is uniform and tuned to a specific white point. However, this typically takes place before the LEDs have gone through an optimum burn-in period of around 500 hours, says Boldt. "As soon as you turn on any display, you get degradation," he says. "In our displays, we make an initial characterization, then when degradation occurs we can increase the pulse-width modulation cycle to bring the display back to the point where we characterized it."
The internal calibration is achieved by means of an optical feedback loop. Inside the Visioneered display, every single LED has its own detector, a silicon IC that sits inside the reflector cup about 400 microns from the LED chip. "The signal-to-noise ratio of the system is about 2000:1, and the accuracy of the measurement circuit is about 0.1%," says Boldt. "The human eye can only perceive difference in brightness of around +/-0.5%."
Provided that the measurement is repeatable, the absolute value of the measurement is not important, since the system only has to return each LED to the originally measured brightness level.
Optics provide headroom
In addition to the measurement feedback loop for each LED, the second important aspect of the Visioneered approach is to use diffractive optics to keep the light within the optimum viewing angle for each particular application. Crucially, this provides the overhead that allows constant brightness to be maintained.
Typical super-oval LED lamps used in LED displays have a viewing angle of 50 degrees vertical by 110 degrees horizontal. For a billboard positioned 10 m above the ground, at distances beyond 140m, the total angle from the center of the display to the viewer is about 5 degrees.
"Standard LED lamps waste a huge amount of light," says Boldt. "We position an optimized chip, reflector and sub-lens inside each LED module. Well over 50% of output from the die is confined to a beam that is only about 8-10 degrees wide. That provides the input power to our diffractive optic." The optical system can be tuned to produce an output ranging from 15 degrees circular to 20 degrees vertical by 100 degrees horizontal (see figure).
The diffractive optics are produced in sheet form on a web process, and the cost is over an order of magnitude less than the hundreds of dollars per square foot a few years ago. "To select different viewing angles for different applications, all we have to do is vary the sheet optics," says Boldt. "The optics can even provide a 5 degree downward tilt. In contrast, some manufacturers physically tilt their LEDs downwards."
Concentrating the light output also results in extremely bright displays. "Our displays can produce about 15,000 nit," says Boldt. "Under bright light conditions, we can either put out more than enough light to give a broad color gamut, or we can operate at 6000 nit and run the LEDs at half to one-third of their normal input power. This results in reduced degradation and extended lifetime."
Visioneered has demonstrated its technology at several conferences, and recently signed off on a substantial funding round. "We are expecting to field our alpha board in first quarter of 2005," says Boldt, who is confident of the success of this next-generation display technology. "We received comments saying that if we make it work then everyone else will be a distant second," he says.