Instrument fine-tunes large LED screens

Light-emitting-diode (LED) screens with millions of pixels are now common at large stadiums and arenas, channeling images of replays to sports fans and pop-singer grimaces to concertgoers.

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Light-emitting-diode (LED) screens with millions of pixels are now common at large stadiums and arenas, channeling images of replays to sports fans and pop-singer grimaces to concertgoers. An LED billboard was recently erected by Coca-Cola at Piccadilly Circus in London; the billboard image can be made to ripple when it's windy and look wet when it rains.

But all these spanking-new screens have a problem—they don't age gracefully. Even if well-calibrated initially, a large LED screen will degrade, and will do so unevenly. In addition to random variations between LEDs of the same color, the red, green, and blue LEDs making up a full-color screen all will degrade at different rates. Local temperature effects can cause additional variations.

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An optical telescope images a portion of the Seahawks Stadium LED screen onto a CCD camera. The data is processed by calibration software, which computes correction factors to upload back into the screen electronics, resulting in a screen that is uniform in color and brightness.
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Because the LEDs in a screen can be adjusted in brightness pixel by pixel, it is possible in theory to tweak them all so that they have the same color and brightness when reacting to the same signal value. This technique has been brought to practical fruition by engineers at Radiant Imaging (Duvall, WA), who have developed an on-site LED screen corrector consisting of a scientific-grade CCD camera and optics, along with calibration software. The instrument was recently used to calibrate the two large full-motion-video LED screens at the Seahawks Stadium (Seattle, WA) from a spot across the stadium (see figure). "It's similar to when you go to an audiovisual store and look at TV sets," says Ron Rykowski, the company's president. "Some just look better, even though you can't quite determine why. What we did was effectively improve the picture quality."

The stadium's south screen is 25 m wide × 6.33 m high, with 1600 × 456 pixels and three LEDs per pixel, for a total of 2,188,800 LEDs—all of which were resolved and measured individually. The screen is made of 250 panels, each containing 12 LED modules of 16 × 16 pixels. Many of the modules were mismatched, making the screen look like a checkerboard, says Rykowski. In addition, pixel-level variations gave the screen a grainy look.

In the calibration process, custom optics image a single LED panel (which measures 1 m wide by 0.66 m high) onto a 14-bit cooled CCD camera (1536 × 1024-pixel resolution) from 200 m away, providing a clear image of every LED on the panel. "We measure it at red, green, and blue separately, and have special software to extract the chromaticity coordinates and brightness of each LED from the image," explains Rykowski. "From this information, we can compute correction factors to upload back into the screen electronics to achieve a target brightness and color for each pixel. This process involves pointing the camera at each of the 250 panels. This is done at night to avoid high ambient light from affecting our measurements. Once we have processed all the 250 panels, then we capture an image of the entire screen with a different lens, and extract from the image the chromaticity coordinates and luminance of each block. We then calculate slight adjustment factors to ensure that each of the 3000 modules matches the target brightness and color."

The blue LEDs tend to degrade faster than green, and green faster than red. Also, each individual LED degrades at a different rate, which causes the screen to shift in color and become nonuniform over time. "We estimate that the screens need recalibration every one to three years depending on usage and environment," says Rykowski. Initially Radiant Imaging was first asked only to calibrate the stadium's north screen, but once the north screen was completed, Radiant Imaging was asked to calibrate the south screen as well.

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