Displays and signs call for improvements in green and white LEDs

Speakers at a recent conference described trends and challenges in the full-color video display and signage markets.

Feb 17th, 2005
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At Strategies in Light, Tony van de Ven, founder and former CEO of display manufacturer Lighthouse Technologies, discussed the market drivers for full-color video LED screens. In his opinion, LEDs are the ideal technology for screens with a diagonal of 100-1000 inches and more, and a brightness in the 1000-10,000 nit (cd/m2 range). "I don't see any technology to supplant LEDs in this range," he said.

For sports stadia and events, full-color video displays showing replays and magnified views have become a requirement, if not a necessity. Even high schools in the US are starting to deploy these products, which are also a feature of music events, trade shows and many other locations requiring video shows with excellent color reproduction.

The industry has seen many outdoor "spectaculars"- very large screens containing millions of LEDs. "This is a growing market, and as chip costs continue to fall, the new screens are staying large," said van de Ven. Typical costs can exceed $5 million for high resolution screens measuring in excess of 200 m2.

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The so-called "killer app" is electronic billboards, which are typically about 30 sq. meters in size, with 15-25 mm pitch. Van de Ven said that such displays have now passed through an important price point of $5000/m2. In many locations, zoning regulations prevent the use of video and flashing effects, and the image is usually a series of JPEG images.

Monoblocks, which are small screens measuring up to 120 inches (diagonal) are non-modular, unlike larger displays - essentially they are like large plasma screens. Clearchannel has deployed 85 such screens at street level outside subway stations in New York City; these have a brightness of 3000 nits and are wireless networked. Such screens can command 3-6 times the revenue of a backlit poster, justifying the higher cost, although return on investment still takes a few years.

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Large screens consist of modules which typically measure about 1 foot square and contain 16 x 16 pixels. In turn, each pixel contains red, blue and green LEDs. Van de Ven explained that since blue and red LEDs are more efficient than green, pixels often use 2 green LEDs. In some cases, the two green LEDs are treated as separate pixels and driven individually; they "share" the brightness of the red and blue LEDs, which result in an increase of the apparent resolution of the screen. "We'd like to see some improvement in the performance of green LEDs," said van de Ven.

The biggest problem for full-color video screens is the need to dynamically adjust the signal to adjust for the variation in LED performance within bins. "Also, red, green and blue LEDs all degrade at different rates, and blue LEDs from the same bin also degrade differently," said van de Ven.

In terms of future development, reduced LED cost will help video screens to penetrate new markets, but this is not the biggest challenge, said van de Ven: "We have enough brightness, what we need now is reliability and uniformity."

Signage seeks cost reduction for white

According to Jim Sloan, president and CEO of SloanLED, the US market for LED-based signs is dominated by channel letters, which have an 84% share. This includes reverse channel letters, where LEDs are used to project light from the rear of the sign onto the building or other mounting surface, creating a halo effect around the letter. Border and perimeter lighting makes up a further 13% of the market.

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As can be sign by driving in any urban area, red is by far the dominant sign color, accounting for 69% of all signs. Bue, green and amber all have 4-6% shares, while white has 16% of the market. "There would be many more white signs, but cost is an issue," said Sloan. In contrast, he explained, 2004 marked the point at which red LED channel letters became lower in cost than their neon counterparts.

Other challenges for using LEDs in the sign market include selecting the correct spacing: generally there is a need for even light distribution without the appearance of spots of light, and this introduces a complex trade-off with the LED power and the system cost.

Sloan also highlighted the need for standardized systems (in terms of appearance, voltages etc), particularly across large projects, for example corporate identity signs on multiple retail outlets or gas stations. However, there are no standards in place: "Companies work in the hope that by winning large programs they will be able to establish standards that other suppliers follow," said Sloan.

Another LED challenge in this market is matching the LED colour to the lens, which is crucial in projects that portray a corporate image colour. The final challenge is to optimize the voltage, line loss and output consistency; customers generally want runs to be as long as possible, but the distance from the first LED to the power supply can vary considerably.

For the future, Sloan is looking towards standardization for the operating voltage of signs - possibly at 12V DC. Also, he predicts an increase in ruggedness and ease of use (including power optimization), and an increase in viewing angles together with optimized dispersion patterns.

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