One of the buzzwords in the display industry right now is LED, thanks much in part to the rise of ultra-thin consumer screens that owe their one-inch depths to LED side-lighting technology. But long before being used in the home, LED-lit screens were being used commercially, and have since dramatically changed digital signage, particularly outdoor installations.

For most of their history, LCD screens have worked like this: A panel of cold cathode fluorescent light (CCFL) tubes, not unlike those lighting many offices, are placed behind two panels of glass filled with liquid crystal matter. When turned on, the CCFLs illuminate the image and it can be seen by the viewer.

Commercial-grade LED-backlit models, are quite similar to their CCFL counterparts except an array of light-emitting diodes (LEDs) replaces the CCFLs (Consumer grade LED models are lit from the side, thus allowing an ultra-thin enclosure). And although the concept and the look and feel are generally the same, the differences in performance are significant.

These differences have caused LED-backlit screens to emerge as the hardware of choice for any outdoor digital signage application, based on several characteristics.

Reliability

According to Brad Gleeson, CEO of Vertigo Digital Displays, reliability is key trait of LED-backlit displays, but moreover, it is reliability over time. The best deployment opportunities seem to be for five to 10 year contracts with companies like Clear Channel, JC Decaux and Titan, which Gleeson says require equipment that is going to last the life of the installation.

The ability to have a screen that operates at 2,000, and even up to 4,000 nits, is significant when it comes to combating direct sunlight and ambient light, which tend to wash out images. The brighter the display, the better contrast the screen will have under those conditions.

Vertigo outdoor display on Vancouver's SkyTrain.

“If you look at a typical TV panel, it has 450-500 nits, and even digital signage panels designed for higher ambient light operate at about 600-700 nits,” said Scott Birnbaum, vice president, LCD group, Samsung Digital Solutions business. “LED backlit panels are actually designed to operate at 1,500-2,000 nits for a much brighter image. It opens up the amount of digital signage applications, so instead of just being inside of a window, it can now be fully outdoors.”

Temperature

Kaszycki says the main advantage that LED backlighting has over CCFL backlighting for outdoor applications is that it handles extreme temperatures better.

“Really the biggest problem with CCFL is the temperature,” he said. “Ninety-nine percent of people don’t read the fine print on these systems where it says that the screen produces 1,500 nits of brightness based on a surface temperature of 77 degrees Fahrenheit. But how often is it comfortably 77? When you get in the hot summer months, the brightness can drop 30 percent when it gets to be about 95 degrees. It can drop 25-30 percent when it gets near 35 degrees.”

While LEDs are less susceptible to extreme temperatures than CCFLs, the LED back-lit screens themselves can run fairly hot, presenting a climate-control challenge.

“For outdoor viewing, you have the problem of direct sunlight, as well as the problem of thermal load,” Gleeson said. “So in addition to the output from the electronics, you’re getting something like 75 watts/m2 per hour of thermal load that’s getting added to the enclosure that has to be managed. In outdoor settings with direct sunlight where you’re trying to run upwards of 2,000 nits, thermal management becomes a challenge.”

The Wedge is a bus shelter equipped with two LED backlit displays.

A significant installation

One of most significant installations of LED-backlit signage was announced in June 2008. As part of a contract between Outdoor Promotions and MRI, 18 bus shelters along the Las Vegas Strip were redesigned and outfitted with 70-inch LED-backlit displays as part of Outdoor Promotions’ 20-year agreement with the Regional Transportation Commission of Southern Nevada to supply digital signage for bus shelter curbside billboards on the Las Vegas strip, maintain the signs and manage the advertising placed on display. The shelter design, called the Wedge, housed back-to-back screens capable of producing 2,000 nits.

“We viewed Las Vegas as being one of the most hostile digital signage environments for several reasons,” Kaszycki said. “For one, the sun load is tremendous and temperatures can exceed 100 degrees. The vandalism factor is also very high.”