Why LED signs go dim

led signThe non-uniform aging, and dimming of LEDs is an inherent problem over time. Outdoor LED displays are a fairly recent addition to the electronic sign world. Improvements in brightness that first made LED signs competitive with incandescent displays did not occur until the early 1990s. Also, outdoor heat and humidity degrades the aluminum gallium arsenic (AlGaAs) diodes that were used in early LED sign models.

According to Peter Lemme, vice president of engineering at Marktech Optoelectronics (Menands, NY), “When first introduced, the AlGaAs LEDs in outdoor products were promoted as having 100,000-hour lifetimes. That wasn’t really the case. In actual use the light output of the AlGaAs LEDs decreased by about 50 percent after 15,000 to 40,000 hours, rendering them unusable in direct sunlight”

Vincent Forte, Marktech’s chief engineer, recently shared that another problem relates to how the products are used. Because electronic signs commonly repeat the same textual messages, a specific group of LEDs is extensively used, while other LEDs in the display matrix are not. Thus, when the predominant message is changed after a certain time period, the display’s “aged” LEDs appear dim beside the other LEDs. This variable aging has a similar effect on incandescent lamps used for electronic signs.

Chart depicting a typical falloff in brightness for an LED over time.

Chart depicting a typical falloff in brightness for an LED over time.

He further explained that the more modern aluminum gallium indium phosphide (AlGaInP) LEDs used in today’s electronic signs are much more durable than the previous generation. Whereas AlGaAs LEDs reach their outdoor half-lives (50-percent reduction of initial brightness) after as few as 15,000 hours of operation, testing shows that AlGaInP LEDs exhibit half-lives of 170,000 to 180,000 hours.

Electronic-sign users should be aware, however, that variable aging of LEDs and incandescent lamps is inherent to applications that display identical messages for extended time periods.

This concern is now spreading outside the signage world, with the move afoot to replace both incandescent and CFL bulbs with LED powered lights. It’s of course, the long life and low electric consumption levels that make LEDs the “great hope” in the lighting industry. The European Commission (EUC) is pushing a phase out of inefficient, older, incandescent bulbs by 2015. They claim that alternatives…specifically, LED lighting, save enough energy to power millions of households every year, as well as significantly reducing CO2 emissions.

Vendors like Philips, GE, Sylvania, Cree, Silan and more also claim that the extended life of LED bulbs gives them a reduced cost of ownership when compared to traditional lighting, including CFL’s. The great unknown, of course, is that nobody has operated an LED bulb for 25 years, as these products have only recently become available. Early testing shows spotty quality, which skews the projections in a big way. One vendor, Laborelec had this to say: “Right now, there are terrific LED products, but also, some terrible products…It’s difficult for a customer to make an informed decision.”

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