Yesterday we brought you news that Apple’s new iPad runs significantly hotter than its predecessor, the iPad 2. While initial reports put the temperature increase around 10 degrees, further testing showed that the new iPad could actually get as much as 13 degrees warmer than the iPad 2, reaching temperatures of 116 degrees under heavy use situations (e.g., gaming). Apple responded to the reports rather curtly, reminding us of all the new features the new iPad sports and stating that it did all this amazing stuff “while operating well within our thermal specifications.”

Requests for clarification concerning exactly what Apple meant by “thermal specifications” went unanswered, meanwhile speculation about the cause of the temperature increase abounded. Many suspected that the new iPad’s A5X chip, which sports a quad-core GPU, was to blame. Now, however, it looks like there may be another culprit: the LEDs in the new iPad’s much-touted retina display. DisplayMate Technologies CEO Raymond Soneira suggested in an interview with CNet that the LEDs might be to blame for the new iPad’s heat issue.

He pointed out that the new iPad has twice as many LEDs as the iPad 2. While a single LED light – or even a handful of them – produces very little heat, that changes when you start cramming them into a small space like the case for the new iPad. Soneira also pointed out that when the iPad is at full brightness, the display’s power efficiency is lower. Though the number of LEDs and pixels has been doubled, the new display actually requires 2.5 times the amount of power to achieve maximum brightness. This, in turn, puts a greater strain on the battery, which has to send more power to the display to achieve the same effect. Greater demands on the battery means a warmer battery. Soneira also pointed out that the IGZO display technology that Apple originally wanted for the new iPad’s display was not ready by the time the iPad went into production, forcing Apple to use traditional (and less energy efficient) LCD display technology.

Of course, that doesn’t mean that the LEDs are the sole culprit. Remember that the heat on the new iPad was not evenly distributed. The temperature was highest near the lower right corner, where the processor is. That suggests that the new A5X is also partly to blame for the problem.

Whatever the cause, though, the new iPad’s heating issue highlights an interesting challenge facing the technology industry: unlike the computer on your desk (or your lap) smartphones and tablets have no built-in cooling system. There’s no fan to cool your iPad or iPhone down when it gets hot. As smartphone and tablet makers start cramming more and more new technologies – better displays, faster processors, and bigger batteries – into their devices, the heat those components generate has the potential to become a significant problem.