What is the useful life of LEDs?

When you buy a package of light bulbs, you'll see a number on the box that says something like “Life: 1125 hours.” This is defined as the time it takes for 50% of test samples to burn out. But there's really not a standard definition like this for LEDs—they gradually decrease in brightness instead of “burning out.” The number 100,000 hours (about 11 years) gets mentioned a lot. Since an LED is a diode, even after it ceases to produce light, it will still use power.

So it becomes more useful to discuss “lumen maintenance” with LEDs. Manufacturers often publish a curve of light output vs. time, which describes the LED's lumen maintenance profile. Indicator LEDs in clear epoxy packages can drop to 80% light output within a few thousand hours due to yellowing of the package material. The degradation occurs due to heat generated in the junction. So lumen maintenance is an important spec for a designer building arrays of indicator LEDs for illumination. Improvements in packaging and heat sinking are improving the lumen maintenance characteristics of illumination-grade LEDs. But this is a significant area of difference between arrays of inexpensive indicator LEDs and illumination-grade LEDs. You may start out with the same number of lumens, but the indicator types will typically degrade more rapidly. 　　

How are infrared LEDs different?

There are a couple key differences in the electrical characteristics of infrared LEDs versus visible light LEDs. Infrared LEDs have a lower forward voltage, and a higher rated current compared to visible LEDs. This is due to differences in the material properties of the junction. A typical drive current for an infrared LED can be as high as 50 milliamps, so dropping in a visible LED as a replacement for an infrared LED could be a problem with some circuit designs.

IR LEDs aren't rated in millicandelas, since their output isn't visible (and candelas measure light in a way weighted to the peak of the visible spectrum). They are usually rated in milliwatts, and conversions to canelas aren't especially meaningful.