Despite being a 50-year-old technology, IR LEDs still have new and interesting applications.
Infrared LEDs have been around for over half a century and have been put to use in myriad applications since their development. Before we take a look at some of these uses, though, here are some fun facts.
Infrared (IR) radiation was discovered in 1800 by Sir William Herschel, an amateur astronomer with the distinction of having discovered the planet Uranus. Herschel devised an experiment to confirm his hypothesis that different wavelengths of visible light might register differing levels of heat. Using a common prism to separate sunlight into its characteristic colors and a thermometer to measure the temperature of each, Herschel noticed that indeed, the measured temperature increased as he moved the thermometer bulb from violet through the other colors of the spectrum and wondered if there might not be radiation just beyond red. He placed his thermometer just beyond the red portion of the spectrum and viola, infrared light was discovered. Slightly more than half of the sun's rays fall into the infrared portion of the electromagnetic spectrum. As a refresher, here's a pictorial:
The development of the first IR LED is credited to a researcher at the Radio Corporation of America, Rubin Braunstein, who reported in the September 1955 edition of the American Physical Society journal that gallium arsenide diodes could emit IR. Six years later, Bob Biard and Gary Pittman of Texas Instruments received the first patent for an IR LED.
IR radiation is still being used in industry, science, the military, law enforcement, and medicine, and because of this, manufacturers are still actively engaged in developing new IR LED packages. As a case in point, Osram has recently introduced a new offering in its line of IR LED packages for use in security cameras and access control.
Osram's new addition, the SFH4718A infrared LED, features a narrow beam angle of ±25°, which they claim eliminates the need for secondary optics, allowing security cameras to be smaller and lower cost. The new package provides radiant intensity of up to 1000W/steradian at 850nm, a wavelength that is difficult for the human eye to discern but still adequate for camera sensors. While the IR light being emitted is not visible, at 850nm, the LEDs will discernably glow, a feature which may serve to encourage the bad guys to move on to a more accommodating target. The operating temperature range is specified at −40°C to 125°C.
This new package, like the other two in the same product line, uses chips with two emission centers, one on top of the other. This arrangement, according to Osram, provides 80 percent higher optical output as compared to a typical infrared LED and enables illumination of distances up to 100 meters.
But there are lots of other applications for IR LEDs as well. Let's take a look at just a few, starting with the mundane and ending with the perhaps less well-known ways in which IR LEDs are put to use.