Infrared LEDs are still going strong after all these years

Article By : Yoelit Hiebert

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.

Osram's SFH4718A infrared LED package (3.85×3.85×2.29 mm) Source: Osram

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.

Remote controls are ubiquitous in our homes and offices, but perhaps coming to the end of their place in our lives because of smart home technology. It uses IR at 940 nm (typical) to transit a pulse-width modulated code to the receiving device, and communication protocols differ by manufacturer.
Machine vision uses IR wavelengths of 850 nm and 940 nm. In addition to reducing glare and reflection, IR also diminishes the color of irradiated objects. The longer wavelength is also able to penetrate to a greater depth and even pass through certain materials like paper, cloth, or plastic. IR-based machine vision is used for inspection purposes, e.g., checking the fill levels of packaged goods, and for identification of flaws or defects not apparent under visible light.
Tissue healing: IR LEDs are used in everything from endoscopy to blood analysis, but increasingly, IR is being used in therapeutic devices, an outcome of the observation that IR radiation accelerates the healing process for astronauts in space. Wavelengths of 680 nm to 880 nm pass through both skin and muscle tissue, accelerating deep tissue healing through the stimulation of mitochondria in tissue cells. Used alone, or simultaneously with hyperbaric oxygen, this technology can help relieve pain and promote healing of war wounds and other serious injuries.
Iris recognition is widely used in government and law enforcement. Iris recognition scanners illuminate the eye with both visible white light and infrared light. IR in the 700 – 900 nm range is used to both enhance the features of dark colored eyes (brown being by far the most common eye color worldwide) and to block reflections off the cornea. The resulting photos are analyzed using an algorithm to identify a set of approximately 240 data points. This information is, in turn, converted to a 512-digit code that is essentially unique to each person.
Despite being a 50-year-old technology, IR LEDs are still going strong, with new and interesting applications still being found.
Yoelit Hiebert has worked in the field of LED lighting for the past 10 years and has experience in both the manufacturing and end-user sides of the industry.
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