Forrest Mims - Using LEDs As Narrow Band Light Sensors

Using LEDs As Narrow Band Light Sensors

Among Forrest Mims many accomplishments, he was the first person to realize that LEDs (Light Emitting Diodes) had the ability to not only emit light, but also to sense light. This dual-action (emission/detection) of LEDs or “Mims Effect” was unknown before his discovery.

Mims' interest in LEDs began in 1962, when he was experimenting with photosensitive devices and discovered the inverse effect. In the "Backscatter" section in an online issue of The Citizen Scientist, Mims describes this himself:

While a high school senior in 1962, I first got the idea that light sensors should be able to double as light detectors. So I connected an automobile ignition coil to a cadmium sulfide photoresistor, switched on the power, and observed bright flashes of green light emitted by the semiconductor. The green flashes were distinctively different from the yellow flashes of an electrical arc.

Mims also continued his investigations into the dual use of LEDs while in college:

While studying government (my major) in college, I found that certain silicon photodiodes can emit near-infrared radiation that can be detected by similar photodiodes. I managed to send modulated tones between such photodiodes. In 1971 I demonstrated the ability of many LEDs to detect light while experimenting with an optical fiber communication system. By placing a single LED at each end of the fiber, it was possible to send signals both ways through the fiber with only a single, dual purpose semiconductor device at each end of the fiber.

In 1980, Mims demonstrated the dual use concept of LEDs by building a bi-directional LED voice-communication circuit that allowed two people to transmit speech optically through the air and also through a 100-meter section of optical fiber. This demonstration was done at 1325 L Street in Washington D.C. —the same site where Alexander Graham Bell invented lightwave communications 100 years earlier. Present for the demonstration, which was sponsored by the National Geographic Society, were representatives from National Geographic, the Smithsonian Institution and Bell Labs. Bell first demonstrated his Photophone on 3 June 1880.

In addition to utilizing the dual-mode use of LEDs for communication, Mims decided to utilize the dual use of LEDs to perform measurements on specific properties of the atmosphere. In a paper published in Applied Optics (1992), entitled “Sun Photometer with light-emitting diodes as spectrally selective filters, Mims describes how LEDs can function as light detectors.

Among his many electronics books written for Radio Shack, Mims developed several electronics kits for them. One kit in particular made use of the "Mims Effect" of LEDs, by utilizing 5 LEDs acting as narrow band light sensors to perform atmospheric analysis. Dubbed the Sun & Sky Monitoring Station, this kit — of which 12,000 units were sold — allowed the user to make sophisticated scientific measurements, including measuring the amount of sunlight, atmospheric haze, atmospheric water vapor, amount of PAR (Photosynthetic Radiation), and the ET (Extraterrestrial Constant). The Sun & Sky Monitoring Station is no longer carried by Radio Shack.

Mims discovery of the dual use of LEDs opened up a whole world of applications, including a commercially available instrument called MicroTOPS II which uses narrow band filters and photodiodes as detectors.