Using Light-Emitting Diodes as Sensors
Vice President, Engineering
Connecticut Analytical Corporation
When we were kids, many of us experimented with using an audio output transducer as an audio input transducer, or, more simply stated, using a speaker as a microphone. If you connect a common coil-type speaker to an amplifier, you can effectively use it as a microphone. The speaker is now a microphone!
The question that now comes to mind is can we use a light-emitting diode (LED) as a light sensor?
The short answer is yes we can! In fact, it was SAS’s own Forrest M Mims III who was the first person to realize this and actually do it! I like to refer to the dual-action (emission/detection) of LEDs as the “Mims Effect.” Until Forrest’s discovery, LEDs were used only as light emitters.
So what can you do with an LED operated as a narrow-band light sensor? Well, how about two-way optical communications for starters. Some of Forrest’s early experiments were done in 1971, when he utilized two LEDs to perform bi-directional communication. In 1980, Forrest demonstrated bi-directional LED voice communication through the air using near-infrared (940 nm) LEDs and also through a 100-meter section of optical fiber (650 nm). This demonstration was done at 1325 L Street in Washington D.C. – the exact same site where Alexander Graham Bell invented lightwave communications exactly 100 years earlier! Present for the demonstration, which was sponsored by the National Geographic Society, were representatives from the Geographic, the Smithsonian Institution and Bell Labs. Bell first demonstrated his Photophone  on 3 June 1880.
Forrest’s interest in LEDs began when he was experimenting with photosensitive devices back in 1962. I managed to find a The Citizen Scientist "Backscatter" letter  in which Forrest 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.
"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.”
Using LEDs as Narrowband Sensors for Atmospheric Studies
Continuing in this tradition, Forrest decided to utilize these same principles to measure the atmosphere. In a paper published in Applied Optics (1992), entitled “Sun Photometer with light-emitting diodes as spectrally selective filters”  Forrest describes how “Light-emitting diodes (LEDs) can function as light detectors with a spectral bandpass similar to the diode's spectral emission band, typically 25-35 nm at the half-maximum points. This means that LEDs can serve as detectors in miniature sun photometers that measure precipitable water and atmospheric turbidity at wavelengths from 555 to 940 nm.”
When an optical sensor is designed, very careful attention must be given to the filtering used to guarantee a response for a specific wavelength. A typical method would be to use an expensive optical sensor with even more costly interference filters. Even when this is done, the response may not be as narrow as desired and may be unstable over time. A simple and inexpensive solution was realized by Forrest by using LEDs, for the emission and detection wavelengths of LEDs have very narrow bandwidths. The peak wavelength for detection is slightly below that of the peak emission wavelength. The amount of shift is typically around 25 to 35 nm below the peak emission wavelength.
Perhaps you were fortunate to have purchased one of the Sun & Sky Monitoring Stations  designed by Forrest for Radio Shack, which sold 12,000 of them. Sadly, this device is no longer carried by Radio Shack. The Sun & Sky Monitoring Station shown in Fig. 1 contained four LEDs that were used as narrowband sensors for specific wavelengths of solar radiation. I bought one of these several years ago and created a series of Excel based worksheets to be used in conjunction with its data .
For some additional details concerning the Sun & Sky Monitoring Station, I will once again defer to Forrest by quoting from his Sun & Sky Station instruction manual:
“The spectral tuning available by using different kinds of LEDs forms the basis for the Radio Shack Sun and Sky Monitoring Station, which uses four different kinds of LEDs to detect wavelengths of about 505 nm, 625 nm, 816 nm and 930 nm. This very broad range of wavelengths from very simple, inexpensive LEDs permits this instrument to measure photosynthetic radiation (PAR) and to detect aerosols and the total column water vapor.”
Figure 1. Instruction manual for Radio Shack Sun and Sky Monitoring Station designed by Forrest M. Mims III.
The basic circuit for utilizing the dual-action Mims effect of LEDs as narrowband sensors is diagrammed in Fig. 2.
Figure 2. Basic 2-way optical communication link using a single LED as both a source and detector of modulated light. (Figure 5.22, LED Circuits and Projects by Forrest M. Mims III, Howard W. Sams & Co., Inc., 1973.)
This simple circuit was the basis of many great remote sensing experiments using LEDs contained within Forrest’s Circuit Scrapbook .
Other Applications for LEDs as Light Sensors
So now that we know that an LED can be used as a narrowband light sensor, what else can we do with it? Following upon Forrest’s early work with LEDs as narrowband sensors, he developed the TOPS (Total Ozone Portable Spectrometer)  instrument that can measures the ozone layer. Although the TOPS unit did not utilize LED’s operating in the Mims effect, it was a prototype for the current commercial version (MicroTOPS II) , an experimental version of which uses LEDs as detectors. Some of the many examples of Forrest’s idea of using LED’s as narrow band light sensors include the following circuits and devices:
A pulsed light receiver 
Design, calibration, and performance of MICROTOPS II handheld ozone monitor and Sun photometer 
An optical sensing approach based on Light Emitting Diodes 
LED Based Photometer (Nuts & Volts Magazine) 
Using LED’s to Detect Vegetation (The Forrest Mims Circuit Scrapbook, Volume 1, pp. 24-26, 2000.
LED Based Sun Photometer 
Last, but certainly not least, there is a cool video from New York University showing the Mims effect LED touch switch  in operation.
I hope that in this two-part series
(see "An Inside Look at Light Emitting Diodes (LEDs)," The Citizen Scientist, January 2009)
I have answered most of your questions concerning LEDs and their wide variety of uses. If not, below I have included a generous list of additional references to supplemental material. I have skimmed over many topics, but how many of you would read an online article that was the equivalent of a major novel?
I hope that this has given you some new avenues to explore when you are considering your next project or experiment for a science fair. There are many good articles on using LEDs as vegetation detectors, water detectors, haze measurements, and remote sensing applications – all written by Forrest M Mims III.
: Mims III, Forrest M., Circuit Scrapbook – Volume I
: Mims III, Forrest M., Circuit Scrapbook – Volume I
Light Measurement Handbook - http://www.intl-lighttech.com/services/light-measurement-handbook
Planck’s constant Photon energy - http://www.colorado.edu/physics/2000/quantumzone/photoelectric2.html
What is Light – http://chem.ps.uci.edu/~potma/COSMOS/lecture7.ppt
Principles of Semiconductor Physics - http://ece-www.colorado.edu/~bart/book/book/title.htm
LEDs, Still popular after all these years - http://pdfserv.maxim-ic.com/en/an/AN1883.pdf
Sun photometer with light-emitting diodes as spectrally selective detectors -
LED used as sensors - http://ledmuseum.home.att.net/lightsen.htm
LED History - http://www.nanophotonics.org.uk/niz/publications/zheludev-2007-ltl.pdf
LED Wafer - http://ledmuseum.home.att.net/ledwafer.htm
LEDs used as narrowband sensors - http://laser.physics.sunysb.edu/~tanya/report2/
3D LED cube video - http://www.youtube.com/watch?v=Aj3_v7xCyJ0&feature=related
Introduction to LED’s - http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html
Wavelength Engineering - http://www.tf.uni-kiel.de/matwis/amat/semi_en/kap_5/backbone/r5_1_4.html
LED Design Principles - http://material.eng.usm.my/stafhome/zainovia/EBB424e/LED1.ppt
LED Communication - http://www.merl.com/reports/docs/TR2003-35.pdf