An Inexpensive LED Light Sensor

Light irradiance measurements are important for students grappling with abstract optical phenomena such as the inverse square law,1 polarization, diffraction, interference, and spectroscopy. A variety of commercial light sensors are available from scientific vendors such as the CI-6504A from PASCO scientific2 and the LS-BTA from Vernier Software and Technology.3 These sensors, in combination with data acquisition interfaces, allow students to digitally record relative irradiances. Many experiments in elementary lab situations, however, do not require this level of sophistication, and a simple LED connected to a voltmeter performs admirably as a readily available and inexpensive photosensor. LEDs are frequently used in the laboratory as light sources,4,5 but their use as photodetectors6,7 is relatively unknown in the physics teaching community. Here we introduce a few of the basic laboratory applications of common LEDs as photosensors.