The GLOBE sun photometer uses a light- emitting diode (LED) as a sunlight detector. What is an LED?
A light-emitting diode is a semiconductor device that emits light when an electrical current flows through it. The actual device is a tiny chip only a fraction of a millimeter in diameter. In the GLOBE sun photometer, this chip is housed in an epoxy housing about 5 mm in diameter. You can find these devices in a wide range of electronic instruments and consumer products. The physical process that causes LEDs to emit light also works the other way around: if light shines on an LED, it produces a very small current. The electronics in your sun photometer amplifies this current and converts it to a voltage. Generally, the wavelength of light detected by an LED is shorter than the wavelength of light emitted by the same LED. For example, certain red LEDs are relatively good detectors of orange light. The LED in the GLOBE sun photometer emits green light with a peak value at about 565 nm. It detects light with a peak at about 525 nm, which is a little farther toward the blue part of the lig
