1. Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black bodylight emission resulting from heat , from a chemical reaction, sound, or other mechanical action.
2. Electroluminescence (EL) imaging takes advantages of the radiadive inter-band recombination of excited charge carriers in solar cells. For electroluminescence investigation the module is operated as a light emitting diode and the emitted radiation due to recombination effects can be detected with a sensitive Si-CCDs camera. For electroluminescence images, the solar cells are supplied, via their metal contacts, with a defined external excitation current while the camera takes an image of the emitted photons at wavelength > 850 nm. Since EL is a low light source, a dark environment is necessary in order to decrease the background noise during the measure. Damaged areas of a solar module appear dark or shine less than good areas. Electroluminescence imaging can be used to detect a multitude of defects in crystalline silicon solar cells, furthermore EL technique provide images with very high resolution that enable to resolve details lesser in size than IR-images that should be hardly perceptible to the eye, such as: micro cracks, bad finger contacts, Crystallization faults in cell material.
https://en.wikipedia.org/wiki/Electroluminescence (1.); http://www.cepsolar.com/electroluminescence-imaging (2.)