Define Barrier layer cells (photocells)
They are the very simple of the detectors. It contains the bottom support layer of conductive metal like iron. A photosensitive layer of the selenium /cadmium (the positive electrode) rest on the top of the metal support. This light sensitive materials is covered along with a transparent thin layer of gold (the negative electrode). Light passing by the transparent layer to the selenium, causes the release of electrons from selenium. These emitted electrons move to the clear conductive layer and make a weak electric current as they complete a circuit by flowing to the metal support. So the current produced is a direction function of the light intensity striking the photocell.