Photo emissive Light sensors Photoemissive light sensors are so named because they contain materials whose surfaces emit electrons when struck by light radiation. The electrons are emitted when the photons of the incident light are able to transfer enough energy to the electrons to break them free from both their atomic bonds and the forces of the entire maternal lartice Materials in which this phenomenon takes place easily enough to produce many electrons when struck by visible light are called photoeminive materials Because the photoemissive material is usually housed in a glass tube, photoemissive devices are also often known as phototubes.