What is selenium?
Selenium is a non-metallic element that is often used as a semiconductor material, as a rectifier or in xerography. There are three forms. In a crystalline form, selenium is metallic gray; in vitreous form, it's black; as a powder, it's red.
In 1873, it was discovered that selenium's electrical conductivity varies in response to exposure to light. This discovery meant both that it was possible to convert image data to electrical signals and that the data could be transmitted.