Video technology originated with cathode ray tube television systems but has since expanded. Standards for TVs and computer monitors evolved independently but advances in digital technology are converging them. The basic principles of image reproduction through still and motion cameras are similar - light is focused onto a photosensitive material to record images that are then developed and printed or projected. In video cameras, light is converted to electrical signals that are scanned and recorded onto magnetic tape then reconverted to images for viewing. Common video recording standards include NTSC, PAL and SECAM which differ in aspects like lines of resolution and frame rates. Film and video outputs take narrative, experimental, animated and documentary forms and have expanded to include music videos, commercials