At the beginning of the 20th century, projection was split into two classes: "If the light traverses the object, the projection is said to be diascopic, if by reflected light, episcopic."
I begyndelsen af 1900-tallet var billedprojektion inddelt i to kategorier: " Hvis lyset går igennem objektet, siges projektionen at være diaskopisk, hvis lyset er reflekteret, episkopisk." .