Hellschreiber or Hell is a so called fuzzy mode. It was invented by Dr. Rudolf Hell in the 1920s. In this mode, the message is transmitted as raw pixels, with the operator viewing the received image and mentally filtering out interference.
Hellschreiber saw limited use until World War II. During World War II the standard mode of Hellschreiber was used by the German Army and was nicknamed Feldhell.
Recently it has become popular with a subset of the amateur radio community.