Physical Considerations The xerographic process is based upon the electrical characteristics of certain semiconductors such as selenium which, although normally good insulators, become charge conductors under the action of light or ionizing radiation. If a metal plate is coated with such a material, the resultant product can be used in numerous applications as a substitute for conventional photographic emulsion. A xerographic plate consists of a metal surface coated with a thin layer of selenium. An electric charge may be uniformly deposited upon the selenium layer. If the plate is then placed in a light-proof box into which is introduced a cloud of powder which has been given an electrical charge of polarity opposite to that of the plate, the powder will be attracted to the plate and uniformly deposited upon it. If a freshly charged plate is exposed to x-rays, the charge will leak off the selenium to the metal plate directly behind. The amount of charge which leaks off is related to the incident x-ray ex...