Photo emulsion is a commonly use misnomer for a photosensitive substance used in screen printing that hardens when subjected to ultraviolet light. To prepare a screen for printing, it is coated with liquid photo emulsion and allowed to dry. Opaque, monochromatic artwork (known as a stencil or a positive) is transferred onto a transparent medium such as glass or film which is placed over the emulsified screen. Ultraviolet light is shown on the screen causing the emulsion to become hard and insoluble, except in areas that are covered by the opaque artwork. The entire screen is then washed in water or solvent, allowing any emulsion not hardened by the light to rinse away, leaving a representation of the artwork on the screen.