A suspension of a light-sensitive salt of silver (especially silver chloride or silver bromide) in a colloidal medium (usually gelatin, which is placed as a coating on rigid or flexible materials such as glass plates, plastic film, or paper to be used in photography. It is usually referred to simply as the emulsion, although it is not a true emulsion. Types in common use are panchromatic (producing black and white pictures), colour negative (producing pictures in colours complementary to those of the object), colour positive (producing pictures in colours the same as those of the object), infrared-color (producing pictures in which infrared radiation from the object is represented by colours in the picture), and infrared black and white (producing pictures in which the density of a spot in the picture is proportional to the amount of infrared illumination radiation that has illuminated that spot).