Photographic emulsion is a layer of light-sensitive material coated onto a substrate. In Silver-gelatin photography, the emulsion consists of silver halide crystals suspended in gelatin, and the substrate may be glass, plastic film, paper or fabric.
In proper chemical terminology, photographic emulsion is a suspension of solid particles in a fluid, and not an emulsion, which refers to a dispersion of one liquid in another, but the misnomer is established usage in the fields of photography and photographic science. The term emulsion is also used to refer to products such as film and paper. Sensitizing solutions for non-silver gelatin processes, such as dichromated colloid processes, cyanotype and kallitype are also called emulsion.
Photographic emulsion consists of a fine supension of insoluble light-sensitive crystals in a colloid sol, usually containing gelatin. The light-sensitive component is one of several silver halides, either silver bromide, chloride, iodide or more likely a combination of them. The gelatin is used as the binder since it acts as a semi-permeable membrane, allowing processing agents (e.g., developer, fixer, toners, etc.) in aqueous solution to penetrate the colloid without dislodging the crystals. Other polymer macromolecules are often blended, but gelatin has not been entirely replaced. The light-exposed crystals are reduced by the developer to black metallic silver particles that form the image and the photographic "grain." The emulsion of colour films is usually multi-layered. Dye couplers are added to the emulsions of colour films and papers, and dye layers may be coated between emulsion layers to act as optical filters.
Photographic emulsion is made by adding a solution of silver nitrate into a warm gelatin solution containing potassium bromide, sodium chloride or other alkali metal halides at a suitable proportions. The reaction precipitates fine crystals of an insoluble silver halides that are the light-sensitive component of the emulsion.
Although in the introduction, this fine silver halide in gelatin is called a suspension, it is really a dispersion in technically correct parlance. The silver halide is actually being 'peptized' by the gelatin. Selection of gelatin type and amount influences the final emuslion properties greatly.
Peripheral to these main ingredients, pH buffer, crystal habit modifier, metal dopants, ripener, ripening restrainer, surfactants, defoamer, emulsion stabilizer and biocide are used in emulsion making.
Emulsion making usually takes several steps, and most modern emulsions require the emulsion be "washed" to remove some of the reaction byproducts (potassium nitrate and excess salts). Such "washing" or desalting step can be performed by ultrafiltration, dialysis, coagulation (using acylated gelatin), or classic noodle washing method.
Emulsion making also incorporates steps to increase sensitivity by using chemical sensitizing agents, as well as sensitizing dye.