Variations on a Theme
The Ambrotype was a variation on the Collodion process, popular in the US, in which the (normally negative) image was made to look positive by placing it over a very dark background - the silver particles in the darkened plate would then reflect more light than the background so that a positive image was produced. Around the same time the Tintype or Ferrotype process became popular - these were broadly the same as Ambrotypes except that the collodion was spread over a sheet of blackened or enameled iron to provide the dark background. The results were not as fine as Ambrotype or Collodion images.
Dry Plates
The Collodion process was overtaken in the 1870's by the 'dry plate' technique, which used an emulsion of silver bromide and gelatine on glass. These plates were much more sensitive than the collodion plates, although somewhat grainier. This fact was tolerated since most prints were made by contact processes that would hide any graininess in the negative. In fact the improved sensitivity was a disadvantage for some photographers because their cameras were not equiped to make the necessary short exposures, and their studios were bright enough to cause the plates to fog.
Flexible Film
The first flexible photographic film was marketed in 1889 by George Eastman. This was a film of cellulose nitrate which was sold as a roll and which could be loaded into mass-produced simple box cameras. The cameras were sold with the film pre-loaded, and when used the whole camera was sent back to the factory where the film was extracted and processed. The camera was reloaded with film and returned to the customer. Cellulose nitrate is highly flammable, with similar properties to guncotton, hence its use was superseded by triacetate films, and in the 1960's by polymer based plastic films.
Experiments in Color
Color photography had first become a reality in 1861, when the physicist James Clerk-Maxwell experimented with colored filters. He found that by taking three exposures of the same scene through red, green and blue filters and then projecting each through its respective filter so that all three images overlapped, he could recreate the full range of color information present in the original scene.
Advanced Color - the Autochrome Process
In 1906 the Lumiere brothers patented a color photography technique in which multicolored grains of potato starch (dyed red, green and blue) were painted over a photographic plate, followed by a dusting of lamp black to fill in the spaces between the microscopic grains. When exposed to a colored scene, the grains acted as colored filters so that minute patches of the underlying film were exposed only to certain colors of light. When developed and viewed by transmitted light through the same colored starch grain filters, the original color was restored and a full color photograph could be seen. This was known as the Autochrome technique and produced remarkably good results, especially if the plates were subjected to pressure before exposure to flatten the starch grains.
Modern Color - Kodachrome
Kodachrome, the first modern three-layer dye-based color film, was not produced until 1936. This contains three separate layers, each sensitive to either red, green or blue light, allowing a single exposure to produce a full color photograph. Developing a color film is a more complex procedure, in which the silver particles in each layer are replaced with different colored dyes to recreate the original color.
Self-Developing Film - Polaroid
Polaroid had produced the first instant black and white film in 1948, and in 1963 produced their first instant color film. These films are distributed in a pack containing the necessary developing chemicals which are released when the film is squeezed by a roller after the photograph has been taken. The chemicals are present in multiple layers within the film, as well as in a mass that is spread over the film by the rollers and sets the development process in motion. Complex reactions involving diffusion and neutralisation of the various substances ensure that the film is developed in the correct sequence and for the correct amount of time to produce a good image.