What's Inside E6 Transparency Film?

Text and Drawings by George Carlson
 

Everybody knows you don't have to know much about how an automobile engine operates in order to be a good driver. But, if the car is not working properly, or you want to get more performance from the car, knowing what's happening under the hood is essential. Photography is much the same way. There are plenty of good photographer's out there that know little about how their material functions, or what to do if there is a problem. The really great photographers study the craft as carefully as they pursue the art.

Our eyes are capable of seeing the entire range of color from a deep red all the way through violet. A rainbow of the light coming from a prism shows the entire range of visible colors that are contained within white light. It was discovered, very long ago, that our eyes can be "fooled" into seeing almost any color by carefully mixing just three primary colors, Red, Blue, and Green, or pigments, Cyan, Magenta, and Yellow. This is the phenomena that makes it possible to see so many colors on this video screen. The system using Red, Green, and Blue (RGB) is an additive system. The three colors of light are added together to form the desired color. Another system, using Cyan, Magenta, and Yellow (CMY), operate as a subtractive system. Various densities of these colors can be stacked as a filter to produce the desired color. Color film uses the additive properties of the RGB system to record the colors in an image, and the subtractive system, CMY, to reproduce the colors.

The original color photographs were made by shooting the scene through three filters using three sheets of film. The three sheets were each processed and used to print a single color. The printing of the each color was carefully done one on top of the other, so registration was critical. This was a very complex and difficult process to do. A camera that could record a scene on three sheets of film was difficult to carry around. Variations on this Three-Color process are still used today, but for the printing industry, not the photographer in the field. So the challenge was to design a system that the photographer in the field could use to easily capture the colors of nature.

In the Thirties, Kodak invented a process they called Kodachrome. This was truly a major event in photography. Kodachrome use a film made up of three light sensitive layers, each sensitive to a different color (RGB). This is called the Integral Tripack Color Film. In 1947, Kodak introduced Ektachrome. Ektachrome was a big advance over Kodachrome because all the chemicals necessary to form the dyes in the three layers, were contained in those layers. This made it possible for almost anyone to safely process their own film. Kodachrome is still available today, but there are only a few labs in the world where it can be processed.

 

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Shown above is a crude diagram of the structure of a Tripack color film. Both color reversal and color negative film use this same structure. I think Fuji Velvia uses a total of seventeen layers in their film. The extra layers are used to control grain size, processing and color balance.

The top layer is an emulsion sensitive only to Blue light. All silver photographic emulsions are sensitive to Blue light, so below the Blue Sensitive layer is a Yellow Filter that prevents the blue light from going to the other layers. Below the Yellow Filter is a layer designed to be sensitive to Green light. The bottom layer of emulsion is sensitive to Red light. These are all coated on a clear plastic film. The back of the film has a dark water soluble antihalation coating that prevent light from being reflected back through the film and cause a reduction in sharpness. All three layers are very similar to the emulsion used on B&W film, except that they also have the ability to produce color dyes during development. More about that later.
 

Example Exposure
 In the example exposure above, we have a Red, Green, and Blue sources of light, as well as a source of white light.
 

Results of First Developer
 The Red light exposes the silver halides in the red layer, the green and blue layer respond accordingly. The white light exposed all layers. The first developer is essentially a B&W developer, and if the film were fixed at this point, it would look very much like a B&W negative. When photographic emulsions are developed, the silver halide grains that have been exposed to light reduce to pure silver. The unexposed grains remain intact. Notice the Antihalation Backing and the Yellow Filter have dissolved in the first developer.
 

After Color Developer
 After the first development and a negative image formed in the film, the film is chemically fogged in the reversal bath. In the older processes, such as E3, the film is actually exposed to white light to fog the undeveloped silver halides. Because about half the silver halides were used-up during the first development, the silver produced during the color development produces a positive image. During development, the exposed grains of silver halide release oxidation products. In the Color Developer, the oxidation products activate color couplers that are contained in the layers of the film. Each layer has a different color coupler. The coupler produces and insoluble dye in the layer whereever development occurs. The Blue layer produces a Yellow Dye, The Green Layer, a Magenta dye, and the Red layer, a Cyan dye. Since all the silver halides in the film have now been reduced to silver, the film is completely opaque. The silver must be removed without affecting the dyes that have been generated.
 

After Bleaching Out Silver
 The bleaching step converts the silver to soluble salts and washes it away. This results in a transparent set of filters produced by the second development in the color developer. After bleaching, a fairly conventional fixer is used to remove the last traces of silver and harden the emulsion to make it more resistant to handling.
 

Projecting Finished Slide
 

As you can see in the diagram above, the resulting set of filters work on the light from the slide projector in a subtractive manner.

A few notes:

Color negative film is almost identical to slide film. When color negatives are made, the process (C41) starts with a color developer. No reversal in done. Less silver is needed in color negative film, so it is generally cheaper. For strange results, you can try processing C41 film in E6.

Kodachrome, why is special processing needed?

In Kodachrome film, there are no dye couplers in the individual layers of emulsion. Kodachrome processing starts out with a first developer, just like E6, but there is where the similarities end. Each layer of the film must be reversed using a special light source. The film is then put in a color developer that operates on only one target layer. This means there are three separate and very carefully measured re-exposures and color developments. Don't try this at home.