Color cast reduction

Almost all color transparency films, color print papers and color negative films use a multilayer subtractive colour construction similar to that shown above. The uppermost layer contains a yellow dye whose density is proportional to the amount of blue light, the middle layer has a magenta dye with density proportional to green light, and the lowermost layer contains a cyan dye with density proportional to the red light during exposure. A yellow filter layer prevents blue and ultra-violet light from reaching the two lower layers. A positive orange masking layer is also usually present in modern colour negative films to correct for imperfections in the response of the cyan dye.  All colour dyes have limited lifetimes since their molecules break down under the influence of temperature, light and reaction with adjacent materials in the layers in the film. The cyan dye which is responsible for the red colour in an image was especially unstable in films manufactured prior to new developments in the mid-1980's.

Henry Wilhelm, in his monumental work on the stability of colour images classifies the effects into several different categories. The graphs shown below are taken from his book. In Chapter 5 he writes:

"Once a color photograph has been properly processed, the most important factors in determining the useful life of a color film or color print are the inherent dye stability and resistance to stain formation during aging that have been built into the product by its manufacturer. While it is true that even the most unstable materials can be preserved almost indefinitely in humidity-controlled cold storage, only a small fraction of one percent of the many billions of color photographs made around the world each year will ever find their way into a cold storage vault."

CastCor has been written to aid in dealing with the majority of images which have not made their way to an archival vault. It is an imperfect solution to the problem, since there is no way of restoring information which has vanished utterly, but it does help if there is enough left for the methods offered to be reasonably effective.

1) Dark fading 

which affects each dye separately. Here is an example which shows the deterioration of the cyan dye with time on several different films and papers:

2) Dark staining

produced by deterioration of the residual otherwise colorless dye couplers in various films and papers:

Wilhelm writes: 

"Dark fading of course is not caused by darkness (light fading, on the other hand, is caused by light and UV radiation). Dark fading simply refers to the fading and staining that take place in a color material during storage when light is not present. Given the inherent dark fading stability characteristics of a particular material, the rate of dark fading and staining is determined primarily by the ambient temperature and, usually to a lesser extent with modern materials, by relative humidity. Air pollution and contamination from unsuitable storage materials can also play a part in the deterioration of color photographs, but these factors are usually much less important....Improper processing of color materials can also impair image stability; for example, use of non-recommended, exhausted, or contaminated chemicals, inadequate washing, omission of the proper stabilizer bath when one is called for, and so forth."

"The principal cause of yellowish stain formation in Ektacolor and other types of chromogenic prints with similar magenta dye couplers has been attributed by Robert J. Tuite of Kodak and others to discoloration of unreacted magenta coupler; the amount of magenta coupler that remains after processing is inversely proportional to the amount of magenta dye present in an image....

Historically, stain formation during dark storage has been a problem with all chromogenic materials except Kodachrome. (From its inception in 1935, Kodachrome has been an "external-coupler" product in which the color-forming dye couplers are placed in separate cyan, magenta, and yellow developer solutions, instead of being anchored in the film emulsion itself. After processing and washing, no unreacted couplers remain in Kodachrome; for this reason the film remains completely free of stain, even after prolonged storage under adverse conditions.)"

Light fadingdue to exposure of an image to light, shown here for a representative material artifically aged with strong illumination:

Wilhelm writes: 

"Actually, the slow but inexorable chemical processes involved in "dark fading" and "dark staining" continue whether or not a color photograph is exposed to light on display or during projection. Light fading is a separate process altogether. When a color photograph is exposed to light on display, both light fading and dark fading occur simultaneously. The fading and staining that afflict a photograph over time are in fact a combination of these two basic types of deterioration."

Light fading and dark fading also differ in the way that they affect the appearance of the image. In light fading, a disproportionate loss of density occurs in the lower densities and highlights. Visually dark parts of an image can remain more or less intact while lighter areas can become totally washed out. With modern materials, light-induced stain formation (distinguished from light-induced fading) is of less concern when prints are displayed than is staining when the prints are stored in the dark. In dark fading, highlight detail is not lost but an overall color shift occurs, caused by the cyan, magenta, and yellow dyes fading at different rates, and is exacerbated by an ever-increasing level of yellowish stain. In addition, there is both an overall loss of contrast and a discoloration caused by stain that is most objectionable in highlight and low-density areas 

.... A further feature of dark fading versus light fading is that a dye with good stability in the dark may be comparatively unstable when exposed to light. In Kodak Ektacolor papers, for example, the magenta dye is the most stable of the three dyes in the dark, but is the least stable in light under typical indoor display conditions."

"Because color negatives are not viewed directly, but rather are used to make prints, analysis of color negative fading (and the ramifications of d-min stain or density losses) in the future will be based on the effects they have when printed. A certain amount of negative density loss and color imbalance can be satisfactorily adjusted for during printing, but more severe negative deterioration cannot. Historically, both still camera and motion picture color negative films have had particularly poor dark fading stability - the logic being, one might suppose, that most color negatives are printed soon after processing so that fading of the negative in later years will not matter in most cases. ...

Kodachrome clearly is the most stable transparency film in dark storage; the film is especially outstanding in terms of its total freedom from yellowish stain, even after extended aging. In spite of Kodachrome's unequaled dark storage stability, it has the worst projector-fading stability of any slide film on the market."

As can be seen from the diagram below,  the changes in density are also a function of the original starting density, so that appearance varies considerably from image to image.  Correction methods must adapt to this conditions.

Dark fading and staining usually produces a yellow-orange cast which CastCor can reduce with several different methods.

Light fading usually produces a blue-green cast along with an overall lightening of the image which CastCor can reduce with the ACE method.

In addition to defects due to age and dye changes, gray cast due to fog or poor visibility can also be reduced with ACE and Sharpen.

© Rog Palmer