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A photographic image is comprised of small discrete particles of dye, silver metal or silver compound held in suspension in the gelatin binder.
This type of image forming material is created during processing. During development the developing agent oxidises in a reaction with the exposed silver halides. The oxidised developer then further reacts with a coupler to form a dye, fig 5.1.
The dyes act by absorbing light, as energy, and moving this energy through the bonds in the dye molecule. Dyes that absorb blue light have longer chains, with the chain length becoming progressively shorter as the dyes absorbance shifts towards the red end of the spectrum.
In practice the dyes do not precisely absorb all the specific wavelengths and may have secondary absorption characteristics. To improve the performance of the dyes 'masking layers’ are incorporated in colour negative materials. The masking layers use dyes that are also formed during processing. This is noticeable as an orange appearance to colour negatives.
Dye fade occurs when one or more of the bonds between the atoms in the dye molecule are broken. This can be as simple as a double bond becoming a single bond and does not require the molecule to divide in to smaller, different chemicals.
This is the difficulty in restoring colour chemically, targeting the particular bonds that have broken, which may not be at the same bond location on all of the same colour dye molecules within the image. The resultant chemicals formed during fade may still have some dye effect but not of the intended colour. This may be noticed as a stain or discolouration within the image.
Typically, the colour film dyes used in motion picture films before the 1980s had very poor stability. Dyes will fade under dark or light storage conditions.
Dark fading is a temperature/relative humidity driven reaction and light fading is additionally influenced by light energy. Even when a photographic image is stored under light conditions the dark fading mechanism will continue.
Concern by users pressured manufacturers to improve the stability of photographic dyes. Dyes developed during the 1980s and 90s have much lower rates of fade. The improvement has led to photographic dyes now being used as benchmarks for other colour technologies such as inkjet and thermal dye transfer colour printers.
As mentioned previously dye fade is largely dependent on temperature and humidity. To maximise the storage life of dye image materials cool and dry conditions are essential. The current ANSI Standard IT9.11-1993 recommendations for extended term storage are given in Table 5.1.
|Maximum Temperature||Relative Humidity(%RH)|
|2||20 - 30|
|-3||20 - 40|
|-10||20 - 50|
table 5.1: ANSI IT9.11-1993 Extended term storage of colour photographic materials.
Exposure during projection is a consideration for dye fade. Although the duration is very short the intensity is great and the heat absorbed by the dense parts of the image can accelerate the fade reaction.
The silver image in motion picture film is formed as a result of exposure of the silver halide by light and its subsequent development. In a developed image the silver forms as thin tangled filaments only a few molecules in diameter, fig 5.3.
Silver is a reactive metal and is readily attacked by oxidising agents and other pollutants, such as acids or sulfurs. The silver metal reacts with these in a variety of ways.
Oxidising agents will form silver oxides which are mobile through the emulsion.
The effect can be noticed when the silver oxides reach the surface and are reduced back to metallic silver, giving the darker parts of the image a metallic sheen when viewed by reflected light. This is known as 'silvering out’ (fig 5.4).
Acids, notably nitric acid formed during the decomposition of nitrate film, will attack the silver to form silver compounds that may be coloured or nearly transparent causing bleaching, (Figure 5.5a). However, acetic acid is considered a weak acid and will not actively affect the silver in the image.
Sulfur compounds will also readily react with the silver to form yellowish or brownish compounds.
Washing during processing is an important step in ensuring the stability of the silver in the image. Residual processing chemicals contain sulfur compounds and, if these are not removed by washing, will rapidly deteriorate the image. Testing the level of residual fixers is carried out by the Methylene Blue Test. This test, however, must be carried out within two weeks of the film having been processed.
The Methylene Blue Test is described in ANSI/ISO 417-1993.1 The standard does not prescribe a specific level of residual chemical that is acceptable for long term storage as the effect of residual chemicals can differ with differing products. From Annexe A Appraisal of keeping characteristics of the standard:
'It is not possible to establish a universally applicable level of residual chemicals that will result in the longest life for all products because of differences among types of products, differences between products of the same type and variations in the combinations of chemicals, even when they contain the same level of residual chemicals.’
Ironically, a certain quantity of residual thiosulfate is required to form a passivating layer over the silver. If all the thiosulfates are removed after processing, the silver is more susceptible to attack from oxidising agents.
1 ANSI/ISO 417-1993 Standard, Methylene Blue Test
Over the years a number of methods have been used commercially to introduce colour to the projected image.
A tint is an even layer of dye added across the image which does not change in colour. Tinting was performed in two ways:
Variations on tinting included handcolouring and stencil colouring the image (Pathecolor). These labour intensive processes were in use until the late 1920s.
Toning used a chemical process to alter or replace the silver metal image with an inorganic compound or a dye colourant. Simple examples of this process is a sepia tone where the silver was reacted with sulfides to deliberately form a brownish silver sulfide image.
Occasionally tints and tones were applied to the same piece of film to produce multicoloured results.
Bipack printing used a stock coated on both sides with an emulsion. The original photography was shot on black and white film through complementary filters (an orange/red and green/blue). The emulsion on each side was separately toned by floating the film across the surface of each toning solution,orange/red and green/blue.
The two layers toned in complementary colours were able to represent a variety of colours on projection.
Dufaycolor was another popular process and was a variation on the earlier Lumiere Autochrome process. Dufaycolor used a panchromatic black and white reversal film with a reseau, or grid, of fine lines of semi-transparent orange, cyan and green printed onto the film base. The film was exposed through the reseau. After reversal processing the projected image gave a good representation of the colours of the original scene.
Lenticular colour gained a degree of popularity in home movies. A filter with three bands of red,green and blue was fitted to the lens of the camera, (Fig 5.11). The lenticluar film was exposed through the base which was embossed with minute cylindrical strips, lenticles, that acted as lenses. Each lenticle formed an image of the banded filter so that the colour separations of each point of the image were recorded.
To view the full colour record the filter was placed over the projecting lens and the projected image would appear in colour, (Fig 5.12b).
The most commercially successful pre-1950 colour process was dye transfer or imbition printing. This process used three dye layers, one for each colour separation, each applied in register to a film base carrier. The best known example of this process is Technicolor.
Because of the wide range of process and formulations used before beginning any treatment on one of these early colour processes testing needs to be carried out on a test section.