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)|
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.
- ANSI IT9.11-1993 Standard Processed Safety Photographic Films – Storage