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Modeling Light Fading of Hard Copy Media

Conventional chromogenic photographic papers have evolved over a period of at least fifty years and much research has been carried out to improve their light stability. One of the earliest technologies for obtaining photographic quality output from digital data was by the thermal dye diffusion process. Original versions of this medium had very poor light stability when compared with chromogenic media and this limitation was also found in ink jet printing systems. Whilst much effort has been given to providing more light-stable hard copy output from digital media, little work has been published on modelling their light fading characteristics. This paper is concerned with the provision of a simple model for the light-fading kinetics of a layer consisting of a molecular dispersion of dye in a transparent binder. It is assumed that the product of fading is nonabsorbing and that reflection is ideal. The results obtained using this model are compared with those obtained experimentally for a selection colorants and substrates. The model provides effective quantum yields for light fading as a means of bench marking and predictions of the fade curves of density plotted against time. Modifications to the proposed model are discussed which have become necessary due to deviations between predicted and measured fading curves and better agreement obtained. These modifications predict that enhanced stability to light can be obtained by assuming that the image microstructure is composed of lines or hard-edged dots and gaps whilst increased fade rates occur if the microstructure is of uneven or noisy density.