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CREDIT: Simon Hill HonFRPS

The Science Behind Ilford Multigrade

Dr Tony Kaye ASIS FRPS

Having read Simon Hill’s news article about Frank Forster Renwick HonFRPS and his work on Ilford Multigrade Paper you may be wondering a little about the imaging science that underpins how Ilford Multigrade paper works and how it has evolved since its introduction in 1940.

Silver halide emulsions are inherently sensitive to blue light. To make them sensitive to light of longer wavelengths sensitising dyes need to be added during their manufacture. In 1873 Hermann Wilhelm Vogel found that adding aniline dyes to freshly prepared emulsions could extend the sensitivity of the emulsions to the longer wavelengths of green and red light. In 1912 Rudolf Fischer patented the idea of using both blue sensitive emulsions and green sensitised emulsions of different contrasts to produce a black and white paper that varied in contrast depending upon the colour of the light used to expose it.

It took the work of Frank Forster Renwick at Ilford to commercialise the first multigrade paper in 1940. He used an unsensitised lowish contrast emulsion that was only sensitive to blue light mixed with a high contrast emulsion that had been green sensitised through the use of an appropriate sensitising dye. Thus, depending upon the relative amounts of green and blue light used at the time of printing the contrast of the paper could be varied. This was achieved via the use of filters. At the time of introduction there were four filters, a blue one and three yellow ones of increasing density. The use of the blue filter would yield the lowest contrast as no green light would expose the high contrast green sensitised emulsion. As the density of the yellow filters went up you would progressively get less blue light exposing the unsensitised emulsion and more green light exposing the high contrast green sensitised emulsion, with the darkest yellow filter producing the highest contrast result.

Since 1940 Ilford have refined considerably their multigrade paper. It was on its fourth generation when Ilford discontinued it in 1969. In 1978 Ilford reintroduced the product to the market and it has continued to evolve since then. Since 1994 Ilford have been using three emulsions and not two as in the first-generation papers. The way current Ilford Multigrade paper works is similar to but in some senses very different to the original paper introduced by Frank Forster Renwick. The similarity is that blue sensitive and green sensitised emulsions are used, but the difference is that all the emulsions are now of the same contrast. Quoting from the Harman Technology Ilford Technical information sheet:-             

MULTIGRADE papers are coated with an emulsion which is a mixture of three separate emulsions. Each emulsion is a basic blue sensitive emulsion to which is added different amounts of green sensitising dye. Thus, part of the mixed emulsion is sensitive mainly to blue light, part to blue light with some sensitivity to green light and part to both blue and green light. All parts of the emulsion have the same contrast. They also all have the same speed to blue light, but naturally, the part of the emulsion with only a small amount of green sensitising dye has a low speed (that is, is less sensitive) to green light. When the paper is exposed to blue light, all parts of the emulsion react and contribute equally to the final image. This image is of high contrast because of the additive effect produced by three emulsions with the same speed and contrast. The resultant curve has a narrow exposure range and is thus of high contrast.

When the paper is exposed to green light, only the parts of the emulsion with the larger amounts of green sensitising dye react initially. This is because the three emulsions have very different sensitivities to green light. This image is of low contrast because of the additive effect produced by three emulsions with different speeds to green light, but with the same inherent contrast. The resultant curve has a very much wider exposure range and is thus of low contrast. By varying the proportion of blue to green light, a contrast range between these two extremes can be obtained.

Like the first-generation paper the colour of the light reaching the emulsion during exposure is modified by the use of filters. However, as the density of the yellow filter goes up the contrast now goes down unlike the first-generation paper where the highest density yellow filter produced the highest contrast. So while the implementation may have changed, the underlying principle hasn’t, the use of emulsions sensitised to different wavelengths of light can be combined to produce a multi-contrast emulsion, where the colour of the exposing light governs the contrast.