Colour Sensitivity

Ordinary

All of the early processes were principally sensitive to the blue/violet end of the spectrum. In contrast the human eye is most sensitive to yellow/green light. As a result a false, or at least different, colour rendering was produced in a photograph; blue objects appeared much lighter and yellow objects much darker. The first gelatine dry plates suffered from this problem, after the introduction orthochromatic plates, the older emulsions continued to be produced, generally known as 'ordinary'.

Orthochromatic

These plates, also called Isochromatic, were sensitive into the green, and in part yellow, spectrum range. The idea of using sensitizers (dyes) to increase the range of colour sensitivity was first announced by Dr. H.W. Vogel in 1873. The first ready-sensitized gelatine silver bromide plates to be sold were by Attout, trading as Tailfer and Clayton, in 1882/83. In Britain these plates were produced under licence by B.J. Edwards. Prior to this the sensitizing chemicals had to be applied by the photographer. J.M. Eder published details of an improved sensitizer, erythrosin, in 1884, soon after this orthochromatic plates were sold by most of the major manufacturers.¹

Self-screen Ortho

This referred to plates that contained an additional dye to balance colour rendering, other orthochromatic plates required the use of a yellow filter on the lens.²

Vogel's Discovery of Optical Sensitizers

Vogel was working with collodion dry plates produced by Colonel Stuart Wortley, he noticed that the sensitivity extended further than normal into the green range of the spectrum, thinking this was due to the developer used he carried out tests that proved negative. He then isolated the coating applied to the plates as an anti-halation measure as being the cause. Along with a number of preservatives the coating contained a dye, corallin, to prevent light penetrating through the film to the glass plate. It was this dye that caused the additional colour sensitivity.

Vogel's announcement of optical sensitizers, as he called them, met with a great deal of hostility, other photo-chemists carried out experiments and were unable to reproduce his results. Carey Lea published an extensive report on experiments he carried out, this was published in the British Journal of Photography (BJP) as 'On Reduction by Light as Influenced by Colour'. John Spiller also found results contrary to Vogel's as did, most importantly, Monckhoven. This led to some quite nasty comments being published in the BJP, a regular columnist referred to Vogel's 'foolish notion' another said he had not understood basic experimental results. Late in 1874 Becquerel, using chlorophyll as a sensitizer, confirmed Vogel's experiments. Vogel made the important finding that the sensitizer acted in addition to the original sensitivity of the plate, thus creating a second peak of sensitivity rather than stretching the existing sensitivity. Shortly after his use of corallin he used aniline green to extend the sensitivity into the orange area.³

In 1875 Major J. Waterhouse announced the use of eosin as a colour sensitizer which, from then on, was widely used.

Panchromatic

This is emulsion sensitive into the red end of the spectrum. Vogel had produced near-panchromatic plates called Azaline in 1884 using cyanine as a sensitizer, these were slow and required a dark filter be used on the lens. Sensitizers, including ethyl red, were produced by Miethe & Traube in 1902, from when panchromatic emulsions can be said to exist. Plates using their sensitizers were sold by Perutz and by Wratten & Wainwright (1906). It was, though, many years before panchromatic emulsions replaced orthochromatic.⁴