Ferdinand Hurter (1844–1898) and Vero Charles Driffield (1848–1915) are universally regarded as the fathers of sensitometry, the science that studies the behaviour of light-sensitive materials.

Both were scientists who had become friends and shared a passion for photography. However, they lived in an era when photographic practice was largely based on empirical rules. There were no exposure meters, and exposure was determined through more or less subjective assessments. Negatives were developed “by sight,” using a safelight, for just long enough to allow the image to appear.

Hurter and Driffield did not share such an approximate approach and sought to address the issue with scientific rigour. They conducted numerous studies and experiments to determine the extent to which the density of a negative was related to light exposure and development time.

The result of their research was the article Photo-Chemical Investigations and a New Method of Determination of the Sensitiveness of Photographic Plates, published in The Journal of the Society of Chemical Industry in May 1890. This article, later included in The Photographic Researches of Ferdinand Hurter and Vero C. Driffield¹, begins with the statement:

“The production of a perfect picture by means of photography is an art;
the production of a technically perfect negative is a science.”

— Ferdinand Hurter and Vero Charles Driffield

Since any scientific theory relies on the measurement of physical quantities, it was first necessary to define a method for measuring the density of a photographic plate—that is, how much light it could transmit through transparency (the higher the density, the less light is transmitted). To this end, the article describes an ingenious instrument that allowed for the measurement of this density through visual comparison.

Additionally, a reference illumination was required, which was chosen as that produced by a standard candle at a distance of one metre. This made it possible to define a standard exposure unit, called the candlemeter-second (C.M.S.).

At this point, Hurter and Driffield conducted numerous tests, exposing plates at various levels of exposure and, after development, measuring their densities. This enabled them to rigorously and repeatably determine the relationship between the amount of exposure received by a negative and its resulting density. Hurter and Driffield also investigated how this relationship varied depending on different developing formulas and the varying amounts of silver salts deposited on the photographic plates, in some respects anticipating the concept of sensitivity—that is, the modern ISO.

Hurter and Driffield’s study laid the theoretical foundation for the determination of the characteristic curve, which had already been devised a few years earlier by James B Spurge², but which would become better known as the Hurter-Driffield curve (or D-log H curve).

References

1. http://www.archive.org/details/memorialvolumeco00hurtiala, The photographic researches of Ferdinand Hurter and Vero C. Driffield – Memorial della Royal Photographic Society of Great Britain, edited by W.B. Ferguson, 1920[↩]
2. https://rps.org/shop-items/mr-driffield-dr-hurter-their-lives-times-in-pictures/, Ronald M Callender, FRPS – Mr Driffield and Dr Hurter, Their lives & times in picture – A valuable booklet resulting from the meticulous research conducted by Ronal M Challender FRPS on the lives and work of Ferdinand Hurter & Vero C. Driffield. Published by the Royal Photographic Society and available for purchase online[↩]