1 By: Dr. Drew Myers, Chemistry Coordinator Consultant in Surface, Colloid, and Polymer Science myersd@itc.com.ar CHEMISTRY OF PHOTOGRAPHY While it is easy to make comparisons between …

While it is easy to make comparisons between the pupil of the eye and the f-stop of a camera or between the retina of the eye and photographic film, once we get past the basic similarities of the optics of the two systems comparisons begin to rapidly break down. The eye is not only much more complex than a camera and its film, but the two imaging devices function by chemically different mechanisms. The photographer (or the automatic exposure system of the camera) regulates the f-stop opening and time of exposure of her camera to match the sensitivity of film, while the iris and retina sensitivity of the eye adjust to correspond to the light level of the scene. While science is slowly putting together the pieces to explain the functioning of our vision system, the basic nuts and bolts of classical photography have been known for years, although certain details remain the subject of some discussion. Just as in the human eye, classical photographic systems are composed of two separate, but interrelated processes – the basic black and white image structure and the finer points of color reproduction. This first installment on the chemistry of photography is intended to introduce in a simplified way the basic concepts of silver halide photography. It will not delve into the physics of optics, the functioning of cameras and lenses, photographic techniques, non-silver processes, or the artistic aspects of photography. Nor will it go beyond a cursory mention of color photographic processes, which will be left for the future. Anyone interested in more detail is referred to the selected bibliographic material cited at the end. A Brief History of Black and White Silver Halide Photography. Perhaps the earliest reference to the concept of silver-based black and white photography is that of J. H. Schulze who observed in 1727 that a mixture of silver nitrate and chalk darkened on exposure to light. The first more or less permanent images were obtained in 1824 by Nicéphore Niepce, a French physicist, using glass plates coated with a dispersion of silver salts in bitumen (a coal derivative). In the early 1830’s, Niepce’s partner, Louis Daguerre, discovered by accident that mercury vapor was capable of developing an image on a silver- plated copper sheet that had been previously sensitized by iodine vapor. The image, which was called a daguerreotype, could be made…

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