American cinematographer (Jan-Dec 1931)

Laboratory Department Conducted by EMERY HUSE, A. S. C. Principles of Sensitometry and Their Practical Application [This is the second installment of the instructive article on practical application of sensitometry which has been written by Mr. Huse, Technical Editor of this Journal. The first part appeared in the May issue. — H. H.] |T WOULD be well at this point to give some consideration to I the processes of film manufacture. In the preceding article mere mention of the discovery of film base was made, but it is of major interest to know those things which go into the manufacture of film base. The base of all film is a cellulose product and the most necessary and important ingredient in it is cotton. One of the first steps in preparing film base is the washing and drying of the raw cotton, which process takes weeks. Following this washing process the cotton is treated with a mixture of nitric and sulphuric acids, which process renders the cotton soluble in alcohol. This treatment gives us what is technically known as "cellulose nitrate." This treatment with the two acids, although it does not alter the physical appearance of the original cotton, does, however, change it chemically so that it will be soluble in the various mixtures which would have no effect upon the unnitrated product. Wood alcohol is the chief solvent used in this process. This action again covers a rather long period and the solution which finally results is a relatively clear one having the consistency of honey. This solution, often referred to as "dope," is then poured on the surface of great polished wheels which run consistently night and day. These are the dope casting machines and give film base in sheets which are approximately 40 inches wide and 2000 feet in length. The standard thickness of film base is from .005 to .00525 of an inch, and the degree of accuracy obtained is such that the variation does not exceed .00025. This base is handled in large rolls in a manner similar to which rolls of printing paper are handled, and in this condition is sent to be coated with the light sensitive emulsion. The photographic emulsion is that light sensitive surface which reacts to the action of light. This emulsion, of course, can be varied in its chemical setup to give the various types of film available on the market — positive, negative, duplicating, and sundry others. The chief element in the emulsion is silver. The bars of silver are dissolved in nitric acid in porcelain dishes and after crystallization pure crystals of silver nitrate are obtained. Other ingredients of the emulsion are potassium iodide, potassium bromide, and gelatin. Gelatin is the substance in which the ingredients of the emulsion are prepared so that this emulsion may be subsequently coated onto the film base. Photographic gelatin is usually prepared from calf skin by soaking the skins in lime water and subsequently extracting with hot water. The gelatin is dissolved in water and the bromide and iodide solutions carefully mixed with it. This mixture, heated to the correct temperature, has added to it the silver nitrate solution. The precipitate of the sensitive silver salt is held in suspension by the gelatin, and because of this the term "emulsion" is derived. The foregoing has been discussed for the prime purpose of giving the reader some idea of how photographic emulsions are made. An Historical Review of Sensitometry Sensitometry literally means a measure of sensitivity. As early as 1848 Claudet devised an instrument for determining the speed of the daguerreotype plate, which instrument was termed a "photograph meter.'" By the aid of this meter one was able to determine the exposure necessary to produce a visible impression on the sensitive material. This method was extremely crude and was not very reliable, but it no doubt laid the foundations for the work which was carried on some years later by two men in England, Hurter and Driffield, who were amateur photographers, but whose prime interest in photography was the production of images which were true to nature. In January, 1891, Ferdinand Hurter states in the opening sentence of his paper, "The Action of Light On the Sensitive Film," that "the function of photography is the production of permanent images of material objects as true to nature as possible." Hurter's use of the words "sensitive film" must not be taken literally, as he used the word film to represent that layer of sensitive material which was coated on a glass plate. Ferdinand Hurter was a Swiss who began the study of chemistry at an early age, which later led him to be apprenticed to a dyer, in which practical field of chemistry he achieved notable success. He went to England some years later where he eventually became chief chemist and technical adviser of the United Alkali Company. Vero C. Driffield, an Englishman, though intending to become an engineer, became interested in the practice of photography. His engineering studies, however, led him eventually to join the same firm with which Hurter was connected, and the two men became great friends. Hurter acquired his interest in photography due to Driffield's continual experiments in this general field, and for several years these two men worked together in an attempt to study the underlying principles of the action of light on a light sensitive material. It must be remembered that at this time the collodion plate was practically the only sensitive material at the disposal of the photographer. It was known generally that the photographer had to expose his plate to suit the light, and great difficulty was experienced in the early stages of photography in the estimation of the correct exposure. Naturally, there was much guess work connected with photography of that day. Hurter and Driffield's first problem, as they saw it, was to devise some means of accurately measuring the actinic power of daylight. This work led to the discovery of their actinometer, data on which is incorporated in a specification drawn up by Hurter on the 23rd of April, 1881. For several years the attention of these two men was absorbed by the general subject of actinometers. In May, 1890, the first joint work of Hurter and Driffield was published under the title "Photochemical Investigations and a New Method of Determination of the Sensitiveness of Photographic Plates." This paper led to a discussion of negative density, opacity, and transparency; means of measuring densities; study of development; gradation, which was referred to by these men as the "ratio of the densities," intensification, and reduction; ending finally with speed determination of sensitive plates. It was Hurter and Driffield who devised the means of graphically showing the action of light on a photographic emulsion by plotting density produced on a negative against the exposure causing these densities. This constitutes the origin of the so-called H and D curve, which letters refer specifically, of course, to Hurter and Driffield. 21 j