American cinematographer (Jan-Dec 1926)

Kighteen AMERICAN CINEM ATOGR APHER June, 1926 DESENSITIZERS (Continued from Page 11) to inspect it satisfactorily during development, even though the sensitivity may be slightly decreased when wet with developer. Motion picture negative film, on the other hand can be inspected quite freely with a red light such as is given by the Series 1 safelight. 2. Desensitizing. A. Purpose of a Desensitizer. A photographic desensitizer is a substance which has the property of greatly diminishing the sensitivity of a photographic emulsion toward light action. To be of practical use in development it must not affect a latent image already present nor interfere with its subsequent development. . . The most important reasons for using a desensitizer are: (a) to permit the inspection of panchromatic film during development, (b) to give much greater freedom in the use of safelights during the development of ordinary film, and (c) to prevent serial or oxidation fog. In a previous communication3 it has been shown that the presence of a desensitizer in a concentration of 1-500 000 or even 1-1,000,000 in a developer which has a tendency to produce aerial fog, is sufficient to prevent such fogging action. This is of considerable value in the machine development of motion picture film, and tor such use it has been found possible, by adding one part in a hundred thousand of phenosafranine, to use a dilute elon-hydroquinone developer with much less sulphite than would otherwise be necessary. It has been stated in the literature that in some cases a desensitizer also diminishes ordinary development or tank fog. This effect may be a decreased oxidation fog within the developer, but with certain developers tank fog is apparently diminished. This is discussed more fully in another section. In the present paper it is proposed to show the extent to which the use of a typical desensitizer will permit greater safelight illumniation during development. B. Methods of Use. Densensitizing dyes are used either as a preliminary bath or in the developer itself. As a preliminary bath a concentration of 1-5000 or 1-10,000 is commonly used, and the film is dipped in the desensitizing solution for one or two minutes just previous to development. This operation must, of course, be carried out with proper safelights or in the dark. When used in the developer the concentration usuallv recommended is 1-25,000 or less and the film is left in the developer for one or two minutes before exposing it to a safelight stronger than usual. In most cases the same concentration of dye desensitizes much more powerfully in the developer than in a separate water solution.'' C. Considerations in Selecting a Desensitizer. Many dyes and other substances are known which greatly reduce the sensitivity of emulsions. However, in finding a substance suitable for practical use there are many factors involved, the most important of which will be considered briefly. 3 Merle L. Dundon and J. I. Crabtree, "Investigations on Photographic Developers, II, The Fogging Properties of DevelopAmer Phot. 18, 742. (1024), Rev. Franc. Phot. 5, 320, 71, 701. 1. Desensitizing Power. Desensitizing power is, of course, the first consideration. With desensitizers now known the speed of an' ordinary fast emulsion to white light can be reduced several hundred times, while the decrease in sensitivity of panchromatic emulsions to certain safelights may reach several thousand times. Different desensitizers vary considerably in their ability to decrease the relative color sensitivity of panchromatic materials, and this variation also depends on the particular dyes used to give color sensitiveness to the emulsion. 2. Effect on the Latent Image. To be of practical use a desensitizer must not remove to any extent a latent image already present on a film within a reasonable length of time. Most desensitizing dyes will destroy a latent image if the desensitized film is exposed to strong red light, and Carroll6 has reported that pinakryptol green will destroy a latent light image even in the dark if allowed to stand several hours, before development. This fact has been confirmed in this investigation. 3. Effect on Development. Desensitizing dyes generally decrease the induction period of certain developing agents such as hydroquinone and pyro and so may change the Watkins factor of a developer. Some desensitizers retard development. It is, of course, desirable that the addition of a desensitizer shall not affect the time of development nor change the shape of the characteristic curve of the developed image. 4. Fogging Action. Some of the most powerful desensitizing substances known, such as methylene blue, have an independent fogging action which entirely prevents their use for this purpose. No appreciable fogging action can be tolerated, although certain commercial desensitizers have a slight tendency in this direction. J). Staining Action. •Some of the desensitizers in use stain not only the gelatin of the film and the trays in a very disagreeable manner, but also the fingers of the person who uses them. The stain is most persistent in the hardened gelatin on the back of a non-curling film. A desensitizer which does not stain gelatin or which washes out very easily is desirable. The color or absorption region of the stain produced is also of importance because if it does not transmit blue light the printing time of a stained negative may be affected. 6. Color in Relation to Safelight. If desensitized films are to be inspected by transmitted light during development the color of the desensitizer with which a film is stained must be such that it does not absorb the light transmitted by the safelight. Otherwise the whole film will appear fogged or too dense to examine satisfactorily. For instance, phenosafranine appears black in a green light and colorless in a red light. 7. Solubility in a Developer. The concentration in which desensitizers can be added to a developer is often limited by the fact that they form an insoluble precipitate with certain developing agents, especially hydroquinone. In extreme cases a precipitate may form in an emulsion when it is put into a developer after a preliminary desensitizing bath. i. i 1 925), B. .1. Phot. ers (1924), Sci. Ind. Phot I 10 2 4). 4. A. E. Amor, "The Prevention of Tank Fog." B 72, 183, (1925). 7 10. J. Phot. 5. A. Huble. "Contributions to Development in Blight Light." Phot Rund. 02, 1 1 4. ( 1025). 6. B. H Carroll. "Solarlzation and Photographic Reversal by Desensitizers," J. Phys. Chem. 29, 093, (1925).