American cinematographer (December 1933)

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310 American Cinematographer • December 1933 A LTHOUGH the beginning of the photographic in- vestigation of the Red and Infra-Red regions of the spectrum dates back to almost half a century, it has been only in the last fifteen years that it has emerged from the research and experimental laboratory and has en- tered the field of practical photography. Modern physics has shown a marked tendency to express by the term “Electromagnetic Radiations” those radiations that travel through space with the velocity of Light and according to the wave theory of propagation. The Electro Magnetic Spectrum comprises radiations ranging from a wave length of a few tenths of a millimicron to that of several miles. At the shorter wave length end of the Spectrum are the Gamma Rays used in Radiology and the still shorter Cosmic Rays which have been brought to public attention in latest years mainly through the investigations of Dr. Millikan of the California Institute of Technology. At the long wave end of the Spectrum are the waves used in wireless and the still longer waves generated by rotating a coil in a magnetic field. A very small portion of the Electromagnetic Spectrum comprises the Radiations that produce the sensation of Light and is commonly known as the Visible Spectrum. These Radiations are confined within the limits of wave lengths from 393 to 759 millimicrons, from the Oxygen A to the Cadmium K Fraunhofer lines. Beyond the two limits of visible radiations are other in- visible radiations which can be detected photographically and which are termed Ultra-Violet or Infra-Red radiation according if their wave length is shorter or longer than that of the limiting visible radiation. The short wave portion of the invisible Spectrum that can be detected through its photochemical action extends to wave lengths as short as approximately 0.006 millimi- crons (X-Rays) while the long wave portion is nowadays limited to wave lengths of approximately 1129 millimi- crons, in the neighborhood of the Mercury line correspond- ing to the 1 128.8 millimicrons wave length. From the very inception of photography, it was recog- nized that the photochemical effect upon the silver halides was limited in the longer wave length end of the spectrum to little beyond the 500 millimicrons wave length and there- fore the yellow radiations between 587 and 589 did not have any effect upon the light sensitive material. It was in 1873, before the introduction of gelatine emul- sions, that Vogel, while experimenting with a view to elimi- nate halation, mixed some dyes with the Collodion emulsion Photography then in use and discovered that this would extend its sen- sitivity to the yellows. Corallin was the first dye used, but by extending his experimentation to the use of a num- ber of dyes he proved that the emulsion was rendered sen- sitive to the radiations that were absorbed by the dyes and in 1875 he discovered the sensitizing properties of Cyanin which extend into the Red region of the Spectrum. The road was open to the Science of Orthochromatics and other experimenters followed it, especially with a view to discover dyes that would have the strongest sensitizing effect together with good keeping qualities and a minimum of fogging propensities. Gelatine emulsions made their appearance, and in 1882 Attout Tailfer introduced the use of Eosin and Erytrosin in conjunction with Ammonia which sensitized the photo- graphic plate for the yellows and green-yellows. Shortly afterwards, Vogel succeeded in sensitizing the emulsion for both the yellow-green and the orange-yellow regions, by mixing Quinoline-Red and Cyanin, but it was only some ten years later that an incursion was made in the Infra-Red region of the Spectrum by Higgs, who, for the scope of photographing the Solar Spectrum, used Coerulin and reached wave length 840. It was at about this time that Eder made known the results of his investigations in the field and expressed the essential requisites and facts to support the theory of dye sensitizing confirming Vogel’s assertion that the sensitiz- ing power of a dye is stronger for the radiations that it absorbs more readily and that the stability of the dye itself has little or no influence on its sensitizing power. Researchers were, however, confronting the fact that all dyes known at that time had a tendency to produce con- siderable fog and also that the plates so prepared had a very limited keeping quality, which almost confined them to the scientific and research laboratory, their commercial application being limited by the knowledge, the patience and the application of the photographer. However, the discovery of the dyes of the isocyanine group first introduced by Miethe (Ethyl Red) and followed by the Pinachrome, Pinaverdol and Orthochrome T discov- ered by Konig in 1904, brought orthochromatics into the commercial realm because of the reliability of their action and of their sensitizing effect in the yellow and orange regions as well as in the green, to which 1 color all emulsions show a definite lack of sensitivity. Two years later in 1906, Pinacyanol, a Red sensitizer of the carbocyanine group, was introduced by Homolka and its discovery marked the advent of Panchromatic emulsions because, when mixed with a green sensitizer, it extended the sensitivity of the of the photographic plate to all the colors of the visible spectrum. In the same year the discovery of Dicyanine permitted a definite entrance into the Infra-Red region of the spec- trum since by using it in conjunction with Ammonia, it was found possible to reach a wave length of 1000 milli- microns and even the Mercury line 1014. Dicyanine did not, however, prove very satisfactory, mainly because of the poor keeping quality of the plates sensitized with it. It was only in 1919 that Adams and Halle discovered Kriptocyanin, a dye that can be either incorporated in the