SOLARIZATION, B S D PHOTOGRAPHIC REVERSAL BY DESENSITIZERS” BY B. H. CARROLL
Several times in the history of photography, it has been reported that red and infra-red radiation tends to reverse the effects of shorter wave-lengths on the photographic plate. The undoubted existence of other photochemical reactions in which there is an antagonistic action of radiation of different wavelengths, tends to support this by analogy. Draper and Herschel reported it lor the daguerreotype,Raterhouse for the wet collodion plate’ and it is important to note that Waterhouse found that the reversing effect was greatly increased by staining the plates with “aniline blue” or “annatto,” and that his photographs of the solar spectrum in the red and infra-red were taken with stained plates. Abney2 supported Waterhouse in his theory that red light has a specific reversing effect, but this theory is denied by two of the highest authorities on photography : Liippo-Cramer3, and Eder4. Yeither of them was able to confirm the results of JVaterhouse when unstained plates were used, and Bancroft; concluded that the effect is simply solarization. Kevertheless, the existence of a specific reversing action of long wavelength radiation on szluer brom ide-gelatine emulsions has been assumed by recent investigators in spectrum photography. Furthermore, experimental evidence on spectral sensitivity for solarization is very limited. Assuming long wave-length radiation to be the most effective in producing reversal, this might indicate either the existence of a specific reversing action as mentioned above, or that the maximum sensitivity for solarization lay in the red or infrared. -411 authorities agree that fast, coarse-grained emulsions are most readily reversed; tests were therefore made with four of the fastest obtainable, namely : Eastman Speedway (Seed Graflex), Cramer Hi-Speed, Eastman SuperSpeed Portrait Film, and Lumi&re Sigma. For spectographic tests, a Hilger E3 spectrograph, with quartz lenses and prism, was used. The slit width, unless otherwise stated, was 0.04 mm.; the source of light a I O O watt gas-filled tungsten lamp; the filament of which was focussed on the slit by a quartz lens of 40 mm aperture and 9 0 mm focal length. Under these conditions, a strong spectral image extending from 0.35 to 0 . js p was produced on all the emulsions by an exposure of one second; while longer exposures gave a range from 0.31 to 0 . 7 p. ITsing backed plates, the four *Published by permission of the Director of the Bureau of Standards of the U. S. Department of Commerce. Waterhouse: Proc. Roy. Soc. 29, 186 (1875). *Phot. J. 32, 318 (1908). Luppo-Cramer: Phot. Corresp. 39, 134 (1902); 46, 344 (1909). Eder: Beitrage zur Photochemie und Spektralanalyse, 2, 172 (1904). Bancroft: J. Phys. Chem. 14, 303 (1910).
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and exposed it behind an Eder-Hecht scale]. On physical development with acid metol-silver nitrate intensifier the plate showed a negative image up to 3 2 on the scale. From 32 to 80 there was a faint reversed (positive) image on the darker ground due to storage fog. Further experiments proved that when latent image on the plate had been produced by light it could be destroyed in the same way as that due to storage. It was at first difficult to obtain the effect with alkaline development; but, if the dye bath is followed by I-2y0 potassium bromide solution, the reversal on silver bromide with alkaline development is much enhanced. Luppo-Cramer was unable to secure reversal on fast plates. A large number of desensitizing dyes were effective, and even some sensitizers such as Rhodamine €3 and erythrosin, when used on silver iodide with physical development; certain of the dyes which produced satisfactory reversal under these conditions were not effective when used on silver bromide with alkaline development. A high relative sensitivity to yellow light was noted. It accordingly becomes evident that dyes which desensitize the photographic plate strongly, restraining the formation of latent image under moderate illumination, are capable of destroying or deactivating that already existing, when sufficient exposure is given. With this in mind, we may consider photography of infra-red spectra by the reversal method. This was first attempted by Millochau2. Using preexposed Lumikre Sigma plates which had been bathed with a saturated alcoholic solution of malachite green, he was able to obtain a reversed image of the solar spectrum to 9325 A.U. Terenin3 has recently improved the method by reducing the concentration of the dye to around I : 2 0 , 0 0 0 (amounts similar to those used in sensitization with cyanine dyes). He was successful with a number of basic dyes of the azine and triphenylmethane groups. In spectrographic exposures, a direct image was produced by light of wave-length below a definite limit which depends on the dye, and reversal above this. Terenin recognized that the difference between dyes is chemical as well as optical. He reported securing a faint image of the mercury line at I I 2 8 0 A.C. on prolonged exposure, but it has been impossible to confirm this. It is evident that the results obtained by Millochau and Terenin depend on a reversal of the same type observed by Luppo Cramer, since the dyes used are known desensitizers. The spectral characteristics should depend on the dye in a manner similar to sensitization in the ordinary sense; as would be expected, Terenin’s best results were obtained with green dyes which have an absorption band, with maximum between 0.6 and o . ~ pand , extending into the infra-red with diminishing intensity. Although solarization takes place most readily with rapid, coarse-grained emulsions (leading Millochau and Terenin 1 The Eder-Hecht “sensitometer” is a neutral wedge, of density radient (Kehlkonstante) 0.4014 per cm, on which there is a millimeter wale; it is also provifed with strips of tricolor and yellow (correction) filters. I t is accurately made, and valuable for preliminary measurements. * AIillochau: Compt. rend. 143 108 (1906); 144, 723 (1907). Terenm: P h y i k . Rer. 4, 1478 (1923).
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to use these), it appears that the opposite type should be most sensitive to reversal when dyed with a desensitizer. In the first place, the amount of dye taken up is much larger, due to the larger specific surface of the grains. For this reason, fine-grained plates show much better relative color sensitivity’ when bathed with ordinary sensitizing dyes, and the same may be expected of the reversal process. In the second place, the natural sensitivity of the emulsion, while greatly diminished by a desensitizer, is not totally destroyed, and fastjplates are relatively less reduced in sensitivity; so that the reversal is opposed by much less residual sensitivity in the case of the fine-grained plate. My first experiments established conclusively that, using pre-exposed plates bathed with desensitizing dyes, with or without bromide, the reversed image was formed on the finest-grained plates with the least exposure. When Speedway and Process plates were pre-exposed, bathed with Desensitol and KBr, and then exposed for the same length of time behind the Eder-Hecht scale, the reversed images extended to 3 2 and 90 respectively. The corresponding ratio of the light intensities necessary to produce an image on the two plates is approximately I 7 5 :I. As the original motive of these experiments was to extend the range of photographic sensitivity further into the infra-red, attention was concentrated on dyes and inorganic compounds which seemed likely to have absorption in the proper region. Desensitizing dyes are largely basic, and confined to the azine and triphenylmethane gioups; from the great number available, choice was made according to the absorption spectra and desensitizing properties, as far as known or predicted. After preliminary experiments to determine the optimum pre-exposure, the dye tests were carried out as follows. Seed 2 j plates were pre-exposed 10-12 seconds to I .4 candle-meters (unscreened tungsten) ; process plates 15-20 seconds to the same intensity. They were then bathed 5-10 minutes at room temperature in the dye solution, at a concentration of I : 2 0 , 0 0 0 unless otherwise stated; rinsed and dried rapidly in a current of air; again bathed for ten minutes, this time in a 1% solution of potassium bromide, again rinsed and dried. Using the spectrograph and light source previously described, one exposure was given without filter, and one with a Jena red glass transmitting practically no radiation below 0.6~.Exposure times varied from one to five minutes, and slit width from 0.04 to 0 . 2 0 mm. on the first tests. The results with dyes are summarized in Table I. Dyes are listed by name, and by number and class in the 1924Colour Index of the Society of Dyers and Colourists. “Desensitol” and “Pinakryptol Green” are the products of Ilford, Ltd., and Meister, Lucius and Bruning, respectively. The former is undoubtedly a safranine. Bindschedler’s Green was synthesized for the purpose in this laboratory. The other dyes were commercial samples in possession of the dye laboratory of the Bureau of Standards. Walters and Davis: Bureau of Standards Science Paper, No.
422.
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