INTERMITTENCY AND THE HERSCHEL EFFECT1 CARLETON C. LONG, F. E. E. GERMANN, A N D JULIAN M. BLAIR
Departments of Chemistry aTd of Phgsics, University of Colorado, Boulder, Colorado Received September 6, 1934 DISCUSSION
Many hypotheses have been advanced to explain the effects produced by exposure of photographic emulsions to radiant energy. One of the more fruitful of these hypotheses assumes that the action of light on the photographic emulsion is photochemical, and postulates the coexistence of forward and reverse reactions. The forward reaction causes the silver halide grains in the emulsion to pass from the original undevelopable state to a developable state. The reverse reaction causes the exposed grains to return to an undevelopable state somewhat similar to that which existed before exposure. When light falls on the emulsion, the forward process causes grains to become developable. As soon as an appreciable number of grains becomes developable, the reverse reaction causes some of them to become undevelopable. After the exposure has proceeded for some time, a statistical equilibrium between the two reactions may be reached when just as many grains are being made undevelopable as are being made developable.2 This condition corresponds to the horizontal portion of the characteristic time-density curve, that is, the portion of the curve where further exposure produces no further increase in density. The magnitude of the equilibrium density attained on any given emulsion is a function of the wavelength and intensity of the radiation. In general the greater the intensity of the incident illumination, the greater will be the resulting equilibrium density. The effect of the wavelength of the incident radiation is dependent on the spectral sensitivity of the emulsion. Exposure to blue light produces high developable density on Paper read before the Twelfth Midwest Regional Meeting of the American Chemical Society, held a t Kansas City, Missouri, May 3, 1934. The syst-m of reactions occurring when energy falls on a photographic emulsion is not always as simple as that outlined above. It has been shown that developable grains are not always capable of being returned to an undevelopable state by the reverse reaction. Such grains are said to be i n a “non-reversible state.” The amount of non-reversible density existent at any one time appears to be a function of the total developable density concomitantly existent. We hope t o discuss this relationship and its implication more fully in a future paper.
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ordinary chloride emulsions, while orange or red light produce low developable densities. It seems then that with blue light the forward reaction greatly predominates over the reverse reaction, while with red light this predominance is not so great. If an emulsion is given an exposure to blue light sufficient to produce a relatively high developable density and is subsequently given an expbsure to red light, the density produced by the first exposure may be somewhat reduced by the second. This reduction in developable density by reexposure to light of the longer wavelength is known as the Herschel effect. By means of the above considerations the Herschel effect may be explained3as being brought about by predominance of the reverse reaction during the exposure to red light. Numerous studies have been made of the relative efficacy of continuous and intermittent primary photographic exposures. Intermittent exposures, to the same illumination and for the same effective length of exposure time, have been found in general to be less effective in producing density than continuous exposures, although it has been found that for exposures to extremely high light intensities and for very short intervals of darkness, intermittent exposures may be more efficient than continuous exposures in building up density. This difference in the density produced by intermittent exposures and by equivalent continuous exposures is known as the intermittency effect. As with the Herschel effect, the intermittency effect may be explained on the hypothesis of simultaneous forward and reverse reactions if we also assume the existence of photochemical lags in these reactions (1, 2, 3). EXPERIMENTAL
It is supposed that, when the illumination is interrupted,-as it is during the periods of darkness in intermittent exposure,-a definite interval of time must elapse before the two reactions cease. Upon this hypothesis Blair and Hylan have suggested that those factors which accentuate the forward reaction, such as (1) strong illumination, (2) short wavelength, and (3) fast emulsion, should make an intermittent exposure more efficient than a continuous one. They also suggest that those factors which accentuate the reverse reaction, such as (1) faint illumination, (2) lon,0 wavelength, and (3) slow emulsion, should make an intermittent exposure less efficient than a continuous one. These suggestions are in keeping with experimental data. Intermittent exposures usually produce less developable density than do equivalent continuous exposures. This fact is For an historical summary and bibliography of the concept of the Herschel effect as a regression phenomenon, see L~~PPo-CRAMER (Proc. 7th Intern. Congr. Phot. (London), p. 45 (1928)). For a more precise treatment, see JOEHNCKAND BLAIR (J. Optical SOC.Am. 23, 67 (1933)), JAMES (J. Chem. Physics 2, 132 (1934)), and the forthcoming paper mentioned in footnote 2.
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explained by supposing that the terminal lag of the reverse reaction predominates over that of the forward r e a ~ t i o n . ~The result, then, is that the density which has been built up during the period of exposure is reduced slightly during the period of darkness. That is, density is not built up at so great a rate by intermittent exposure as by continuous exposure. Moreover, the equilibrium density resulting from intermittent exposure is less than that resulting from continuous exposure. Factors which accentuate the reverse reaction, such as faint illumination, long wavelength, and slow emulsions, tend to produce a reduction of density during the periods of darkness in intermittency. It is ordinarily necessary that these same conditions be present in order that the Herschel effect may be produced. These considerations enable us to predict that, when the above-mentioned factors are present, intermittent exposures will be more effective in producing the Herschel effect than equivalent continuous exposures. In order to test the above-mentioned prediction, a series of experiments was made on a slow, contrasty emulsion which is particularly subject to the Herschel effect (Azo F No. 5 paper). The paper was exposed to white light from an incandescent lamp for 48 seconds, the energy intensity at the surface of the paper being 1550 ergs per square centimeter per second. This exposure produced a high developable density approximating that shown on the ordinate a t t = 0 in the figures. Successive sheets of the exposed paper were then reexposed, some intermittently and others continuously, to a second radiation of intensity 1700 ergs per square centimeter per second. The intermittent exposures were made in a sensitometer employing a sector wheel of 120 degrees opening and rotating at a speed of 28 R.P.M. Three separate wavelength bands of secondary radiation, as given by appropriate Wratten filters, were employed,-orange, red, and short infra-red. The filters employed were No. 72 (600 and 700 mp), No. 70 (700 mp), and No. 88 (over 700 mp). Each filter was used in conjunction with a water-cell which passed 90 per cent of radiation of wavelength 800 mp, 50 per cent of 900 mp, and only 10 per cent of 1000 mp. The sheets were developed for 90 seconds in (Eastman formula) “D-73” developer (diluted 1 part of developer to 3 of water) at 21°C.) fixed in acid hypo, washed, and ferrotyped. The normal reflection densities remaining after stated reexposure intervals were measured with a photoelectric cell reflection densitometer. The results are depicted in figures 1,2, and 3, where the normal reflection densities are plotted against the corresponding intervals of exposure to the “herschelizing” light. It is apparent from the figures that intermittent Davis (2) suggests that, during the period of the terminal lags, the “strength” of both the forward and reverse reactions falls off exponentially, so the resultant effect is a difference between two exponential functions.
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exposure, under the conditions of the experiment, produces a relatively greater Herschel effect than corresponding intervals of continuous exposure to the “herschelizing” light. There is not sufficient data at hand to 16
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FIG.1.
HERSCHEL EFFECT IN ORANGE LIGHT 0,continuous reexposure; A, intermittent reexposure
INTERMITTENCY AND THE
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FIG.2. INTERMITTENCY AND THE HERSCHEL EFFECT IN RED LIGHT 0 , continuous reexposure; A, intermittent reexposure
enable any explanation of why the observed “spread” between the curves for intermittent and for continuous exposure is apparently greater for orange light than for the longer wavelengths. This variation of “spread” with wavelength was found in all of several different determinations.
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Though perhaps not apparent from the extent of the curves, there seems to be a tendency for prolonged infra-red exposure to yield slightly lower equilibrium densities than those obtaining for orange light. In any event absolute density comparisons, for much prolonged exposure times and for low densities, are rendered difficult both by natural regression of the latent image with time (in the dark) and because of the fact that at low densities variations introduced by slight differences in development conditions are relatively more important than a t higher densities. SUMMARY
The interpretation of the Herschel and intermittency effects is briefly discussed on the basis of the familiar hypotheses of simultaneous forward
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FIQ.3. INTERMITTENCY AND THE HERSCHEL EFFECT IN INFRA-RED LIQHT 0, continuous reexposure; A, intermittent reexposure
and reverse reactions with accompanying photochemical lags. From a consideration of certain implications of these hypotheses a new effect is predicted. The existence of this effect-namely, that intermittent exposure to the “herschelizing” light will be more efficient in producing Herschel effect than equivalent continuous exposure-has been amply proven by laboratory results employing three different wavelength bands of “herschelizing” light. REFERENCES SOC.Am.28,353 (1933). (2) DAVIS:Bur. Standards Sci. Papers, No. 528, 21, 95 (1926). (3) WEINLAND:J. Optical SOC.Am. 16, 337 (1927). (1) BLAIRAND HYLAN: J. Optical
