T H E TEMPERATURE COEFFICIENT O F PHOTOGRAPHIC SENSITIVITY. I
Low TEMPERATURES AND THE NATURAL AND OPTICALSENSITIVITIES OF DYEDSILVERHALIDES~ S. E. SHEPPARD, E. P. WIGHTMAN,
AND
R. F. QUIRK
The Kodak Research Laboratories of the E a s t m a n Kodak Co., Rochester, Ne% York Received J a n u a r y $6, I954
The primary object of the present study was to ascertain if the natural and the optical sensitivities of dye-sensitized silver halide were sharply differentiated as regards temperature influence. If this were not so, this would be evidence that the mechanism of latent image formation must be very similar in both cases. On the other hand, a very considerable difference, particularly at quite low temperatures, might strengthen the case for those who support a more or less complicated chemical reaction as the basis of optical sensitizing (14). I n discussing previous work on the subject of temperature influence, we regard it as desirable to deal separately, a t least for the present, with investigations below normal temperature-i.e., below 20°C.-and those above. Since the experiments to be described in this first part deal with the lower range, discussion of relevant literature will be confined to this interval. Much of the work on the effect of temperature on sensitivity appears to be rather discordant. Dewar and Abney (4),for example, reported a lowering of 80 per cent in "sensitivity" a t -180°C. as compared with that a t room temperature, whereas A. and L. Lumi&re (16) reported that it required 350 to 400 times more exposure to produce the same density on plates exposed at - 191°C. as at room temperature, Le., about 99.7 per cent loss. Same have found only a slight (if any) change in sensitivity with increasing temperature (21, 22) from -50°C. to +lOO"C., and others a considerable increase (1, 3, 4,12, 16, 18, 19, 23); some have even found a decrease at the higher temperatures (2, 13). R. J. Wallace (29), plotting density (less fog) against temperature (from -20°C. to +lOO"C.), found that the lower densities (up to about 1.2) passed through a maximum between -20°C. and O'C., and decreased more or less for higher temperatures. 1 I
Contribution No. 528 from the Eastman Kodak Co. 817
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S. E. SHEPPARD, E. P. WIGHTMAN, AND R. F. QUIRK
The first to notice an effect of temperature on contrast (gamma, or gradation of density with exposure over the normal exposure region, for different development times) was Abney (1). He found that for the most part for exposures with time variation, i.e., intensity constant, temperature variation made little, if any, change on the contrast. With intensity varied and time held constant, he found a lowering of contrast with decrease of temperature between +34"C. and - 15°C. King (13) also mentioned the change of contrast with temperature, but did not say in which direction it trended. Wallace (29) found that between -10°C. and +20°C. contrast decreased and above 20°C. it remained constant, under the conditions of his experiments. It should be remembered that his results represented sensitometric curves with fog subtracted uniformly for all densities. Eggert and Luft (5) have tried to explain some of these discrepancies. They found that in plotting the dependence of density for constant energy of (white light) illumination, and (presumably) constant development time, against the teqperature, the curve drawn through the observed points had a maximum between -60°C. and -2O"C., and a minimum between +40"C. and +60"C., somewhat similar to those of Wallace but a t different temperatures. Gammas for the same characteristic curves (for constant development time) from which the densities are taken, decrease to a minimum and then increase again. X-rays, however, produced only a small steady increase in density, for constant energy of illumination, with temperature increase. One cause for the lack of agreement among the various investigators as to the effect of temperature change on photographic sensitivity has been the looseness with which the word "sensitivity" has been used. It has had distinctly different meanings with different workers. We shall use the term to imply the average relative sensitivity of all the silver halide grains of the plate, as modified by their surroundings of gelatin, etc., the relative sensitivity of the individual grains being defined as the exposure (to some standard light source) just sufficient to make the grain developable under some chosen standard conditions of development, which takes into account the developer, the temperature, and the time of development. We shall use the term "speed" to mean 10 X l/i, i being the inertia of the plate, or that exposure on the log E axis a t which the straight-line portion of the D-log E curve extended cuts the axis. This is often called the H . and D. speed, although Hurter and Driffield used a different factor than 10. Another cause for the discrepancies in conclusions concerning the effect of temperature lies in the fact that no two investigators used the same sensitometric procedure. The conclusions were derived from very meager
TEMPERATCRE COEFFICIENT OF PHOTOGRAPHIC SENSITIVITY. I
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data, frequently from just two or three exposures and one development time, the conditions of exposure and development not being specified, nor the type of plate used. Even more important, as indicated by recent unpublished investigations by Dr. J. H. Webb, of the Physics Department of this Laboratory, is the intensity level. Since our results were obtained with a mixed rkgime, Le., an intensity scale of exposures instead of a time scale, they can only be regarded as comparative, but we regard them as significant for the comparison of natural and optical sensitivities. This investigation is by no means complete, having been conducted so far a t only three temperatures, - 180°, -70" and +20"C., and with comparatively few different kinds of plates, but results have already been obtained which it was felt warranted publication. EXPERIMENTAL PROCEDURE
The plates which were cooled to -7O"C., and their controls at room temperature, were exposed in a vertical sensitometer in which they lay on a metal box, in a metal drawer. The temperature of the box mas lowered by partly surrounding it with solid carbon dioxide, and by passing through it acetone cooled by allowing it to flow through a copper coil packed in solid carbon dioxide in a surrounding vessel. The system was well insulated, and was provided with a means of raising the acetone back from a lower reservoir into an upper reservoir after the latter had emptied itself. The temperature at the surface of the plate was determined by pressing against it a calibrated alumel-chrome1 thermocouple, connected with a galvanometer and rheostat, the former having a scale graduated in tenths of a millivolt, and which could easily be estimated to 0.1 to 0.2 millivolt. The thermal junction was flattened so as to make as intimate contact with the plate as possible. The calibration was carried out by burying the thermocouple in liquid nitrogen, liquid oxygen, solid alcohol, solid ethyl chloride, solid carbon dioxide, a soft mush of carbon dioxide in acetone, salt and ice in water, pure ice in water, and water at various temperatures above 0°C. up to 60°C. As the room temperature was fairly constant, in the neighborhood of 20"C., the refinement of operating the above thermocouple against a standard at, say 20"C., was found unnecessary, although the room temperature was recorded. A curve of the galvanometer readings plotted against temperature was drawn, and found to be practically a straight line above -80°C. There was no appreciable condensation of moisture on the plate when placing it in the sensitometer nor during the period it remained there,
820
S. E. SHEPPA-RD, E. P. WIGHTMAN, AND R. F. QUIRK
since practically all moisture inside the sensitometer mas displaced by the rapidly evaporating carbon dioxide, The same sensitometer was used for attaining exposures at - 180°C., but the cooling system was modified, in that liquid air was passed directly from its upper container through an insulated tube to the metal box which was partly surrounded by either liquid air or liquid nitrogen, and from the metal box to a lower container, and the whole system was more thoroughly insulated. The temperature of a plate lying on the metal box when the latter was surrounded with liquid nitrogen was very little, if any, different from that when liquid air was used as the surrounding medium, because liquid air was flowing rapidly through the inside of the box all the while, The difference in speed and in contrast a t -180°C. (noted below), when liquid nitrogen was used around the box in the drawer, as compared with liquid air, was therefore not due to temperature difference but perhaps to the presence of a small amount of ozone in the oxygen vaporized from the liquid air. The ozone would no doubt react with the dye in the panchromatic plate, which, it was observed, does very quickly lose its orchid color in the vapor above liquid air, even without the action of light. Dewar has shown that liquid air, when evaporating, does form some ozone, especially when acted on by light (4). Because the rubber tube connections would become hard and brittle a t that low temperature, the metal drawer containing the metal box was fastened securely to supports screwed to the work bench, and the sensitometer was put on a platform on wheels and rolled back and forth to open and close it. The sensitometer itself (27) consisted of a balopticon with a shortfocus lens, which focused, for the desired time, the image of a step tablet onto a 4 x 5 inch plate lying on the metal box in the metal drawer, by opening and closing a metal flap in front of the lens. The exposure time was either 1 minute or 2 minutes. The light from the sensitometer lamp (which was operating at about 110 volts) before reaching the plate, passed through filters. With panchromatic plates, either a Wratten No. 39 glass filter + a 15 per cent copper sulfate solution in a l-cm. deep glass cell, or a Wratten No. 9 + a Wratten No, 29 gelatin filter were used to give blue-violet or red light, respectively. The former combination has a maximum transmission a t about 380 mp t o 400 mp and no transmission of wavelengths longer than 510 mp. The latter filters cut off all light of wavelength shorter than 600 mp, With the process plates, not dye-sensitized, we substituted for the red filter a Kodak No. 4 filter, transmitting only beyond 480 mp. After the exposure at the low temperature, the plate was placed in a 4 x 5 inch plate box. Of course, some moisture condensed on it in trans-
TEMPERATURE COEFFICIENT OF PHOTOGRAPHIC SENSITIVITY. I
.
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5
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821
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822
S. E. SHEPPARD, E. P. WIGHTMAN, AND R . F. QUIRK
ferring it to the box, but this, it was known from previous experiments on the effect of moisture on the sensitivity and latent image (27), had no appreciable effect on the latter. The plate was not developed until it had attained room temperature again and the moisture had evaporated. The plate which had been cooled was developed along with an uncooled one, which had previously been exposed in the sensitometer at room temperature (about 20°C.) (or in some cases with one which had been cooled after exposure) in a standard pyro-soda developer2 at 20°C. Development time was varied from 2 minutes to 60 minutes. Some experimen ts were also done using p-aminophenol d e ~ e l o p e r . ~ EXPERIMENTAL RESULTS
The full results are best presented as the actual sets of characteristic curves obtained at each temperature and for all times of development, TABLE 3 Comparison o j relative speeds o j process plates for blue-violet and bluish green-yellow light at different temperatures using pyro-soda developer COLOR OF ILLUMINATION
TEMPERATURE
SPEEDS FOR D E V E L O P XENT T I M E S OF
~min.
1
AVERAFE SPEED
Omin.
degrees
Control. . . . . . . . . . . . . . . . ..\I
J
Blue-violet Green-yellow
+20 $20
Liquid nitrogen. . . . . . . . . . .
Blue-violet Green-yellow
- 180 -180
84 11.5
89 12.6
5.3
3.4
0.63
0.63
87 12 4.4 0.63
but this is likely to be regarded as taking too much space. We give, therefore, tables of sensitometric variables taken from these curves, as well as graphs derived from these. I n tables 1 to 3 are given the data on total speeds4 a t 20"C., -7O"C., and - 180°C. for Wratten panchromatic Standard pyro-soda developer: Solution A : NazS03(anhydrous). . . . . . . 256 g. NaHSOs . . . . . . . . . . . . . . . . . . . 70 g. Pyrogallol.. . . . . . . . . . . . . . . . 80 g. Water t o . . . . . . . . . . . . . . . . . . 4 1. 3 Standard p-aminophenol developer: p-Aminophenol hydrochloride Na2SO3(anhydrous). . . . . . . . . .
2
Solution B : NazCOs (anhydrous). . . . . . 300 g. KBr. ..................... 4 g. Water t o . . . . . . . . . . . . . . . . . 4 1.
. . . . . . . . . . . . . . . . . 7.275 g.
................. 50.
g. 50. g. 1. 1. For practical photographic purposes, "speed minus fog" 4 Not subtracting fog. or a similar value, is more useful, but not for sensitivity studies.
...................................... ......................................
TEMPERATURE COEFFICIENT OF PHOTOGRAPHIC SENSITIVITY. I
823
plates, with blue-violet and red radiation, respectively, with both a pyrosoda, and a p-aminophenol developer, as also the value of y (gamma), the slope of the characteristic curve. Data are also given for a process plate for blue-violet light (maximum a t 380 mp to 400 mp) and for bluish green-yellow rays longer than 480 mp, of which only the shorter ones are effective. These represent data on wavelength effect within the natural absorption region of the silver iodobromide, and not for optically (dye) sensitized material. TABLE 4 Wratten panchromatic plate BPEED N U M B E R 5 TEMPERATURE
Blue-violet
I
Red
Pyro-soda developer degrees C . 20 -70 -180 - 180
(Nitrogen) (Air)
20
- 180
(Air)
I
-
425 = 100 per cent 389 92 43 10 27 6.3
324 100 per cent 256 79 10.6 3.5 10.4 3.3
400 83
311 100 17.4 5
2
100 20
per cent
per cent
Process plate TEMPERATURE
I
