Method for producing a long scale direct-positive photographic emulsion

A direct positive emulsion is produced by nonuniform fogging. The latter is conducted by varying the addition of the emulsion to fogging agent or by variable quenching of the fogging reaction. Films using such emulsion have extended exposure latitude or long scale, and the Density vs. Log Exposure curve is smooth and continuous.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a fogged direct positive emulsion, and 
particularly a method of nonuniform fogging to produce a long scale 
(extended latitude) emulsion having a smooth and continuous Density vs. 
Log of Exposure (Log E) curve. 
2. Description of the Prior Art 
The simplest route to positive-positive photographic reproduction is 
provided by certain types of silver halide emulsions that are fogged in 
manufacture, and, on exposure and conventional development, yield direct 
positive images of the original subject matter. These emulsions are called 
prefogged direct positives. To give faithful reproductions it is desirable 
that the Density vs. Log Exposure curves of such emulsions have an 
extended exposure latitude, or long scale. In addition it is desirable 
that the curve shape be smooth and continuous. 
Smith et al., in U.S. Pat. No. 3,615,573 "Direct-Positive Composition 
Containing Individually and Differently Fogged Silver Halide Emulsions" 
addressed the problem of extending the exposure latitude of a fogged 
direct positive emulsion by separating an unsensitized emulsion into two 
or more portions, individually fogging them to different levels, and 
applying these to a support, either in separate layers or as a blend. The 
result was a Density vs. Log E curve having one or more discrete 
high-contrast steps of exposure range of the photographic composition. 
FIG. 1 of the drawings is a reproduction of a highly preferred embodiment 
of Smith et al, viz. FIG. 3 of U.S. Pat. No. 3,615,573. This illustrates 
how three separate emulsions of different degrees of chemical fogging can 
be combined to provide extended exposure latitude. While illustrating the 
steps which are produced by the combination of emulsions, this Figure also 
shows that techniques which involve mixing of emulsions of different 
sensitivities give Density vs. Log E curves which contain breaks, i.e., 
there are sharp changes in direction. Prior art techniques of mixing 
emulsions are characterized by such breaks since the net curve is really a 
superpositioning of the curve shapes of the different emulsions used and 
the break is representative of a transition from one emulsion to another. 
However, given the complex nature of photographic emulsions, it is 
difficult to control the fogging process for even a single emulsion, much 
less maintain proper control over several emulsions so that, if desired, 
the transition from one step to another is more or less "smooth". Also, 
the blending of two emulsions of different speeds to produce a step or 
flat spot in the Density vs. Log E curve causes a region of reduced 
contrast in the midtones of the duplicate. The present invention proceeds 
in the opposite direction and provides a simpler means to extend exposure 
latitude, using only a single emulsion to obtain an essentially smooth 
curve. 
SUMMARY OF THE INVENTION 
The present invention provides a single chemically fogged direct positive 
emulsion having a multitude of photographic sensitivities within the 
emulsion, along with its process of manufacture. In a further embodiment 
the invention provides a photographic film characterized by (1) a Density 
vs. Log E curve which exhibits no discrete steps in the curve, and (2) an 
extended exposure latitude. 
The chemical fogging may proceed by combining separate portions of emulsion 
and fogging agent over a period of time, using either constant or varying 
rates of addition of one to the other. In one embodiment, double jet 
fogging may be employed, i.e., a stream of unfogged silver halide emulsion 
is continuously pumped from a supply vessel to a receiving vessel, and at 
the same time a stream of chemical foggant is continuously pumped into and 
mixed with the emulsion in the receiving vessel. Continuously variable 
fogging of the emulsion by means of "double jet" metering the emulsion and 
foggant into a separate vessel over a period of time will produce low 
contrast and long scale without a flat spot in Density vs. Log E curve. A 
refinement to the method would be to vary the rate of addition of foggant 
while keeping the silver halide (emulsion) rate of addition constant, 
e.g., 25 ml/min. for 10 minutes, and after each 10 minutes reducing the 
rate by 5 ml/min. for the foggant while keeping the emulsion rate at 25 
ml/min. Another variation is to combine the emulsion and fogging agent and 
continuously draw off the emulsion into a separate vessel which is below 
digestion temperature or contains a quenching solution. A further 
refinement is to meter the unfogged emulsion into a separate kettle while 
in-line injecting the foggant, or in-line injecting a quenching solution 
into the fogged emulsion. The end result of any of these techniques is to 
provide a single emulsion in which the silver halide grains have 
experienced nonuniform fogging, thereby providing extended exposure 
latitude (long scale) when the emulsion is used in a photographic element.

DETAILED DESCRIPTION OF THE INVENTION 
Silver halide grains useful for the present invention may be produced by 
techniques well known in the art. They may be heterodisperse or 
monodisperse, produced by splash, double jet, conversion, or core-shell 
techniques, and may incorporate metal ion dopants to modify photographic 
response. Sensitizing dyes, stabilizers, antifoggants, surfactants, and 
other photographic addenda may be used in conjunction with silver halide 
grains prepared according to the present invention. 
A preferred reducing agent is cesium thiadecaborane, used in combination 
with gold salts to produce the nonuniform fogging of the present 
invention. The preferred emulsion grains are monodisperse. A preferred 
method of variable fogging employs a constant rate of addition of emulsion 
along with a gradually decreasing addition of the fogging agent. 
Electron trapping cyanine dyes are particularly useful in these fogged 
direct positive emulsions. Organic halogen compounds as taught in Belgium 
Pat. No. 876,734 are also useful with the present invention. 
A practical advantage of the present invention is that only a single 
emulsion need be produced in order to obtain the extended latitude or long 
scale response. As an alternative, it is possible to adapt the present 
invention to a continuous emulsion fogging and quenching process, whereas 
prior art techniques require blending different emulsions and cannot be 
employed for a single emulsion production. While the prior art teaches 
that separate emulsion batches must be prepared with discrete photographic 
properties determined by grain size and/or degree of fogging, the present 
invention introduces the concept of a spectrum of sensitivities within the 
grains which make up the single emulsion. 
Whereas conventional prior art fogging techniques provided a chemical 
environment and digestion reaction which was uniform for all the silver 
halide grains within a particular batch, the present invention subjects 
the silver halide grains to a changing chemical environment and different 
degrees of digestion. As a consequence of the delibrate alteration of the 
fogging conditions the grains do not have uniform sensitivity to exposure. 
Conceptually, if one were able to examine individual grains from an 
emulsion prepared by the present invention, there would be a wide range of 
sensitivities from one extreme to another. Yet because there are also 
grains which differ in sensitivity by only very small increments, the net 
result is that there are no steps or breaks in the curve shape. The smooth 
and continuous curve shape of emulsion made by the present invention can 
be attributed to the nonuniform fogging which produces the extended 
exposure range. 
The methods of carrying out the present invention including the best mode 
will be made clear by the following examples. 
EXAMPLE 1 
A monodisperse cubic grained gelatin iodobromide emulsion (1.4% iodide) was 
prepared by double jet precipitation. The emulsion contained 20 mg rhodium 
chloride per mole of silver halide to increase gradient. The emulsion was 
dispersed in gelatin and the pH adjusted to 7.6. 
A portion of this emulsion was used as a control and was digested for 90 
minutes at 73.degree. C. after the addition of 25 micrograms of cesium 
thiadecaborane and 47 micrograms of gold chloride per mole of silver 
halide. After digestion the emulsion was cooled to 35.degree. C. and the 
pH adjusted to 5.4; cetyl betaine was added as a coating aid, and 
formaldehyde as a hardener. The emulsion was coated on a film support and 
samples were tested by exposing for 1.6 sec. with an EK101 sensitometer 
through a .sqroot.2 wedge followed by a 90 second development at 
27.degree. C. in 24 DL (commercial developer available from Du Pont). FIG. 
2 represents the photographic response obtained from this control test, 
and can be characterized as a typical prefogged direct positive Density 
vs. Log E curve shape produced by a single emulsion in which the silver 
halide grains have been uniformly sensitized. 
An experimental portion of the emulsion received the same 47 micrograms of 
gold chloride as the control but the cesium thiadecaborane was added by a 
novel method. Referring to FIG. 7, the emulsion was continuously pumped 
over 80 minutes from a holding vessel 1, through line 4, to a reaction 
vessel 3 where the internal temperature was maintained at 73.degree. C. 
over a period of 90 minutes. The 25 micrograms of cesium thiadecaborane 
was added from container 2 through line 5 as aliquot of solution in the 
following manner: at reaction time zero, 96 ml; after 10 minutes, 84 ml; 
after 20 minutes, 72 ml; after 30 minutes, 60 ml; after 60 minutes, 24 ml; 
and after 70 minutes, 12 ml was the final addition. The digestion was 
cooled to 35.degree. C. at the end of the 90 minute reaction period, the 
pH adjusted to 5.4, and cetyl betaine and formaldehyde added as for the 
control. When a film was prepared and tested as for the control, the FIG. 
3 curve was produced. FIG. 3 can be characterized as a representation of a 
single emulsion nonuniformly fogged according to the present invention. By 
comparison with FIG. 1, it has the extended latitude or long scale, but 
without the steps or breaks in the curve. By comparison with FIG. 2, it 
has the same smooth continuous curve, but with lower gradient and extended 
latitude or longer scale. 
The following Table contains a comparison of the results with and without 
gold addition. 
TABLE 1 
______________________________________ 
Log 
Gold Average 
Exposure 
Addition Fogging .gamma. Gradient 
Range 
______________________________________ 
No Std(Control) 
1.9 1.7 1.20 
No Aliquot foggant 
1.2 1.0 1.90 
Yes Std(Control) 
2.7 2.0 1.05 
Yes Aliquot foggant 
1.2 1.0 2.00 
______________________________________ 
These results illustrate the improvement in exposure latitude or longer 
scale produced by the nonuniform introduction of foggant to the emulsion. 
As the emulsion enters the reaction vessel 3 at different times the amount 
of foggant is changing due to a depletion by consumption, and new addition 
of aliquots. Also, depending on when the emulsion enters vessel 3, it will 
be held at digestion temperature for varying times. Thus it can be seen 
that the silver halide grains will experience a wide range of spectrum of 
reaction conditions. Grains which entered the vessel initially have been 
present for all aliquot additions. Grains which entered the vessel with 
the end of the 80 minute addition have been present for the minimum 
reaction period under fogging conditions. Since all the aliquots have been 
previously added, the activity of the foggant has already been diminished 
by prior reaction. Between these extremes are grains with intermediate 
sensitivities to provide the smooth-continuous curve characteristic of the 
present invention. 
EXAMPLE 2 
The speeds of both the control portion and the experimental portion of 
Example 1 were increased without adversely affecting the extended latitude 
when a desensitizing dye of the following formula was included in both 
emulsions: 
##STR1## 
EXAMPLE 3 
The speeds of both the control portion and the experimental portion of 
Example 1 were increased without adversely affecting the extended latitude 
when tribromoquinaldine was added at the end of digestion as taught in 
Belgium Pat. No. 876,734. 
EXAMPLE 4 
A control emulsion was prepared as in Example 1 but the experimental 
emulsion was prepared by adding aliquots of both emulsion and foggant to 
the reaction vessel 3. Equal portions of emulsion were added every 10 
minutes over the 80 minute period so that the total amount of emulsion was 
present for the final 10 minutes of the digestion. The foggant was added 
in milliliters as follows: 
______________________________________ 
Add 1 96 
Add 2 84 
Add 3 72 
Add 4 60 
Add 5 48 
Add 6 36 
Add 7 24 
Add 8 12 
______________________________________ 
The curve shape obtained is illustrated in FIG. 3. An examination of this 
result reveals that there is no disadvantage to using aliquot or portion 
addition of the emulsion relative to the continuous addition described in 
Example 1. 
EXAMPLE 5 
Control and experimental emulsions were prepared which contained the 
emulsion of Example 1, the desensitizing dye of Example 2, and the 
tribromoquinaldine of Example 3. All emulsion contains 47 micrograms of 
gold chloride per mole of silver halide prior to the fogging reaction. 
Referring to Table 2, in the Control (Col. #1) the amount of emulsion 
employed in line 1 of the data (Time Min.=0) was 900 units, and was zero 
thereafter. The emulsion was added in 9 equal portions of 100 units each 
in the experimental emulsions (Cols. #2-5), along with the indicated 
amount of foggant. The curve of FIG. 4 demonstrates how different modes of 
nonuniform fogging can vary the resulting curve shape. Curve (a) 
represents the mode of adding the foggant at a constant rate. Curve (b) 
represents the mode of gradually reducing the rate of addition of the 
foggant. Curve (c) represents the mode of starting with a very high rate 
of addition of foggant and then rapidly decreasing the rate of foggant 
addition until it reaches a very low level toward the end of the fogging 
reaction period. These smooth and continuous curves illustrate that it is 
possible to maintain the extended latitude advantages of the invention 
while altering the shape of the curve. 
TABLE 2 
______________________________________ 
Control 
Time 1 Experiment 
Min. Foggant Amount 
2 3 4 5 
______________________________________ 
0 70 18 9 25 30 
10 0 13 9 8 10 
20 0 10 9 7 10 
30 0 8 9 7 5 
40 0 6 9 7 5 
50 0 5 9 7 5 
60 0 4 9 6 3 
70 0 3 9 6 3 
Dmax 2.3 2.2 2.3 2.2 2.1 
.gamma. 
2.7 0.95 1.25 1.0 0.85 
Ave. Grad. 
2.0 0.75 0.95 0.85 0.75 
Total Scale 
1.05 2.1 1.5 1.8 1.8 
______________________________________ 
Experimental emulsion 2, characterized by the addition of equal portions of 
emulsion and gradually decreasing portions of foggant, gives a curve shape 
which gives a very close match to the .gamma.=1 curve and is illustrated 
in FIG. 5. This represents the best mode contemplated for practice of the 
invention. 
EXAMPLE 6 
A control and experimental emulsion were prepared as in Example 2 except 
that the foggant was pumped from container 2 at a continuously decreasing 
rate. Since both emulsion and foggant were pumped, this illustrates what 
has been referred to as double jet fogging. 
These double jet experiments are illustrated by FIG. 6 in which curve 1 
represents the control and curve 2 represents the same emulsion with 
double jet fogging. This represents the advantage obtained by the present 
invention. Curve 1 shows the normal curve response of grains fogged in a 
conventional manner. Curve 2 shows the response obtained with the same 
grains when they are nonuniformly fogged, using a double jet fogging 
reaction wherein both emulsion and foggant are continuously added to a 
reaction vessel. 
EXAMPLE 7 
A chloride precipitated bromide-converted emulsion was prepared by the 
conventional splash technique and exhibited heterodisperse grains instead 
of the monodisperse grains used in the previous examples. The grains were 
nonuniformly fogged as in Example 4 except that the emulsion and foggant 
were added at 5 minute intervals as follows: 
______________________________________ 
Add 1 48 
Add 2 42 
Add 3 37 
Add 4 30 
Add 5 24 
Add 6 18 
Add 7 12 
Add 8 6 
Add 9 0 
______________________________________ 
As in Ex. 4, the results are illustrated by the curve shape of FIG. 3. This 
illustrates that the technique is not limited to monodisperse grains but 
is generally applicable to any silver halide grains useful in direct 
positive emulsions. 
EXAMPLE 8 
Two portions of emulsion were prepared as in Example 1 which contained 
identical amounts of gold and foggant. One of these served as a control 
and was digested for 90 minutes at 73.degree. C. as in Example 1. 
The other portion had eight aliquots of emulsion removed each 10 minutes 
and cooled to 48.degree. C. during the digestion so that at the end of the 
90 minutes the final aliquot was cooled or quenched. The experiment showed 
extended exposure latitude relative to the control as in FIG. 6. The 
experiment is illustrated schematically in FIG. 8 of the drawings. Fogged 
emulsion was removed from vessel 9 at 73.degree. C. as soon as fogging had 
started, in aliquots, through line 10, over the period of time which is 
stated, to vessel 11, which was maintained at a temperature of 48.degree. 
C. Here the fogging reaction was stopped (quenched) by the reduced 
temperature in vessel 11. 
Alternatively, fogging could be stopped in vessel 11 by the addition of a 
quenching agent instead of by low temperature. Also, the fogged emulsion 
could be pumped through line 10 at a constant rate instead of in aliquots. 
EXAMPLE 9 
Emulsions were prepared using the aliquot method of Example 4. For one 
experiment the foggant additions were varied while the emulsion additions 
were kept constant, while for the other experiment the foggant additions 
were kept constant while the emulsion additions were varied. Table 3 
contains a comparison of the methods of addition. 
TABLE 3 
______________________________________ 
Experiment A Experiment B 
Emulsion 
Foggant Emulsion Foggant 
Portion 
Portion Portion Portion 
______________________________________ 
Time Zero 
100 16 40 6 
10 min. 100 14 40 6 
20 min. 100 12 60 6 
30 min. 100 10 80 6 
40 min. 100 8 100 6 
50 min. 100 6 120 6 
60 min. 100 4 140 6 
70 min. 100 2 160 6 
80 min. 100 0 190 0 
______________________________________ 
Both experimental emulsions gave results comparable with curve 2 of FIG. 6 
while a control using the same emulsion with all the foggant and emulsion 
added at the beginning of the 90 minute digestion gave the standard curve 
1 of FIG. 6. This illustrates that the nonuniform fogging can be carried 
out in a variety of ways and that it is possible to obtain the low 
contrast and long scale in more than one manner.