Emulsions with the highest speeds compatible with low granularity

A radiation-sensitive emulsion is disclosed capable of producing the highest attainable speed compatible with low granularity. The silver halide grains (a) have a mean equivalent circular diameter of less than 0.6 .mu.m; (b) have a face centered cubic crystal lattice structure of the rock salt type; (c) have six {100} faces; (d) contain from 95 to 99.5 mole percent bromide ions and from 0.5 to 5 mole percent iodide ions, based on silver; (e) contain in the face centered cubic crystal lattice structure from 5.times.10.sup.-8 to 1.times.10.sup.-6 mole per silver mole of an iridium dopant comprised of Ir.sup.+3 ions forming coordination bonds with at least five halide ions occupying adjacent crystal lattice positions; and (f) contain in the face centered cubic crystal lattice structure from 1.times.10.sup.-5 to 3.times.10.sup.-4 mole per silver mole of a speed enhancing dopant comprised of divalent Group 8 metal ion chosen from among Fe.sup.+2, Ru.sup.+2 and Os.sup.+2 and at least one coordination ligand more electron withdrawing than fluoride ion; (g) the speed enhancing dopant is located in at least a portion of an internal region of the grains formed after 70 percent and before 90 percent of total grain silver has been precipitated and (h) the iridium dopant is located in at least a portion of a central region of the grains and is separated from the speed enhancing dopant by an interposed region containing at least 10 percent of the total grain silver.

FIELD OF THE INVENTION 
The invention relates to photography. More specifically, the invention 
relates to silver halide emulsions for use in photographic elements. 
DEFINITIONS 
The term "cubic" grain when referring to grains having a face centered 
cubic crystal lattice structure of the rock salt type is employed to 
indicate a grain having six {100} faces. 
The term "ECD" refers to the diameter of a circle having an area equal to 
the projected area of a silver halide grain. 
In referring to silver halide grains and emulsions containing two or more 
halides, the halides are named in order of ascending concentrations. 
The term "dopant" is employed to indicate any occlusion within a silver 
halide grain crystal lattice structure other than silver and halide ions. 
The term "low intensity reciprocity failure" (also referred to as LIRF) is 
employed herein to indicate a lower speed for exposures of longer duration 
in comparing emulsion samples receiving the same exposure, but over 
differing time periods ranging from 1/50th to 0.5 second. When the law of 
photographic reciprocity is satisfied (i.e., there is no reciprocity 
failure), the speed of a photographic emulsion remains the same for all 
equal products of I.times.ti produced by varied values of I and ti, where 
I is exposure intensity and ti is exposure time. 
All references to the periodic table of elements are based on the format 
adopted by the American Chemical Society, published in the Feb. 4, 1985, 
issue of the Chemical and Engineering News. 
Research Disclosure is published by Kenneth Mason Publications, Ltd., 
Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. 
BACKGROUND 
In classical silver halide photography a photographic element, commonly 
referred to as a taking film, containing at least one silver halide 
emulsion layer coated on a transparent film support is imagewise exposed 
to light, producing a latent image within the emulsion layer. The film is 
then photographically processed to transform the latent image into a 
silver or dye image that is a negative image of the subject photographed. 
The resulting processed photographic element, commonly referred to as a 
negative, is placed between a uniform exposure light source and a second 
photographic element, commonly referred to as a photographic paper, 
containing at least one silver halide emulsion layer coated on a white 
paper support. Exposure of the emulsion layer of the photographic paper 
through the negative produces a latent image in the photographic paper 
that is a positive image of the subject originally photographed. 
Photographic processing of the photographic paper produces a positive of 
the subject image. The image bearing photographic paper is commonly 
referred to as a print. 
While both negatives and prints rely on radiation-sensitive silver halide 
emulsions for image capture, the choices of emulsions for these separate 
applications are quite different. Silver halide emulsions used in 
photographic paper to produce prints are usually subjected to high 
intensity, short duration exposures from a controlled light source. The 
silver halide emulsions chosen for prints typically contain high (&gt;50M % 
and, more typically, &gt;90M %) proportions of silver chloride and low (&lt;5M % 
and, more typically, &lt;1M %) silver iodide to facilitate rapid processing. 
As compared to the silver halide emulsions employed in taking films, the 
speeds of silver halide emulsions used to form prints are limited. Speed 
limitations can be tolerated, since the light source for exposure is 
entirely under control. 
The silver halide emulsions employed in taking films are usually chosen to 
realize under available lighting conditions the highest attainable speeds 
compatible with image quality (e.g., granularity) requirements. To 
maximize speed and speed in relation to granularity, taking films almost 
universally employ silver iodobromide emulsions. 
Since the early 1980's the very highest speed taking films have 
increasingly relied upon tabular grain emulsions. These emulsions provide 
superior speed-granularity relationships at moderate to high photographic 
speeds. However, nontabular grain silver iodobromide emulsions have 
remained the emulsions of choice for most taking films used to meet image 
quality (e.g., low granularity) requirements dictating mean grain ECD's to 
0.6 .mu.m or less. 
A wide variety of dopants have been employed to modify the properties of 
the silver halide emulsions. A summary of silver halide grain dopants is 
included in Research Disclosure, Vol. 365, Sep. 1994, Item 36544, Section 
I. Emulsion grains and their preparation, D. Grain modifying conditions 
and adjustments, sub-paragraphs (3), (4) and (5). In looking through 
dopant selections for silver halide emulsions it is apparent that grain 
halide content, shape, size and intended modes of exposure and processing 
all influence dopant selections. Dopant selections are in most instances 
carefully tailored to serve specific photographic applications. 
Kuno U.S. Pat. No. 5,051,344 discloses silver iodobromide emulsions 
containing 0.1 to 4 mole percent iodide and, as grain dopants, 
5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iridium compound and 
5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iron compound per mole 
of silver. The grains are of a core-shell structure with the core 
containing a higher iodide content (at least 3 mole percent greater) than 
the shell. Kuno specifically prefers both the iridium and iron to be 
present in the shell. The object is to achieve high contrast with 
high-illuminance short-duration exposure, rapid processing, and better 
safe-light handling. The latter requirement, better safe-light handling, 
is a requirement of increased low intensity reciprocity failure. In other 
words, the emulsions are intended to be responsive to high intensity, 
short duration exposures, but relatively unresponsive to the low levels of 
illumination provided by safe-lights. Kuno recognizes that iridium reduces 
both high and low intensity reciprocity failure. Kuno's purpose in adding 
iron is to eliminate the effect of iridium in reducing low intensity 
reciprocity failure. 
SUMMARY OF THE INVENTION 
In one aspect this invention is directed to a radiation-sensitive emulsion 
comprised of silver halide grains containing metal dopants wherein the 
grains (a) have a mean equivalent circular diameter of less than 0.6 
.mu.m; (b) have a face centered cubic crystal lattice structure of the 
rock salt type; (c) have six {100} faces; (d) contain from 95 to 99.5 mole 
percent bromide ions and from 0.5 to 5 mole percent iodide ions, based on 
silver; (e) contain in the face centered cubic crystal lattice structure 
from 5.times.10.sup.-8 to 1.times.10.sup.-6 mole per silver mole of an 
iridium dopant comprised of Ir.sup.+3 ions forming coordination bonds with 
at least five halide ions occupying adjacent crystal lattice position; and 
(f) contain in the face centered cubic crystal lattice structure from 
5.times.10.sup.-6 to 3.times.10.sup.-4 mole per silver mole of a speed 
enhancing dopant comprised of divalent Group 8 metal ion chosen from among 
Fe.sup.+2, Ru.sup.+2 and Os.sup.+2 and at least one coordination ligand 
more electron withdrawing than fluoride ion; (g) the speed enhancing 
dopant is located in at least a portion of an internal region of the 
grains formed after 70 percent and before 90 percent of total grain silver 
has been precipitated; and (h) the iridium dopant is located in at least a 
portion of a central region of the grains and is separated from the speed 
enhancing dopant by an interposed region containing at least 10 percent of 
the total grain silver. 
The limited ECD's of the emulsions of the invention assure low levels of 
granularity while grain composition, configuration and dopants combine to 
make possible the highest compatible imaging speeds. The emulsions are 
particularly characterized in that high speeds are realized while also 
reducing or eliminating entirely low intensity reciprocity failure. Thus, 
the emulsions of the invention are particularly suited for incorporation 
in taking films that must convert imagewise exposures of less than high 
intensity (e.g., ambient or augmented ambient illumination) into images of 
high quality definition. The emulsions can be employed in black-and-white 
(silver image) taking films, such as common camera films, reduced format 
(micro) films and intensifying screen exposed radiographic films. The 
emulsions are useful in emulsion layer units that contain both the 
emulsions of the invention and faster emulsion layers, such as tabular 
grain emulsion layers. It is specifically contemplated to employ the 
emulsions of the invention as the slower emulsion component(s) of 
multi-emulsion layer units, such as those of extended exposure latitude 
films and color negative films. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The emulsions of the invention can be realized by doping during their 
precipitation emulsions silver halide grains which 
(a) have a mean ECD of less than 0.6 .mu.m; 
(b) have a face centered cubic crystal lattice structure of the rock salt 
type; 
(c) have six {100} faces; and 
(d) contain from 95 to 99.5 mole percent bromide ions and from 0.5 to 5 
mole percent iodide ions, based on silver. 
The purpose in restricting the mean ECD of the grains as stated in (a) to 
less than 0.6 .mu.m is to limit the granularity of the emulsions. The 
purpose of requiring the bromide and iodide content as stated in (d) is to 
insure the highest attainable photographic speeds compatible with the 
restricted maximum ECD of (a). All silver halide grains satisfying 
composition requirements (d) have a face centered cubic crystal lattice 
structure of the rock salt type in satisfaction of (b). The purpose of (c) 
in further restricting the grains to those having six {100} faces (i.e., 
cubic grains) is that in the grain sizes permitted by (a) cubic grains 
provide higher speeds when optimally sensitized than grains bounded by 
exclusively by {111} faces, such as octahedral grains and tabular grains 
with {111} major faces. In other words, before dopant selection to improve 
further photographic properties, discussed below, emulsion grain 
characteristics have already been selected to achieve the highest levels 
of speed compatible with the desired low levels of image granularity made 
possible by limiting grain size. 
Mean grain ECD can take any convenient value less than 0.6 .mu.m. At lower 
mean ECD's granularity and speed are proportionately lowered. A minimum 
mean grain ECD of 0.1 .mu.m is contemplated, since few photographic 
applications are capable of benefiting by smaller grain sizes. A range of 
grain sizes that provides a preferred balance of photographic speed and 
granularity occurs when mean ECD ranges from about 0.2 to 0.4 .mu.m. 
Silver bromide, which constitutes at least 95 percent of the grain 
structure, based on silver, forms a face centered cubic crystal lattice 
structure of the rock salt type. The limited displacement of bromide ions 
with iodide ions in the crystal lattice structure places strains in the 
crystal lattice structure required to accommodate the larger iodide ions 
and is well recognized to enhance photographic speed. Low levels of 
iodide, preferably at least 0.5 mole percent, are well recognized to 
increase photographic speed. Generally speed enhancements are fully 
realized with as little as 1 mole percent iodide, based on silver forming 
the grains. Higher levels of iodide can be employed for other effects, 
such as enhanced native blue sensitivity and interimage effects in color 
photographic elements. However, higher iodide concentrations slow 
development and therefore unnecessarily high levels of iodide 
incorporation are usually avoided. For most photographic applications 
iodide levels of up to 5 mole percent, preferably up to 4 mole percent, 
based on silver are contemplated. For radiographic imaging applications 
customary rapid (&lt;90 second) processing requirements dictate still lower 
maximum iodide concentrations of less than 3 mole percent and, optimally, 
1 mole percent or less. Chloride ion concentrations compatible with the 
bromide and iodide concentrations noted are contemplated, but in the 
preferred form of the invention the grains are silver iodobromide grains. 
The emulsions of the invention require no particular placement of iodide 
within the grains. Iodide is typically uniformly or substantially 
uniformly distributed within the grains. Increasing iodide concentrations 
toward the center of the grain are common and can be employed to advantage 
to increase development rates. Common practices of adjusting silver ion 
concentrations in the dispersing medium at the conclusion of precipitation 
can produce small variances in surface iodide concentrations. Further, it 
is a well known practice to add potassium iodide to an emulsion to enhance 
spectral sensitizing dye adsorption. All of these conventional practices 
are compatible with the practice of the invention. 
The grains are precipitated as cubic grains. That is, they exhibit six 
{100} faces. In their simplest form the grains have the appearance of 
cubes when micrographically inspected. The edges and corners of the cubic 
grains can and typically do show some rounding attributable to ripening. A 
common variant is for the grains to be tetradecahedral, having six {100} 
faces and micrographically identifiable {111} faces. Cubic grains have 
three mutually perpendicular axes oriented normally intersecting the {100} 
faces. The grains of the invention preferably and typically have 
perpendicular axes of equal length, but are not precluded from having axes 
of unequal length. 
As precipitated the grains can be polydisperse or monodisperse. It is 
generally preferred that the grains be relatively monodisperse, most 
preferably exhibiting a coefficient of variation (COV) of ECD of less than 
30 percent, optimally less than 15 percent. COV is 100 times the quotient 
of grain size standard deviation (G) divided by mean grain ECD. 
An iridium dopant capable of reducing low intensity reciprocity failure and 
a Group 8 speed enhancing dopant are incorporated in the grains of the 
emulsions of the invention. To realize both a reduction in low intensity 
reciprocity failure and a significant speed increase particular selections 
of both the concentrations and the relative placements of the iridium and 
Group 8 speed enhancing dopants are required. 
The iridium dopant can be located anywhere within the interior of the 
grains to obtain a reduction in low intensity reciprocity failure. 
However, the Group 8 speed enhancing dopant is effective only when located 
within an internal region of the grains. That region has been determined 
to be the portion of the grain that is formed after 70 percent and before 
90 percent of the grain silver has been precipitated. The Group 8 speed 
enhancing dopant can be distributed uniformly throughout the 20 percent of 
the silver forming this internal region or can be located in any 
convenient portion of this region. If the Group 8 speed enhancing dopant 
is placed at or closer to the surface of the grains, the Group 8 speed 
enhancing dopant is ineffective. A possible explanation is that the Group 
8 speed enhancing dopant is competing with surface chemical sensitization 
sites on the grains. On the other hand, if the Group 8 speed enhancing 
dopant is too deeply buried, its contribution to speed increase is also 
diminished. This is believed to be attributable to the increased diffusion 
path length that a photoelectron temporarily trapped by the Group 8 speed 
enhancing dopant must travel before reaching the grain surface and thereby 
becoming available to participate in latent image formation. 
Even if the Group 8 speed enhancing dopant is correctly placed within the 
crystal lattice structure of the grains, it still is ineffective to 
increase speed when the iridium dopant is located (1) in the same region 
of the grain, (2) in an immediately adjacent region, or (3) nearer the 
surface of the grain than the Group 8 speed enhancing dopant. 
It has been discovered quite unexpectedly that both reduced low intensity 
reciprocity failure and speed enhancement are realized only when the 
iridium dopant is located within at least a portion of a central region of 
the grain (i.e., a region of the grain precipitated prior to the internal 
region of the grain in which the Group 8 speed enhancing dopant is 
effective) and at least 10 (preferably at least 15) percent of the silver 
forming the grain separates the central portion containing the iridium 
dopant from the internal region containing the Group 8 speed enhancing 
dopant. Stated another way, after introduction of the iridium dopant is 
completed, at least 10 (preferably at least 15) percent of the total 
silver forming the grain is precipitated before the Group 8 speed 
enhancing dopant is introduced. 
Specific examples of iridium dopants employed to reduce low intensity 
reciprocity failure are contained in Kim U.S. Pat. No. 4,449,751 and 
Johnson U.S. Pat. No. 5,164,292, the disclosures of which are here 
incorporated by reference. A more general survey of iridium dopants 
employed to reduce reciprocity failure and for other purposes is provided 
by B. H. Carroll, "Iridium Sensitization: A Literature Review", 
Photographic Scienceand Engineering, Vol. 24, No. 6, November/December 
1980, pp. 265-267. A still more general survey of dopants, including 
iridium dopants intended to reduce reciprocity failure is provided in 
Research Disclosure, Item 36544, Section I. Emulsion grains and their 
preparation, D. Grain modifying conditions and adjustments, sub-paragraphs 
(3) and (4), cited above. 
In a specifically preferred form the iridium dopant is comprised of 
Ir.sup.+3 ions forming coordination bonds with at least five halide ions 
occupying adjacent crystal lattice positions. The iridium dopant can be 
conveniently incorporated in the form of a hexacoordination complex 
satisfying the formula: 
EQU [Ir.sup.+3 X.sub.5 L'].sup.m (I) 
where 
X is a halide ligand, 
L' is any bridging ligand, and 
m is -2 or -3. 
As the iridium is added during precipitation a convenient counter ion, such 
as ammonium or alkali metal, is associated with the hexacoordination 
complex, but only the anionic portion of formula I is actually 
incorporated within the crystal lattice structure. Also, as introduced, 
the iridium can be in a +4 valence state, as illustrated, for example by 
Leubner et al U.S. Pat. No. 4,902,611. However, the +4 iridium reverts to 
the +3 valence state upon incorporation. Chloride and bromide are 
preferred halide ligands. The bridging ligand L' can also be a halide 
ligand or, alternatively, can take any convenient conventional form, 
including any of the various individual ligand forms disclosed in McDugle 
et al U.S. Pat. Nos. 4,933,272, 4,981,781 and 5,037,732, Marchetti et al 
U.S. Pat. No. 4,937,180, Keevert et al U.S. Pat. No. 5,037,732 and Olm et 
al U.S. Pat. No. 5,360,712, the disclosures of which are here incorporated 
by reference. 
Preferred concentrations of the iridium dopant can range up to about 
5.times.10.sup.-8 to 1.times.10.sup.-6 mole per silver mole. Most 
preferably, the iridium dopant is present in a concentration of from 
1.times.10.sup.-7 to 5.times.10.sup.-7 mole per silver mole. 
The speed enhancing dopant is comprised of a divalent Group 8 metal (i.e, 
Fe.sup.+2, Ru.sup.+2 or Os.sup.+2) and at least one coordination ligand 
more electron withdrawing than a fluoride ion. The speed enhancing Group 8 
dopant can be introduced as a hexacoordination complex satisfying the 
formula: 
EQU [ML.sub.6 ].sup.n (II) 
where 
M is a divalent Group 8 cation (i.e, Fe.sup.+2, Ru.sup.+2 or Os.sup.+2), 
L represents six coordination complex ligands which can be independently 
selected, provided that at least four of the ligands are anionic ligands 
and at least one the ligands is more electronegative than any halide 
ligand (i.e., more electron withdrawing than a fluoride ion, which is the 
most electronegative halide ion), and 
n a is negative integer having an absolute value of less than 5. 
At least four of the ligands are required to be anionic to facilitate 
incorporation of the dopant into the crystal lattice structure of the 
tabular grains. The remaining two ligands can also be anionic or can take 
any convenient conventional neutral form, such as carbonyl, aquo or ammine 
ligands. 
Although only one of the ligands is required to be more electronegative 
than a halide ion, any higher number, up to and including all of the 
ligands can be more electronegative than a halide ion. One common way of 
assessing electron withdrawing characteristics is by reference to the 
spectrochemical series of ligands, derived from the absorption spectra of 
metal ion complexes in solution, referenced in Inorganic Chemistry: 
Principles of Structure and Reactivity, by James E. Huheey, 1972, Harper 
and Row, New York and in Absorption Spectra and Chemical Bonding in 
Complexes by C. K. Jorgerisen, 1962, Pergamon Press, London. From these 
references the following order of ligands in the spectrochemical series is 
apparent: 
##STR1## 
The abbreviations used are as follows: ox=oxalate, dipy=dipyridine, 
phen=o-phenathroline, and phosph 
=4-methyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane. The spectrochemical 
series places the ligands in sequence in their electron withdrawing 
properties, the first (I.sup.-) ligand in the series is the least electron 
withdrawing and the last (CO) ligand being the most electron withdrawing. 
The underlining indicates the site of ligand bonding to the polyvalent 
metal ion. The efficiency of a ligand in providing the electron 
withdrawing characteristic needed for speed enhancement increases as the 
ligand atom bound to the metal changes from Cl to S to O to N to C. Thus, 
the ligands CN.sup.- and CO are especially preferred. Other preferred 
ligands are thiocyanate (NCS.sup.-), selenocyanate (NCSe.sup.-), cyanate 
(NCO.sup.-), tellurocyanate (NCTe.sup.-) and azide (N.sub.3.sup.-). 
When the metal M in the hexacoordination complex is Fe.sup.+2, it is 
preferred that at least five of the ligands L be more electron withdrawing 
than a halide ion. When the metal M in the hexacoordination complex is 
Os.sup.+2 satisfactory speed enhancement is observed with only one ligand 
more electron withdrawing than a halide ion, but at least two such ligands 
are preferred. In a specific, preferred form the hexacoordination complex 
is comprised of Os.sup.+2 and six ligands chosen from among halide and 
cyano ligands with at least one ligand being a cyano ligand. For Ru.sup.+2 
complexes it is preferred that at least three of the ligands be more 
electronegative than a halide ion. In a specific, preferred form the 
hexacoordination complex is comprised of Ru.sup.+2 and six ligands chosen 
from among halide and cyano ligands with at least three ligands being 
cyano ligands. 
The Group 8 coordination complexes when introduced can be associated with 
the same charge balancing counter ions as the iridium complexes, described 
above. Subject to the requirements noted, the ligands L can be selected 
from the same conventional ligands as L', described above (i.e., from any 
of the various individual ligand forms disclosed in McDugle et al U.S. 
Pat. Nos. 4,933,272, 4,981,781 and 5,037,732, Marchetti et al U.S. Pat. 
No. 4,937,180, Keevert et al U.S. Pat. No. 5,037,732 and Olm et al U.S. 
Pat. No. 5,360,712, the disclosures of which are here incorporated by 
reference). 
The following are specific illustrations of Group 8 coordination complex 
dopants capable of enhancing speed when employed in combination with 
iridium dopants: 
______________________________________ 
SED-1 [Fe(CN).sub.6 ].sup.-4 
SED-2 [Ru(CN).sub.6 ].sup.-4 
SED-3 [Os(CN).sub.6 ].sup.-4 
SED-4 [Fe(pyrazine)(CN).sub.5 ].sup.-4 
SED-5 [RuCl(CN).sub.5 ].sup.-4 
SED-6 [OsBr(CN).sub.5 ].sup.-4 
SED-7 [FeCO(CN).sub.5 ].sup.-3 
SED-8 [RuF.sub.2 (CN).sub.4 ].sup.-4 
SED-9 [OsCl.sub.2 (CN).sub.4 ].sup.-4 
SED-10 [Ru(CN).sub.5 (OCN)].sup.-4 
SED-11 [Ru(CN).sub.5 (N.sub.3)].sup.-4 
SED-12 [Os(CN).sub.5 (SCN)].sup.-4 
SED-13 [Fe(CN).sub.3 Cl.sub.3 ].sup.-3 
SED-14 [Ru(CO).sub.2 (CN).sub.4 ].sup.-1 
SED-15 [Os(CN)Cl.sub.5 ].sup.-4 
______________________________________ 
Preferred concentrations of the Group 8 speed enhancing dopant can range up 
to about 5.times.10.sup.-6 to 3.times.10.sup.-4 mole per silver mole. Most 
preferably, the Group 8 speed enhancing dopant is present in a 
concentration of from 1.5.times.10.sup.-5 to 2.times.10.sup.-4 mole per 
silver mole. 
The emulsions contain a dispersing medium, typically including a 
hydrophilic colloid peptizer, such as gelatin or a gelatin derivative. 
Conventional dispersing media for photographic emulsions are summarized in 
Research Disclosure, Item 36544, cited above, Section II. Vehicles, 
vehicle extenders, vehicle-like addenda and vehicle related addenda. A 
further summary of conventional photographic emulsion features, 
photographic element features, exposures and processing is provided in 
dispersing media for Research Disclosure, Item 36544, cited above. In most 
instances the emulsions are surface sensitized employing chemical 
sensitizer and spectral sensitizing dyes. Research Disclosure, Item 36544, 
Section IV. Chemical sensitization and Section V. Spectral sensitization 
and desensitization summarize conventional approaches for effecting 
chemical and spectral sensitization. In most applications at least one 
antifoggant or stabilizer is added to the emulsions. Section VII. 
Antifoggants and stabilizers summarize these types of emulsion addenda.

EXAMPLES 
The invention can be better appreciated by consideration in conjunction 
with the specific embodiments. The notation (C) is employed to designate 
comparative emulsions while the notation (E) is employed to designate 
emulsions that are examples of the invention emulsions. Speeds are 
reported as relative log speeds--e.g., 30 speed units=0.30 log E, where E 
is exposure in lux-seconds. 
Emulsions 1-9 
A series of cubic grain silver iodobromide emulsions were prepared. The 
grains were nearly perfect cubes, were relatively monodisperse, and 
exhibited a mean ECD of 0.27 .mu.m. 
Emulsion Making: 
Solutions were prepared as follows: 
______________________________________ 
Solution A: 
Gelatin 240 g 
NaBr 2 g 
1,8-dihydroxy-3,6-dithiaoctane 
1.74 g 
antifoammant 0.25 mL 
Water to make 6.5 L 
Solution B: 
3.033 N AgNO.sub.3 
Solution C: 
2.954 N NaBr 
0.790 N KI 
Solution D: 
2.954 N NaBr 
0.790 N KI 
1.672 g/L K.sub.4 Ru(CN).sub.6 
Solution E: 
2.954 N NaBr 
0.790 N KI 
0.837 g/L K.sub.4 Ru(CN).sub.6 
Solution F: 
2.954 N NaBr 
0.790 N KI 
0.124 g/L K.sub.4 Ru(CN).sub.6 
Solution G: 
8.3 .times. 10.sup.-5 M K.sub.2 IrCl.sub.6 
Solution H: 
Gelatin 160 g 
3,6-dimethyl-4-chlorophenol 
2 g 
Water to make 1.0 L 
______________________________________ 
Emulsion 1(C): Solution A was added to the reaction vessel and brought to 
pH 3, pAg 8.7 and 68.3.degree. C. Agitation was provided. Solutions B and 
C were run into the vessel at 94.3 mL/minute, with the flow rate of B 
finely adjusted to maintain pAg 8.7. After 3 minutes, the pAg was ramped 
to 8.2 taking 3 minutes and maintained thereafter. After a total of 35 
minutes the additions were stopped. The emulsion was cooled to 40.degree. 
C. and solution H was added. The emulsion was washed until it reached pAg 
7.9, concentrated and chill set. 
Emulsion 2(C): The emulsion was prepared similarly as Emulsion 1(C), except 
60 mL of solution G were added to the vessel from 50-55% of the 
precipitation (percentage of total silver introduced). 
Emulsion 3(C): The emulsion was prepared similarly as Emulsion 1(C), except 
solution D was substituted for solution C during 75%-90% of the 
precipitation. 
Emulsion 4(E): The emulsion was prepared similarly as Emulsion 1(C), except 
60 mL of solution G were added to the vessel during from 50-55% of the 
precipitation and solution D was substituted for solution C during 75-90% 
of the precipitation. 
Emulsion 5(E): The emulsion was prepared similarly as Emulsion 1(C), except 
30 mL of solution G were added to the vessel from 50-55% of the 
precipitation and solution E was substituted for solution C during 75-90% 
of the precipitation. 
Emulsion 6(C): The emulsion was prepared similarly as Emulsion 1(C), except 
60 mL of solution G were added to the vessel during 85-90% of the 
precipitation. 
Emulsion 7(C): The emulsion was prepared similarly as Emulsion 1(C), except 
solution D was substituted for solution C during 50-65% of the 
precipitation. 
Emulsion 8(C): The emulsion was prepared similarly as Emulsion 1(C), except 
solution D was substituted for solution C during 50-65% of the 
precipitation and 60 mL of solution G were added to the vessel from 85-90% 
of the precipitation. 
Emulsion 9(C)): The emulsion was prepared similarly as Emulsion 1(C), 
except solution E was substituted for solution C during 50-65% of the 
precipitation and 30 mL of solution G were added to the vessel from 85-90% 
of the precipitation. 
Emulsions 10-12 
These emulsions were similar to those above, except that the cubic grain 
size was reduced 0.25 .mu.m, and the concentrations of the dopants were 
also reduced. 
Emulsion 10(C): This emulsion was prepared similarly to Emulsion 1(C), 
except the level of 1,8-di- hydroxy-3,6-dithiaoctane in solution A was 
reduced to 1.4 g. 
Emulsion 11(E): This emulsion was prepared similarly to Emulsion 10(C), 
except 20 mL of solution G were added to the vessel from 50-55% of the 
precipitation and solution F was substituted for solution C during 75-90% 
of the precipitation. 
Emulsion 12(C): This emulsion was prepared similarly to Emulsion 10(C), 
except 20 mL of solution G were added to the vessel from 75-90% of the 
precipitation and solution F was substituted for solution C during 75-90% 
of the precipitation. 
The dopant patterns of the emulsions are summarized in Table I. Dopant 
concentrations are reported in molar parts per million parts of silver. 
The location of the dopant is reported in terms of the percentage of total 
silver precipitated at the beginning and end of dopant addition. 
TABLE I 
______________________________________ 
Group 8 Dopant 
Iridium Dopant 
Emulsion mppm Location mppm Location 
______________________________________ 
1(C) 0 -- 0 -- 
2(C) 0 -- 0.50 50-55% 
3(C) 200 75-90% 0 -- 
4(E) 200 75-90% 0.50 50-55% 
5(E) 100 75-90% 0.25 50-55% 
6(C) 0 -- 0.50 85-90% 
7(C) 200 50-65% 0 - 
8(C) 200 50-65% 0.50 85-90% 
9(C) 100 50-65% 0.25 85-90% 
10(C) 0 -- 0 -- 
11(E) 15 75-90% 0.17 50-55% 
12(C) 15 75-90% 0.17 75-90% 
______________________________________ 
Sensitizations and Evaluations 
The emulsion samples were chemically sensitized by melting 1 mole of each 
emulsion at 40.degree. C. To the melted sample were added 26 mg of 
3-methyl-1,3-benzothiazolium iodide, 3 mg of KSeCN, 13 mg of Na.sub.2 
S.sub.2 O.sub.3.5H.sub.2 0, 24 mg Au.sub.2 S and 30 mg of 
4-hydroxy-6-methyl-1,3,3a, 7-tetraazaindene, sodium salt. Each sample was 
heated to 71.degree. C., held for 5 minutes and then cooled to 40.degree. 
C. Each emulsion sample was spectrally sensitized with 
anhydro-3,3'-bis(2-carboxyethyl)-9-methylthiacarbocyanine hydroxide. 
Samples of the emulsions were identically coated on a transparent 
poly(ethylene terephthalate) transparent film support over an antihalation 
layer comprising 18.8 mg/dm.sup.2 gelatin, surfactant and a solid particle 
dispersion of absorbing dyes. The emulsion layer as coated contained 14 
mg/dm.sup.2 Ag, 18.8 mg/dm.sup.2 gelatin and a surfactant. The emulsion 
layer was overcoated with 9.15 mg/dm.sup.2 gelatin plus surfactant. The 
gelatin containing layers were hardened with bis(vinylsulfonyl)methane at 
2 percent by weight, based on the total weight of gelatin. 
The dried coatings were given 1/50 sec. and 1/2 sec. exposures to a 
2850.degree. K. tungsten light source using a calibrated neutral step 
tablet (0-3density range) and processed in a Kodak Prostar .TM. processor 
using Kodak Prostar Plus .TM. processing solutions. Photographic speeds 
were measured at a density of 1.0. 
The results are summarized in Table II. 
TABLE II 
______________________________________ 
Grp. 8 Ir Log Spd. 
Emulsion Location Location 1/50".vertline.1/2" 
LIRF 
______________________________________ 
1(C) -- -- 100.vertline.91 
-9 
2(C) -- 50-55% 101.vertline.96 
-5 
3(C) 75-90% -- 104.vertline.97 
-9 
4(E) 75-90% 50-55% 108.vertline.103 
-5 
5(E) 75-90% 50-55% 105.vertline.101 
-4 
6(C) -- 85-90% 101.vertline.96 
-4 
7(C) 50-65% -- 100.vertline.91 
-9 
8(C) 50-65% 85-90% 101.vertline.98 
-3 
9(C) 50-65% 85-90% 100.vertline.97 
-4 
10(C) -- -- 100.vertline.91 
-9 
11(E) 75-90% 50-55% 107.vertline.104 
-3 
12(C) 75-90% 75-90% 100.vertline.97 
-3 
______________________________________ 
Discussion of Results 
From Table II it is apparent that undoped control Emulsion 1(C) exhibited a 
relative log speed of 100 that dropped to 91 when the same exposure was 
given, but with the time of exposure extended from 1/50" to 1/2". This 
demonstrated a low intensity reciprocity failure of -9 speed units. All of 
the controls lacking an iridium dopant exhibited the same -9 LIRF value. 
When the iridium dopant was present, LIRF was essentially similar (-3 to 
-5), whether or not the Group 8 dopant was present. This further 
demonstrated that the iridium dopant was capable of reducing LIRF at all 
of the tested levels and locations. 
The Group 8 dopant located in the 75-90% silver precipitated grain region 
was effective to increase speed relative to the remaining control 
emulsions in every instance, except one. In control Emulsion 12(C) the 
iridium dopant and the Group 8 dopant were both located in the same region 
of the grains. No speed enhancement was in this instance observed. 
When the Group 8 dopant was shifted to the 50-65% silver precipitated grain 
region and iridium dopant occupied the 85-90% silver precipitated grain 
region, no speed increase was observed to result from Group 8 dopant 
inclusion. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.