Radiographic elements exhibing reduced crossover

A radiographic element is disclosed having a support capable of transmitting blue light and, coated on each of two opposite major faces of said support, a silver halide emulsion layer capable of forming a latent image in response to exposure to blue light. Interposed between at least one of the latent image forming emulsion layers and the support is a blue absorbing silver iodide emulsion layer exhibiting at temperatures below 25.degree. C. an absorption transition wavelength that is bathochromically displaced by at least 20 nm as compared to the absorption transition wavelength of .beta. phase silver iodide.

FIELD OF THE INVENTION 
The invention relates to dual coated silver halide radiographic elements. 
BACKGROUND OF THE INVENTION 
Emulsions comprised of a dispersing medium and silver halide microcrystals 
or grains have found extensive use in photography. Radiation sensitive 
silver halide emulsions have been employed for latent image formation. The 
radiation sensitive silver halide grains employed in photographic 
emulsions are typically comprised of silver chloride, silver bromide, or 
silver in combination with both chloride and bromide ions, each often 
incorporating minor amounts of iodide. Radiation sensitive silver iodide 
emulsions, though infrequently employed in photography, are known in the 
art. Silver halide emulsions are known to be useful in photographic 
elements for purposes other than latent image formation, such as for 
radiation absorption or scattering, interimage effects, and development 
effects. 
In general silver halides exhibit limited absorption within the visible 
spectrum. Progressively greater blue light absorptions are observed in 
silver chloride, silver bromide, and silver iodide. However, even silver 
iodide emulsions appear pale yellow, with their principal light absorption 
occurring near 400 nm. 
The crystal structure of silver iodide has been studied by 
crystallographers, particularly by those interested in photography. The 
most commonly encountered crystalline class of silver iodide is the 
hexagonal wurtzite class, hereinafter designated .beta. phase silver 
iodide. Silver iodide of the face centered cubic crystalline class, 
hereinafter designated .gamma. phase silver iodide, is also stable at room 
temperature. The .beta. phase of silver iodide is the more stable of the 
two phases so that emulsions containing .gamma. phase silver iodide grains 
also contain at least a minor proportion of .beta. phase silver iodide 
grains. 
Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic 
Silver Iodide", Journal of Photographic Science, Vol. 18, 1970, pp. 53-59, 
have reported emulsions containing a third crystalline class of silver 
iodide, the body centered cubic class, hereinafter designated .alpha. 
phase silver iodide. .alpha. phase silver iodide is bright yellow, 
indicating that it exhibits increased absorption in the blue portion of 
the spectrum as compared to .beta. and .gamma. phase silver iodide, which 
are cream colored. The emulsions containing .alpha. phase silver iodide 
studied by Byerley and Hirsch were unstable in that they entirely reverted 
to cream colored silver iodide at temperatures below 27.degree. C. 
Daubendiek U.S. Ser. No. 784,139, filed Oct. 4, 1985, commonly assigned, 
titled ELEMENTS CONTAINING BRIGHT YELLOW SILVER IODIDE discloses silver 
iodide emulsions exhibiting at temperatures below 25.degree. C. an 
absorption transition wavelength that is bathochromically displaced by at 
least 20 nm as compared to the absorption transition wavelength of .beta. 
phase silver iodide. The emulsions are disclosed to be useful for 
absorbing blue light. 
In silver halide photography one or more silver halide emulsion layers are 
usually coated on a single side of a support. An important exception is in 
medical radiography. To minimize patient X ray exposure silver halide 
emulsion layers are commonly dual coated (that is, coated on both opposed 
major faces) of a film support. Since silver halide emulsion layers are 
relatively inefficient absorbers of X radiation, the radiographic element 
is positioned between intensifying screens that absorb X radiation and 
emit light. Crossover exposure, which results in a reduction in image 
sharpness, occurs when light emitted by one screen passes through the 
adjacent emulsion layer and the support to imagewise expose the emulsion 
layer on the opposite side of the support. Loss of image sharpness results 
from light spreading in passing through the support. 
It is quite common in radiography to use blue emitting intensifying 
screens. At the same time radiographic supports used with these screens 
are typically clear or blue tinted; hence, in each instance transparent to 
blue light. 
A variety of approaches have been suggested to the art to reduce crossover, 
as illustrated by Research Disclosure, Vol. 184, August 1979, Item 18431, 
Section V. Research Disclosure is published by Kenneth Mason Publications, 
Etd., Emsworth, Hampshire P010 7DD, England. More particularly it has been 
taught to coat a relatively lower speed silver halide emulsion between the 
support and a higher speed silver halide emulsion layer to reduce 
crossover, as illustrated by Van Stappen U.S. Pat. No. 3,923,515. 
While the art cited above is considered most pertinent to the invention 
claimed, additional art which may be of background interest is identified 
and discussed in the Related Art Appendix following the Examples. 
SUMMARY OF THE INVENTION 
In one aspect this invention is directed to a radiographic element 
comprised of a support capable of transmitting blue light and, coated on 
each of two opposite major faces of said support, a silver halide emulsion 
layer capable of forming a latent image in response to exposure to blue 
light transmitted through the support. Interposed between at least one of 
the latent image forming emulsion layers and the support is a blue 
absorbing layer. The radiographic element is further characterized in that 
the blue absorbing interposed layer is a silver iodide emulsion layer 
exhibiting at temperatures below 25.degree. C. an absorption transition 
wavelength that is bathochromically displaced by at least 20 nm as 
compared to the absorption transition wavelength of .beta. phase silver 
iodide.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, in the assembly shown a radiographic element 100 
according to this invention is positioned between a pair of blue emitting 
intensifying screens 201 and 202. The radiographic element support is 
comprised of a radiographic support element 101, typically transparent or 
blue tinted, capable of transmitting at least a portion of the blue light 
to which it is exposed and optional, similarly transmissive subbing layer 
units 103 and 105, each of which can be formed of one or more adhesion 
promoting layers. On the first and second opposed faces 107 and 109 of the 
support formed by the subbing layer units are blue absorbing layers 111 
and 113, respectively. Overlying the blue absorbing layers 111 and 113 are 
blue recording latent image forming silver halide emulsion layer units 115 
and 117, respectively. Each of the emulsion layer units can be formed of 
one or more silver halide emulsion layers. Overlying the emulsion layer 
units 115 and 117 are optional protective overcoat layers 119 and 121, 
respectively. 
In use, the assembly is imagewise exposed to X radiation. The X radiation 
is principally absorbed by the intensifying screens 201 and 202, which 
promptly emit blue light as a direct function of X ray exposure. 
Considering first the blue light emitted by screen 201, the blue recording 
latent image forming emulsion layer unit 115 is positioned adjacent this 
screen to receive the blue light which it emits. Because of the proximity 
of the screen 201 to the emulsion layer unit 115 only minimal light 
scattering occurs before latent image forming absorption occurs in this 
layer unit. Hence blue light emission from screen 201 forms a sharp image 
in emulsion layer unit 115. 
However, not all of the blue light emitted by screen 201 is absorbed within 
emulsion layer unit 115. This remaining blue light, unless otherwise 
absorbed, will reach the remote emulsion layer unit 117, resulting in a 
highly unsharp image being formed in this remote emulsion layer unit. Both 
blue absorbing layers 111 and 113 are interposed between the screen 201 
and the remote emulsion layer unit and are capable of intercepting and 
attenuating this remaining blue light. Both blue absorbing layers thereby 
contribute to reducing crossover exposure of emulsion layer unit 117 by 
the screen 201. 
In an exactly analogous manner the screen 202 produces a sharp image in 
emulsion layer unit 117, and the blue absorbing layers 111 and 113 
similarly reduce crossover exposure of the emulsion layer unit 115 by the 
screen 202. It is apparent that either of the two blue absorbing layers 
employed alone can effectively reduce crossover exposures from both 
screens. Thus, only one blue absorbing layer is required, although for 
manufacturing convenience dual coated radiographic elements most commonly 
employ identical coatings on opposite major faces of the support. 
The radiographic elements of the present invention offer advantages in 
crossover reduction by employing one or more blue absorbing layers 
comprised of a silver iodide emulsion that is highly efficient in 
absorbing blue light at ambient temperatures--e.g., at temperatures of 
less than 25.degree. C. By a unique preparation procedure set forth below 
in the Examples it has been possible to prepare a silver iodide emulsion 
not heretofore known in the art that is bright yellow at ambient 
temperatures. 
The bright yellow color of the silver iodide emulsion is an important 
quality, since it is visible proof that a higher proportion of blue light 
is being absorbed at ambient temperatures than is absorbed at these 
temperatures by conventional silver iodide emulsions. Silver iodide 
emulsions heretofore observed at ambient temperatures have appeared pale 
yellow. 
The blue light absorption advantage of the bright yellow silver iodide 
emulsions can be quantitatively expressed by observing that the absorption 
transition wavelength in the blue spectrum is bathochromically displaced 
more than 20 nm as compared to the blue spectrum absorption transition 
wavelength of a corresponding silver iodide emulsion in which the silver 
iodide consists essentially of .beta. phase silver iodide. The "blue 
spectrum" is the portion of the visible electromagnetic spectrum extending 
from 400 to 500 nm. The "transition wavelength" is defined as the longest 
blue spectrum absorption wavelength that separates a hypsochromic 20 nm 
spectral interval and a 20 nm bathochromic spectral interval differing in 
that absorption variance is at least 5 times greater in the hypsochromic 
spectral interval than in the bathochromic spectral interval. 
Silver iodide emulsions all show a relatively high absorption at 400 nm and 
a relatively low absorption at 500 nm. A steep transition in absorption 
occurs within the blue spectrum. For silver iodide of differing crystal 
classes the rise from low to high absorptions occurs at differing blue 
wavelengths. The transition wavelength identifies the onset or toe of the 
absorption rise in traversing the blue spectrum from longer to shorter 
wavelengths. As an illustration, in the examples below the silver iodide 
emulsion satisfying the requirements of this invention exhibits an 
absorption variance of about 1% between 520 and 490 nm and an absorption 
variance of about 20% between 490 and 470 nm. For this emulsion coating 
the transition wavelength is 490 nm. The transition wavelength for a 
corresponding emulsion consisting essentially of .beta. phase silver 
iodide grains is 455 nm, since the bathochromic 20 nm interval exhibits an 
absorption variance of about 1% while the hypsochromic 20 nm interval 
exhibits an absorption variance of 14%. In this comparison there is a 35 
nm difference in the transition wavelengths of the two silver iodide 
emulsion coatings. 
The transition wavelength of the emulsions employed in the practice of this 
invention is referenced to the transition wavelength of emulsions 
consisting essentially of .beta. phase silver iodide grains, since this is 
the most readily prepared and most stable form of silver iodide. Emulsions 
which contain .gamma. phase silver iodide also contain .beta. phase silver 
iodide in varying proportions. It is recognized that the presence of 
.gamma. phase silver iodide shifts the transition wavelength 
bathochromically to some extent as compared to the transition wavelength 
of emulsions consisting of .beta. phase silver iodide. However, the 
presence of .gamma. phase silver iodide can not alone account for a 20 nm 
bathochromic displacement of the transition wavelength as compared to 
.beta. phase silver iodide. 
When the transition wavelength of emulsions employed in the practice of 
this invention is at least 20 nm greater than the transition wavelength of 
emulsions consisting essentially of .beta. phase silver iodide grains, the 
transition wavelength occurs at a longer wavelength than any heretofore 
known silver iodide emulsion which is stable at ambient temperatures. In 
preferred embodiments of the invention the emulsions employed are silver 
iodide emulsions exhibiting a transition wavelength which is at least 30 
nm bathochromically displaced as compared to the transition wavelength of 
silver iodide consisting essentially of .beta. phase silver iodide. 
It is to be noted that the transition wavelength of silver iodide emulsions 
varies as a function of average grain size and silver coating coverage. 
Thus, in comparing emulsions containing silver iodide grains of differing 
crystallographic classes corresponding average grain sizes and silver 
coating coverages are necessary. When emulsions of varied grain sizes and 
silver coating coverages differing only in the crystallographic class of 
the silver iodide are compared, the differences in their transition 
wavelengths are remarkably constant. 
The silver iodide emulsions employed in the practice of this invention 
contain silver iodide grains--that is, grains which have an identifiable 
discrete silver iodide phase. Attempts to identify the crystallographic 
class of the silver iodide have been unsuccessful, except to the extent 
that it has been determined that neither .alpha. phase, .beta. phase, 
.gamma. phase silver iodide, nor mixtures of these silver iodide phases 
can account for all the observed properties of the silver iodide emulsions 
prepared and employed. That is, at least a significant portion of the 
silver iodide exhibits properties differing from the three known phases of 
silver iodide. It is, of course, recognized that silver iodide emulsions 
prepared as described below can be blended with conventional silver iodide 
emulsions and still satisfy the requirements of this invention, provided 
transition wavelength requirements of this invention are preserved. 
The bright yellow silver iodide grain population of the emulsions are 
prepared using the general double jet precipitation techniques known to 
the photographic art, as illustrated by Research Disclosure, Vol. 176, 
December 1978, Item 17643, Paragraph I, modified as illustrated by the 
Examples. 
The bright yellow silver iodide grains can be of any convenient size for 
the application undertaken. Since any ripening out of silver iodide grains 
which occurs after their initial formation has the effect of increasing 
the proportion of .beta. or .gamma. phase silver iodide, it is preferred 
to prepare silver iodide grain populations under conditions that are not 
highly favorable to post precipitation ripening. For examle, it is 
generally most convenient for the silver iodide grains to have an average 
diameter of greater than 0.05 .mu.m. Also, it is preferred to prepare the 
emulsions with a minimum of grain heterodispersity. Monodispered silver 
iodide grain populations are preferred. In quantitative terms, it is 
preferred that the bright yellow silver iodide grains exhibit a 
coefficient of variation of less than about 40 and optimally less than 20 
percent, based on grain volume. 
In addition to their increased levels of blue absorption the silver iodide 
emulsions described above are advantageous in that the silver iodide 
grains can be readily removed (i.e., fixed out) in processing concurrently 
with the undeveloped silver halide grains in the latent image forming 
silver halide emulsion layers. This avoids any variance from conventional 
processing and avoids any residual yellowing of the image bearing 
radiographic element, such as can be the case with incompletely removed 
yellow dyes, pigments, and the like heretofore conventionally employed for 
crossover reduction. 
While the silver iodide emulsions heretofore described are preferably 
employed alone for crossover reduction, it is recognized that they can be 
employed in combination with conventional approaches for crossover 
reduction, if desired. A variety of approaches have been suggested to the 
art to reduce crossover, as illustrated by Research Disclosure, Vol. 184, 
August 1979, Item 18431, Section V, cited above and here incorporated by 
reference. 
Apart from the blue absorbing layers 111 and 113 described above, the 
remaining features of the dual coated radiographic elements can take any 
convenient conventional form. Such conventional radiographic element 
features are illustrated, for example, in Research Disclosure, Item 18431, 
cited above and here incorporated by reference. Other conventional 
features common to both silver halide radiographic elements and 
photographic elements are disclosed in Research Disclosure, Vol. 176, 
December 1978, Item 17643. 
Radiographic elements according to this invention having highly desirable 
imaging characteristics are those which employ one or more tabular grain 
silver halide emulsions. It is specifically contemplated to provide dual 
coated radiographic elements according to this invention in which tabular 
grain silver halide emulsion layers are coated nearer the support than 
nontabular grain silver halide emulsion layers to reduce crossover, as 
illustrated by Sugimoto European Patent Application No. 0,084,637. 
Preferred radiographic elements according to the present invention are 
those which employ one or more high aspect ratio tabular grain emulsions 
or thin, intermediate aspect ratio tabular grain emulsions, as disclosed 
by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426, respectively. 
Preferred tabular grain emulsions for use in the radiographic elements of 
this invention are those in which tabular silver halide grains having a 
thickness of less than 0.5 .mu.m (preferably less than 0.3 .mu.m and 
optimally less than 0.2 .mu.m) have an average aspect ratio of greater 
than 5:1 (preferably greater than 8:1 and optimally at least 12:1) and 
account for greater than 50 percent (preferably greater than 70 percent 
and optimally greater than 90 percent) of the total projected area of the 
silver halide grains present in the emulsion. 
To maximize blue light absorption it is preferred to employ a blue spectral 
sensitizing dye adsorbed to the surface of the tabular silver halide 
grains. Preferred blue spectral sensitizing dyes as well as optimum 
chemical and spectral sensitizations of tabular silver halide grains are 
disclosed by Kofron et al U.S. Pat. No. 4,439,520, here incorporated by 
reference. Additional preferred sensitizations, including blue spectral 
sensitizations, for tabular grain silver halide imaging emulsions are 
disclosed by Maskasky U.S. Pat. No. 4,435,501. 
The preferred radiographic elements of this invention are those which 
employ one or more of the crossover reducing blue absorbing layers 
described above in combination with tabular grain latent image forming 
emulsion containing conventional radiographic elements of the type 
disclosed in Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 and 
Dickerson U.S. Pat. No. 4,414,304, here incorporated by reference. By 
employing tabular grain emulsions, which in themselves reduce crossover in 
combination with the blue absorbing layers provided by this invention 
radiographic elements exhibiting extremely low crossover levels can be 
achieved while also achieving high photographic speed, ow levels of 
granularity, high silver covering power, and rapid processing capabilities 
deemed highly desirable in radiography. 
EXAMPLES 
The invention is further illustrated by the following examples. In each of 
the examples the contents of the reaction vessel were stirred vigorously 
throughout silver and iodide salt introductions; the term "percent" means 
percent by weight, unless otherwise indicated; and the term "M" stands for 
a molar concentration, unless otherwise stated. All solutions, unless 
otherwise stated, are aqueous solutions. 
Example 1--Cross Results 
Example 1A--Bright Yellow AgI Emulsion 
A reaction vessel equipped with a stirrer was charged with 2.5 L of water 
containing 35 g of deionized bone gelatin. At 35.degree. C. the pH was 
adjusted to 5.0 with H.sub.2 SO.sub.4, and the pAg to 3.5 with AgNO.sub.3. 
At 35.degree. C. a 1.25M solution of AgNO.sub.3 was added at a constant 
rate over 6 min, consuming 0.0038 mole Ag. The flow of AgNO.sub.3 was then 
accelerated following the profile approximated by the equation flow 
rate=Initial Rate+0.023t+0.00134t.sup.2 (t=time of acceleration in min) 
over a period of 44 min, consuming 0.089 mole Ag. Flow was continued at a 
constant rate over a period of 70 min, consuming 0.312 mole Ag. This was 
followed by acceleration on the same profile as previously over 26 min, 
consuming 0.176 mole Ag. Finally a constant flow over 45 min consumed 
0.424 mole Ag. A total of 1.0 mole Ag was consumed in the precipitation. 
Concurrently with the AgNO.sub.3 a 1.25M solution of NaI was added as 
required to maintain the pAg at 3.5. The pAg was adjusted to 10.15 at 
35.degree. C. with NaI and the pH to 4.00 with H.sub.2 So.sub.4. A 1 L 
portion of the emulsion was washed by the procedure of Yutzy et al, U.S. 
Pat. No. 2,614,929. The final gelatin content was about 44 g/Ag mole. 
X-ray powder diffraction analysis showed some of characteristics to match 
those of .alpha. phase silver iodide, but significant differences from 
.alpha. phase, .beta. phase, and .gamma. phase silver iodide prevented 
positive assignment of any art recognized silver iodide crystalline class. 
Unlike .alpha. phase and .gamma. phase silver iodide emulsions, which are 
pale yellow, this emulsion was bright yellow at room temperature. The 
grains exhibited an average equivalent circular diameter of 0.09 .mu.m and 
a coefficient of variation of 25 percent, based on volumn. 
Example 1B--Coating of the Invention (Lower Level of AgI) 
On each side of a transparent blue tinted polyester support was coated an 
undercoat layer containing 1.08 g/m.sup.2 gelatin and the bright yellow 
AgI emulsion of Example 1A at 0.135 g/m.sup.2 Ag per side. Over this layer 
was coated on each side a sulfur and gold sensitized silver bromoiodide 
emulsion of mean grain size 0.79 .mu.m, 3.4 mole% iodide, at 2.15 
g/m.sup.2 Ag and 1.51 g/m.sup.2 gelatin per side. Over the emulsion was 
coated a protective overcoat at 0.86 g/m.sup.2 gelatin per side. 
Crossover was determined using the method described in Abbott et al, U.S. 
Pat. No. 4,425,425. Two types of screen were used: KODAK X-OMATIC.RTM. 
Regular Intensifying Screens, emitting in the UV at 360-420 nm, and KODAK 
X-OMATIC.RTM. Rapid Intensifying Screens, emitting in the UV at 360-400 
nm, and in the blue at 460-510 nm. The film samples were processed in a 
KODAK RP X-OMAT.RTM. Processor, Model M6-N, using KODAK RP X-OMAT.RTM. 
Developer Starter and Developer Replenisher. The crossover results are 
shown in Table I. 
Example 1C--Coating of the Invention (Higher Level of AgI 
Coating Example 1C was prepared as described for Example 1B but with a 
bright yellow AgI level of 0.27 g/m.sup.2 Ag per side. 
Example 1D--Control Coating (No AgI) 
Coating Example 1D was prepared like Example 1B, but with omission of 
bright yellow AgI from the undercoat layers. 
Example 1E--Control Coating (No Undercoat) 
Coating Example 1E was prepared like Example 1B, but with omission of the 
undercoat layers. 
TABLE I 
______________________________________ 
Percent Crossover 
Example Regular Rapid 
No. Screen Screen Comments 
______________________________________ 
1B 9 13 Invention 
1C 3 8 Invention 
1D 22 23 Control 
1E 22 24 Control 
______________________________________ 
The crossover measurement results of Table I demonstrate the major 
reduction in crossover obtained with the use of undercoat layers 
containing bright yellow AgI. 
Example 2--Comparison of Crossover Reduction with Bright Yellow and .beta. 
Phase AgI Undercoats 
Example 2A--Control Coating (.beta. Phase AgI Undercoat) 
Coating Example 2A was prepared like Example 1B, but with a .beta. phase 
silver iodide emulsion having grains with a mean equivalent circular 
diameter of 0.05 .mu.m forming an undercoat beneath the latent image 
forming emulsion layer. The .beta. phase silver iodide emulsion was 
prepared by a precipitation procedure generally analogous to that 
described below for Emulsion 1. Silver iodide coverages are set out in 
Table II. 
Example 2B--Bright Yellow AgI Emulsion 
Coating Example 2B was prepared like Coating Example 2A, but with the 
bright yellow silver iodide emulsion of Example 1A substituted for the 
.beta. phase silver iodide. 
Example 2C--Control Coating (No Undercoat) 
Coating Example 2C was prepared like Example 1B, but with omission of the 
undercoat layers. 
Example 2D--Control Coating (No AgI) 
Coating Example 2D was prepared like Example 1B, but with omission of 
bright yellow AgI from the undercoat layers. 
Example 2E--Crossover Comparisons 
Crossover was determined as described in Example 1B using Du Pont CRONEX 
.RTM. Screens, which have a broad emission range from about 330 nm to 
about 600 nm, peaking at 430 nm. The results are tabulated in Table II. 
TABLE II 
______________________________________ 
AgI 
Example Percent Coverage 
No. Crossover Type (g Ag/m.sup.2 /side) 
______________________________________ 
2A 24 .beta.-Phase 
0.135 
23 0.270 
22 0.540 
2B 18 Bright yellow 
0.135 
13 0.270 
10 0.540 
2C* 32 None 0 
2D** 30 None 0 
______________________________________ 
*No undercoat 
**No AgI in undercoat 
From Table II a significantly greater reduction in crossover was obtained 
with the bright yellow silver iodide emulsion employed as an undercoat as 
compared to the .beta. phase silver iodide. This demonstrates the 
superiority of the bright yellow silver iodide emulsions employed as 
undercoats for reducing crossover in combination with intensifying screens 
emitting in the blue portion of the visible spectrum. 
Example 3--Comparison of Absorption Transition Wavelengths 
Emulsion 1. .beta. Phase Silver Iodide (Control) 
A reaction vessel equipped with a stirrer was charged with 3.0 L of water 
containing 80 g of deionized bone gelatin. At 35.degree.C. the pAg was 
adjusted to 12.6 with KI and maintained at that value during the 
precipitation. The pH was recorded as 5.50 at 35.degree. C. At 35.degree. 
C. a 5.0M solution of AgNO.sub.3 was added at a linearly accelerating rate 
(3.83.times. from start to finish) over a period of 42.4 min, consuming 
4.0 moles Ag. A 5M solution of KI was added concurrently as required to 
maintain the pAg at 12.6. The pAg was then adjusted to 10.7 with 
AgNO.sub.3. A solution of 80 g of deionized bone gelatin was added. The 
emulsion was washed by the ion exchange method of Maley, U.S. Pat. No. 
3,782,953, and stored at approximately 4.degree. C. 
X-ray powder diffraction analysis showed the composition to be 97.7% .beta. 
phase. The average equivalent circular diameter of the grains was found to 
be about 0.12 .mu.m. 
Emulsion 2. .beta. and .gamma. Phase Silver Iodide (Control) 
A reaction vessel equipped with a stirrer was charged with 2.5 L of water 
containing 40 g of bone gelatin at 35.degree. C. The pH was adjusted to 
6.00 at 35.degree. C. using NaOH and the pAg to 2.45 with AgNO.sub.3. At 
35.degree. C. a 5.0M solution of AgNO.sub.3 was added at a linearly 
accelerating rate (2.62.times. from start to finish) over a period of 20.3 
min, consuming 1.0 mole Ag. A 5.0M solution of KI was concurrently added 
as required to maintain the pAg at 2.45. The pAg was then adjusted to 10.6 
with KI. A solution of 60 g of bone gelatin in 200 cc of water was then 
added. The emulsion was washed and stored similarly as Emulsion 1. 
X-ray powder diffraction analysis showed the composition to be 72% .beta. 
and 28% .gamma. phase silver iodide. The greater part of the silver iodide 
was present as grains of an average equivalent circular diameter of 0.11 
.mu.m . A finer grain population of average equivalent circular diameter 
of about 0.04 .mu.m was also present. 
Emulsion 3. Bright Yellow Silver Iodide (Example) 
A reaction vessel equipped with a stirrer was charged with 2.5 L of water 
containing 35 g of deionized bone gelatin. At 35.degree. C. the pH was 
adjusted to 5.0 with H.sub.2 SO.sub.4, and the pAg to 3.5 with AgNO.sub.3. 
At 35.degree. C. a 1.25M solution of AgNO.sub.3 was added at a constant 
rate over 6 min, consuming 0.0038 mole Ag. The flow of AgNO.sub.3 was then 
accelerated following the profile approximated by the equation flow 
rate=Initial Rate+0.023t+0.00134t.sup.2 (t=time of acceleration in min) 
over a period of 44 min, consuming 0.089 mole Ag. Flow was continued at a 
constant rate over a period of 70 min, consuming 0.312 mole Ag. This was 
followed by acceleration on the same profile as previously over 26 min, 
consuming 0.176 mole Ag. Finally a constant flow over 45 min consumed 
0.424 mole Ag. A total of 1.0 mole Ag was consumed in the precipitation. 
Concurrently with the AgNO.sub.3, a 1.25M solution of NaI was added as 
required to maintain the pAg at 3.36. A 25% deionized bone gel solution 
containing 50 g of gelatin was added. The pAg was adjusted to 10.1 with KI 
and the pH to 4.00 with H.sub.2 SO.sub.4. A 1 L portion of the emulsion 
was washed as described for Emulsion 1, 17 g of gelatin (25% solution) 
added, and the pH adjusted to 4.00. The emulsion was stored at 
approximately 4.degree. C. 
X-ray powder diffraction analysis showed some of characteristics to match 
those of .alpha. phase silver iodide, but significant differences from 
.alpha. phase, .beta. phase, and .gamma. phase silver iodide prevented 
positive assignment of any art recognized silver iodide crystalline class. 
Unlike Emulsions 1 and 2, which were pale yellow, Emulsion 3 was bright 
yellow at room temperature. The grains exhibited an average equivalent 
circular diameter of 0.09 .mu.m. 
ABSORPTION SPECTRA 
For measurement of the absorption spectra, coatings of each emulsion were 
made on an acetate support at 0.86 g/m.sup.2 Ag, 9.77 g/m.sup.2 gelatin. 
The coating melts were adjusted to pAg 5.0 at 35.degree. C. using 
AgNO.sub.3 or NaI as required, and to pH 4.00 at 35.degree. C., using 
H.sub.2 SO.sub.4 or NaOH as required. A sample of Emulsion 3 was coated on 
the same day it was precipitated. Another sample was coated one week after 
precipitation, and still another sample was coated four weeks after 
precipitation. Between precipitation and coating Emulsion 3 was held at 
4.degree. C. Spectra were measured using a DIANO MATCHSCAN.RTM. 
spectrophotometer. From curves plotting percent absorption versus 
wavelength, it was determined that the absorption transition wavelength 
was in each instance 490 nm--that is, invariant as a function of the 
delays in coating. When the transition wavelength of a coating held for 
four weeks at room temperature was compared with the transition wavelength 
of a fresh coating, the transition wavelengths of the two coatings were 
identical. This showed that the silver iodide was in a stable state. 
Absorption spectra were obtained using Emulsions 1 and 2 similarly as 
described above. In each instance Emulsion 1 showed an invariant 
transition wavelength of 455 nm, and Emulsion 2 showed an invariant 
transition wavelength of 465 nm. Although Emulsion 2 exhibited a 10 nm 
bathochromic displacement of the transition wavelength as compared to 
Emulsion 1, this absorption difference was not sustained at wavelengths 
shorter than the transition wavelength. At wavelengths shorter than its 
transition wavelength Emulsion 2 approached the absorption of Emulsion 1, 
exhibiting essentially the same absorption at a wavelength of 420 nm. 
RELATED ART APPENDIX 
Additional art related to silver iodide is listed in chronological order of 
publication: 
1. Steigmann German Pat. No. 505,012, issued Aug. 12, 1930. 
2. Steigmann, Photographische Industrie, "Green and Brown Developing 
Emulsions", Vol. 34, pp. 764, 766, and 872, published July 8 and Aug. 5, 
1938. 
Items 1 and 2 disclose the preparation of silver halide emulsions having a 
green tint by introducing sodium chloride into a silver iodide emulsion. 
3. Carroll U.S. Pat. No. 2,327,764, issued Aug. 24, 1943, discloses the use 
of silver iodide as an overcoat acting as an ultra-violet filter. 
4. Zharkov, Dobroserdova, and Panfilova, "Crystallization of Silver Halides 
in Photographic Emulsions IV. Study by Electron Microscopy of Silver 
Iodide Emulsions", Zh. Nauch. Prikl. Fot. Kine, March-April, 1957, 2, pp. 
102-105. 
5. Ozaki and Hachisu, "Photophoresis and Photo-agglomeration of Plate-like 
Silver Iodide Particles", Science of Light, Vol. 19, No. 2, 1970, pp. 
59-71. 
Items 4 and 5 report silver iodide precipitations with an excess of iodide 
ions, producing hexagonal crystal structures of predominantly .beta. phase 
silver iodide. 
6. James, The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, 
pp. 1 and 2, contains the following summary of the knowledge of the art: 
According to the conclusions of Kokmeijer and Van Hengel, which have been 
widely accepted, more nearly cubic AgI is precipitated when silver ions 
are in excess and more nearly hexagonal AgI when iodide ions are in 
excess. More recent measurements indicate that the presence or absence of 
gelatin and the rate of addition of the reactants have pronounced effects 
on the amounts of cubic and hexagonal AgI. Entirely hexagonal material was 
produced only when gelatin was present and the solutions were added slowly 
without an excess of either Ag.sup.+ or I.sup.-. No condition was found 
where only cubic material was observed. 
7. Maskasky, Research Disclosure, Item 16158, Vol. 161, pp. 84-87, 
September 1977, discloses the preparation of monodisperse hexagonal 
bipyramid silver iodide crystals by a double jet precipitation technique 
which utilized accelerated reactant introduction rates. 
8. Daubendiek, "AgI Precipitations: Effects of pAg on Crystal Growth(PB)", 
II-23, Papers from the 1978 International Congress of Photographic 
Science, Rochester, N.Y., pp. 140-143, 1978, reports the double jet 
precipitation of silver iodide under a variety of conditions. Spectral 
absorption and X-ray diffraction measurements reportedly gave no 
indication of .alpha. phase silver iodide in the precipitated emulsions 
examined. 
9. Maskasky U.S. Pat. No. 4,094,684, issued June 18, 1978, discloses silver 
chloride epitaxially deposited on silver iodide grains. 
10. Maskasky U.S. Pat. No. 4,142,900, issued Mar. 6. 1979, discloses 
conversion of silver chloride epitaxially deposited on silver iodide 
grains using bromide ions. 
11. Maskasky Research Disclosure, Vol. 181, May 1979, Item 18153, reports 
silver iodide phosphate photographic emulsions in which silver is 
coprecipitated with iodide and phosphate. 
12. Maskasky U.S. Pat. No. 4,158,565, issued June 19, 1979, discloses the 
use of grains containing silver chloride epitaxially deposited on silver 
iodide grains in a dye image amplification process. 
13. Koitabashi U.K. Specification No. 2,063,499A, published Feb. 4, 1981, 
discloses silver bromide or bromoiodide epitaxially deposited on silver 
iodide host grains. 
14. Maskasky U.S. Pat. No. 4,459,353, issued July 10, 1984, discloses high 
aspect ratio tabular grain .gamma. phase silver iodide emulsions. 
5. House U.S. Pat. No. 4,490,458, issued Dec. 25, 1984, discloses tabular 
grain silver iodide emulsions employed in multicolor photographic 
elements. 
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.