Forehardened high aspect ratio silver halide photographic elements and processes for their use

Forehardened photographic elements, particularly radiographic elements, intended to produce silver images are disclosed including among hydrophilic colloid layers at least one emulsion layer containing thin tabular silver halide grains. When developed in less than 1 minute to produce a viewable silver image, these photographic elements exhibit increased covering power.

FIELD OF THE INVENTION 
This invention relates to silver halide photography. More specifically, 
this invention relates to forehardened silver halide photographic 
elements, particularly radiographic elements, and to processes for their 
use. 
BACKGROUND OF THE INVENTION 
Black-and-white silver halide photography has relied traditionally upon 
developed silver to produce a viewable image. Although black-and-white 
photography serves a variety of imaging needs, medical radiography, 
described below, illustrates the varied and in some instances competing 
demands that are encountered in silver imaging. 
In medical radiography comparatively large areas of the radiation-sensitive 
material are often required for a single exposure--i.e., large format 
exposures are common. Further, the silver which remains in the element for 
imaging may be unavailable for reclamation for many years. Therefore, it 
is highly desirable to make efficient use of the silver which the 
radiographic elements contain. One measure of the efficiency of silver use 
is covering power. Covering power is herein defined as 100 times the ratio 
of maximum density to developed silver, expressed in grams per square 
decimeter. High covering power is recognized to be an advantageous 
characteristic of not only radiographic elements, but other 
black-and-white photographic elements as well. Covering power and 
conditions which affect it are discussed by James, Theory of the 
Photographic Process, 4th Ed., Macmillan, 1977, pp. 404, 489, and 490, and 
by Farnell and Solman, "The Covering Power of Photographic Silver Deposits 
I. Chemical Development", The Journal of Photographic Science, Vol. 18, 
1970, pp. 94-101. 
One approach to achieving high covering power is to employ relatively fine 
silver halide grains, since it is well recognized that increasing grain 
size will reduce covering power. Unfortunately, in medical radiography 
even more important than achieving efficient use of silver is the need to 
minimize patient exposure to X-radiation. Since silver halide becomes more 
sensitive (increases in speed) as a direct function of grain size, it is 
not then surprising that radiographic elements commonly employ large grain 
sizes. Thus, although attaining high covering power is important, the 
comparatively coarse silver halide emulsions actually employed are not 
well suited to achieving high levels of covering power. 
Other techniques are therefore employed to improve covering power. It is 
known that larger silver halide grain sizes, typically at least about 0.6 
micron in average diameter and larger, are subject to reductions in 
covering power as a function of hardening. To achieve the highest covering 
power compatible with speed requirements (and therefore grain size 
requirements), it is common practice in the art to limit forehardening 
(i.e., hardening during manufacture) to just the degree necessary to 
permit the radiographic elements to be handled (although the risk of 
damage of such materials remains comparatively high). 
Final hardening to the desired level is achieved by incorporating a 
hardener in the processing composition, usually the developer. 
Particularly effective hardeners for use in processing compositions are 
dialdehydes and bis-bisulfite derivatives thereof of the type disclosed in 
Allen and Burness U.S. Pat. No. 3,232,764. Unfortunately, the hardener 
must be kept separate from the developer composition prior to use. 
Further, the presence of such hardener places additional constraints on 
the choice of developer compositions. 
In a typical medical radiographic application a radiographic film is 
employed having relatively coarse silver halide emulsions coated on both 
major surfaces. The emulsion layers are minimally forehardened to achieve 
maximum covering power. The element is more sensitive to light than to 
X-radiation and is therefore typically placed between a pair of 
fluorescent screens which, upon imagewise exposure to X-radiation, 
imagewise fluoresce to expose the radiographic element. Thereafter the 
radiographic element is processed in a developer containing a hardener. To 
provide rapid access to a viewable image, the radiographic element is 
processed at temperatures above ambient (typically about 25.degree. to 
50.degree. C.) and in time periods of less than 1 minute. Development is 
usually complete in about 20 seconds. A typical process of the type 
described above is illustrated by Barnes et al U.S. Pat. No. 3,545,971. 
A great variety of regular and irregular grain shapes have been observed in 
silver halide photographic emulsions intended for black-and-white imaging 
applications generally and radiographic imaging applications specifically. 
Regular grains are often cubic or octahedral. Grain edges can exhibit 
rounding due to ripening effects, and in the presence of strong ripening 
agents, such as ammonia, the grains may even be spherical or near 
spherical thick platelets, as described, for example by Land U.S. Pat. 
Nos. 3,894,871 and Zelikman and Levi Making and Coating Photographic 
Emulsions, Focal Press, 1964, page 223. Rods and tabular grains in varied 
portions have been frequently observed mixed in among other grain shapes, 
particularly where the pAg (the negative logarithm of silver ion 
concentration) of the emulsions has varied during precipitation, as 
occurs, for example in single-jet precipitations. 
Tabular silver bromide grains have been extensively studied, often in 
macro-sizes having no photographic utility. Tabular grains are herein 
defined as those having two substantially parallel crystal faces, each of 
which is substantially larger than any other single crystal face of the 
grain. The aspect ratio--that is, the ratio of diameter to thickness--of 
tabulator grains is substantially greater than 1:1. High aspect ratio 
tabular grain silver bromide emulsions were reported by de Cugnac and 
Chateau, "Evolution of the Morphology of Silver Bromide Crystals During 
Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 
(1962), pp. 121-125. 
From 1937 until the 1950's the Eastman Kodak Company sold a Duplitized.RTM. 
fully forehardened radiographic film product under the name No-Screen 
X-Ray Code 5133. The product contained as coatings on opposite major faces 
of a film support sulfur sensitized silver bromide emulsions. Since the 
emulsions were intended to be exposed by X-radiation, they were not 
spectrally sensitized. The tabular grains had an average aspect ratio in 
the range of from about 5 to 7:1. The tabular grains accounted for greater 
than 50% of the projected area while nontabular grains accounted for 
greater than 25% of the projected area. The emulsion having the thinnest 
average grain thickness, chosen from several remakes, had an average 
tabular grain diameter of 2.5 microns, an average tabular grain thickness 
of 0.36 micron, and an average aspect ratio of 7:1. In other remakes the 
emulsions contained thicker, smaller diameter tabular grains which were of 
lower average aspect ratio. 
Although tabular grain silver bromoiodide emulsions are known in the art, 
none exhibit a high average aspect ratio. A discussion of tabular silver 
bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, 
Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of 
Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", 
The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and 
Smith observed a pronounced reduction in both grain size and aspect ratio 
with the introduction of iodide. Gutoff, "Nucleation and Growth Rates 
During the Precipitation of Silver Halide Photographic Emulsions", 
Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, 
pp. 248-257, reports preparing silver bromide and silver bromoiodide 
emulsions of the type prepared by single-jet precipitations using a 
continuous precipitation apparatus. 
Bogg, Lewis, and Maternaghan have recently published procedures for 
preparing emulsions in which a major proportion of the silver halide is 
present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 
teaches forming silver halide crystals of tabular habit bounded by {100} 
cubic faces and having an aspect ratio (based on edge length) of from 1.5 
to 7:1. The tabular grains exhibit square and rectangular major surfaces 
characteristic of {100} crystal faces. In the example reported the average 
edge length of the grains was 0.93 micron and the average aspect ratio 
2:1. Thus the average grain thickness was 0.46 micron, indicating thick 
tabular grains were produced. Lewis U.S. Pat. No. 4,067,739 teaches the 
preparation of silver halide emulsions wherein most of the crystals are of 
the twinned octahedral type by forming seed crystals, causing the seed 
crystals to increase in size by Ostwald ripening in the presence of a 
silver halide solvent, and completing grain growth without renucleation or 
Ostwald ripening while controlling pBr (the negative logarithm of bromide 
ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994, 4,184,877, and 
4,184,878, U.K. Pat. No. 1,570,581, and German OLS publications 2,905,655 
and 2,921,077 teach the formation of silver halide grains of flat twinned 
octahedral configuration by employing seed crystals which are at least 90 
mole percent iodide. (Except as otherwise indicated, all references to 
halide percentages are based on silver present in the corresponding 
emulsion, grain, or grain region being discussed.) Lewis and Maternaghan 
report increased covering power. Maternaghan states that the emulsions are 
useful in camera films, both black-and-white and color. It appears from 
repeating examples and viewing the photomicrographs published that average 
tabular grain thicknesses were greater than 0.40 micron. Japanese patent 
Kokai 142,329, published Nov. 6, 1980, appears to be essentially 
cumulative with Maternaghan, but is not restricted to the use of silver 
iodide seed grains. Thus, the patents discussed above in this paragraph 
are viewed as teaching the preparation of silver halide emulsions 
containing relatively thick tabular grains of intermediate average aspect 
ratios. 
Wilgus and Haefner U.S. Ser. No. 429,420, filed concurrently herewith and 
commonly assigned, titled HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS 
AND PROCESSES FOR THEIR PREATION, which is a continuation-in-part of 
U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, more fully 
discussed below, discloses high aspect ratio silver bromoiodide emulsions 
and a process for their preparation. 
Kofron et al U.S. Ser. No. 429,407, filed concurrently herewith and 
commonly assigned, titled SENSITIZED HIGH ASPECT RATIO SILVER HALIDE 
EMULSIONS AND PHOTOGRAPHIC ELEMENTS, which is a continuation-in-part of 
U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, more fully 
discussed below, discloses chemically and spectrally sensitized high 
aspect ratio tabular grain silver halide emulsions and photographic 
elements incorporating these emulsions. 
Daubendiek and Strong U.S. Ser. No. 429,587, filed concurrently herewith 
and commonly assigned, titled AN IMPROVED PROCESS FOR THE PREATION OF 
HIGH ASPECT RATIO SILVER BROMOIODIDE EMULSIONS, which is a 
continuation-in-part of U.S. Ser. No. 320,906, filed Nov. 12, 1981, now 
abandoned, more fully discussed below, discloses an improvement on the 
processes of Maternaghan whereby high aspect ratio tabular grain silver 
bromoiodide emulsions can be prepared. 
Abbott and Jones U.S. Ser. No. 430,222, filed concurrently herewith and 
commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED 
CROSSOVER, which is a continuation-in-part of U.S. Ser. No. 320,907, filed 
Nov. 12, 1981, more fully discussed below, discloses the use of high 
aspect ratio tabular grain silver halide emulsions in radiographic 
elements coated on both major surfaces of a radiation transmitting support 
to control crossover. 
Abbott and Jones U.S. Ser. No. 431,910, filed concurrently herewith and 
commonly assigned, titled RADIOGRAPHIC ELEMENTS EXHIBITING REDUCED 
CROSSOVER, more fully discussed below, discloses the use of thin, 
intermediate aspect ratio tabular grain silver halide emulsions in 
radiographic elements coated on both major suirfaces of a radiation 
transmitting support to control crossover. 
Wey U.S. Ser. No. 429,403, filed concurrently herewith and commonly 
assigned, titled IMPROVED DOUBLE-JET PRECIPITATION PROCESS AND PRODUCTS 
THEREOF, which is a continuation-in-part of U.S. Ser. No. 320,908, filed 
Nov. 12, 1981, now abandoned, more fully discussed below, discloses a 
process of preparing tabular silver chloride grains which are 
substantially internally free of both silver bromide and silver iodide. 
The emulsions have an average aspect ratio of greater than 8:1. 
Solberg, Piggin, and Wilgus U.S. Ser. No. 431,913, filed concurrently 
herewith and commonly assigned, titled RADIATION-SENSITIVE SILVER 
BROMOIODIDE EMULSIONS, PHOTOGRAPHIC ELEMENTS, AND PROCESSES FOR THEIR USE, 
which is a continuation-in-part of U.S. Ser. No. 320,909, filed Nov. 12, 
1981, now abandoned, more fully discussed below, discloses high aspect 
ratio tabular grain silver bromiodide emulsions wherein a higher 
concentration of iodide is present in an annular region than in a central 
region of the tabular grains. 
Mignot U.S. Ser. No. 320,912, filed Nov. 12, 1981 and commonly assigned, 
titled SILVER BROMIDE EMULSIONS OR NARROW GRAIN SIZE DISTRIBUTION AND 
PROCESSES FOR THEIR PREATION discloses high aspect ratio tabular grain 
silver bromide emulsions wherein the tabular grains are square or 
rectangular in projected area. 
Maskasky U.S. Ser. No. 431,455, filed concurrently herewith and commonly 
assigned, titled SILVER CHLORIDE EMULSIONS OF MODIFIED CRYSTAL HABIT AND 
PROCESSES FOR THEIR PREATION, which is a continuation-in-part of U.S. 
Ser. No. 320,898, now abandoned, filed Nov. 12, 1981, discloses a process 
of preparing tabular grains having opposed major crystal faces lying in 
{111} crystal planes and, in one preferred form, at least one peripheral 
edge lying perpendicular to a &lt;211&gt; crystallographic vector in the plane 
of one of the major surfaces. Thus, the crystal edges obtained are 
crystallographically offset 30.degree. C. as compared to those of Wey. 
Maskasky requires that the novel tabular grains be predominantly (that is, 
at least 50 mole percent) chloride. 
Wey and Wilgus U.S. Ser. No. 431,854, filed concurrently herewith and 
commonly assigned, titled NOVEL SILVER CHLOROBROMIDE EMULSIONS AND 
PROCESSES FOR THEIR PREATION which is a continuation-in-part of U.S. 
Ser. No. 320,899, filed Nov. 12, 1981, now abandoned, discloses tabular 
grain silver chlorobromide emulsions in which the molar ratio of chloride 
to bromide ranges up to 2:2. 
Maskasky U.S. Ser. No. 431,855, filed concurrently herewith and commonly 
assigned, titled CONTROLLED SITE EPITAXIAL SENSITIZATION, which is a 
continuation-in-part of U.S. Ser. No. 320,920, now abandoned, filed Nov. 
12, 1981, discloses high aspect ratio tabular grain emulsions wherein 
silver salt is epitaxially located on and substantially confined to 
selected surface sites of the tabular silver halide grains. 
SUMMARY OF THE INVENTION 
In one aspect this invention is directed to a photographic element 
comprised of a support and, located on the support, one or more 
hydrophilic colloid layers including at least one emulsion layer 
containing radiation-sensitive silver halide grains. The photographic 
element is characterized by at least 50 percent of the total projected 
area of said silver halide grains in at least one emulsion layer being 
provided by thin tabular grains having a thickness of less than 0.3 
micron. Further, the hydrophilic colloid layers are forehardened in an 
amount sufficient to reduce swelling of the layers to less than 200 
percent. Percent swelling is determined by (a) incubating the photographic 
element at 38.degree. C. for 3 days at 50 percent relative humidity, (b) 
measuring layer thickness, (c) immersing the photographic element in 
distilled water at 21.degree. C. for 3 minutes, and (d) determining change 
in layer thickness as compared to the layer thickness measured in step 
(b). 
In one preferred form the photographic element is a radiographic element 
comprised of a substantially specularly transmissive support having first 
and second major surfaces each coated with one or more hydrophilic colloid 
layers including at least one emulsion layer containing 
radiation-sensitive silver halide grains. The radiographic element is 
characterized by silver halide grain and hydrophilic colloid layer 
features specifically set forth above. 
In another aspect this invention is directed to a process of producing a 
high covering power silver image comprising imagewise exposing a 
photographic element or, specifically, a radiographic element, as 
described above, and developing a viewable image in less than 1 minute. 
The present invention allows a black-and-white photographic element 
intended to form a viewable silver image to be sufficiently forehardened 
that no additional hardening is required in processing and still achieve 
high levels of covering power. The invention satisfies a long-standing 
need in the art for relatively high speed, high covering power 
photographic elements, particularly radiographic elements, that can be 
rapidly processed without encountering the risk of damage due to 
incomplete hardening or requiring the use of a processing bath containing 
a hardener. 
As taught by Abbott and Jones, cited above, the radiographic elements of 
this invention exhibit significantly reduced crossover and therefore less 
reduction in sharpness attributable to crossover, taking other 
photographic characteristics into account. More specifically, the 
radiographic elements of this invention have at least one silver halide 
emulsion layer which, at a selected silver coverage (based on the weight 
of silver per unit area of the emulsion layer) and a comparable 
photographic speed, permit less crossover of exposing radiation. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention is generally applicable to black-and-white 
photographic elements intended for use in forming viewable retained silver 
images having at least one relatively coarse grain silver halide emulsion 
layer containing a hardenable hydrophilic colloid or its equivalent. To 
achieve the advantages of this invention thin tabular grain silver halide 
emulsions are employed to form at least one of the emulsion layers. 
As applied to the silver halide emulsions the term "thin" is herein defined 
as requiring that the tabular silver halide grains have a thickness of 
less than 0.3 micron. In a specifically preferred form the thin tabular 
grain silver halide emulsions have an average grain thickness of less than 
0.2 micron. The covering power advantages of this invention bear an 
inverse relationship to the average thickness of the tabular grains of the 
thin tabular grain silver halide emulsions employed. Typically the tabular 
grains have an average thickness of at least 0.03 micron, although even 
thinner tabular grains can in principle be employed--e.g., as low as 0.01 
micron, depending upon halide content. 
Although thin tabular grain emulsions can achieve advantages in covering 
power at low aspect ratios, in order to achieve other tabular grain silver 
halide advantages, such as those taught by Kofron et al and Abbott and 
Jones, cited above, in combination with covering power advantages, it is 
preferred that the thin tabular grain silver halide emulsions employed in 
the practice of this invention have an average aspect ratio of at least 
5:1. The preferred thin tabular grain silver halide emulsions are high 
aspect ratio thin tabular grain emulsions. High aspect ratio thin tabular 
grain emulsions are those in which the thin tabular grains have an average 
aspect ratio of greater than 8:1 and account for at least 50 percent of 
the total projected area of the silver halide grains. In a preferred form 
of the invention these thin tabular silver halide grains account for at 
least 70 percent and optimally at least 90 percent of the total projected 
area of the silver halide grains. 
Increases in covering power are particularly in evidence when the tabular 
silver halide grains having a thickness of less than 0.3 micron have an 
average diameter of at least 0.6 microns, optimally an average diameter of 
at least 1 micron. 
The grain characteristics described above of the silver halide emulsions of 
this invention can be readily ascertained by procedures well known to 
those skilled in the art. As employed herein the term "aspect ratio" 
refers to the ratio of the diameter of the grain to its thickness. The 
"diameter" of the grain is in turn defined as the diameter of a circle 
having an area equal to the projected area of the grain as viewed in a 
photomicrograph or an electron micrograph of an emulsion sample. From 
shadowed electron micrographs of emulsion samples it is possible to 
determine the thickness and diameter of each grain and to identify those 
tabular grains having a thickness of less than 0.3 micron--i.e., thin 
tabular grains. From this the aspect ratio of each such thin tabular grain 
can be calculated, and the aspect ratios of all the thin tabular grains in 
the same (meeting the less than 0.3 micron thickness) can be averaged to 
obtain their average aspect ratio. By this definition the average aspect 
ratio is the average of individual thin tabular grain aspect ratios. In 
practice it is usually simpler to obtain an average thickness and an 
average diameter of the thin tabular grains and to calculate the average 
aspect ratio as the ratio of these two averages. Whether the averaged 
individual aspect ratios or the averages of thickness and diameter are 
used to determine the average aspect ratio, within the tolerances of grain 
measurements contemplated, the average aspect ratios obtained do not 
significantly differ. The projected areas of the thin tabular silver 
halide grains can be summed, the projected areas of the remaining silver 
halide grains in the photomicrograph can be summed separately, and from 
the two sums the percentage of the total projected area of the thin 
tabular silver halide grains can be calculated. 
In the above determinations a reference tabular grain thickness of less 
than 0.3 micron was chosen to distinguish the uniquely thin tabular grains 
herein contemplated from thicker tabular grains which provide inferior 
photographic properties. At lower diameters it is not always possible to 
distinguish tabular and nontabular grains in micrographs. Thin tabular 
grains for purposes of this disclosure are those silver halide grains 
which are less than 0.3 micron in thickness and appear tabular at 2,500 
times magnification. The term "projected area" is used in the same sense 
as the terms "projection area" and "projective area" commonly employed in 
the art; see, for example, James and Higgins, Fundamentals of Photographic 
Theory, Morgan and Morgan, New York, p. 15. 
Although only one thin tabular grain emulsion layer is required in the 
photographic elements of this invention, the photographic elements can, if 
desired, contain a plurality of such tabular grain emulsion layers. 
Emulsions other than the required thin tabular grain emulsion can take any 
convenient form. Various conventional emulsions are illustrated by 
Research Disclosure, Vol. 176, December 1978, Item 17643, Paragraph I, 
Emulsion preparation and types, here incorporated by reference. (Research 
Disclosure and Product Licensing Index are publications of Industrial 
Opportunities Ltd.; Homewell, Havant; Hampshire, P09 1EF, United Kingdom). 
It is additionally contemplated to employ thin tabular grain emulsion 
layers in combination with thicker high aspect ratio tubular grain 
emulsion layers, such as those having average tabular grain thicknesses up 
to 0.5 micron described by Kofron et al, cited above. 
The silver halide emulsion layers and other layers, if any, such as 
overcoat layers, interlayers, and subbing layers, of the photographic 
elements can contain various hardenable colloids alone or in combination 
as vehicles. As employed herein the term vehicle is inclusive of both 
binders and peptizers. The photographic elements of this invention are 
forehardened. That is, the colloids are sufficiently cross-linked that no 
subsequent hardening is required after manufacture. The hydrophilic 
colloid containing layers are sufficiently forehardened to reduce swelling 
thereof to less than 200 percent. Although a number of similar swell tests 
have been employed, for purposes of providing a specific definition, 
percent swell is herein defined as the percentage determined by the 
procedure of Example 11 of Burness et al U.S. Pat. No. 3,841,872, but with 
an incubation temperature of 38.degree. C. and an immersion temperature of 
21.degree. C. Specifically, percent swell is determined by (a) incubating 
the photographic element at 38.degree. C. for 3 days at 50 percent 
relative humidity, (b) measuring layer thickness, (c) immersing the 
photographic element in distilled water at 21.degree. C. for 3 minutes, 
and (d) determining the percent change in layer thickness as compared to 
the layer thickness measured in step (b). The percentage of swell is the 
product of the difference between the final layer thickness and the 
original (post-incubation) layer thickness divided by original layer 
thickness and multiplied by 100. It is preferred that the photographic 
elements of this invention exhibit less than 100 percent swell. As is well 
understood in the art, the percentage of swell can be controlled by 
adjusting the concentration of the hardener employed. 
It has been surprisingly observed that forehardening of photographic 
elements according to the present invention does not produce the reduction 
in covering power observed in forehardened commercial photographic 
elements lacking thin tabular grain silver halide emulsions, as described 
above, particularly those containing silver halide grains having an 
average diameter based on projected area of at least 0.6 micron. Further, 
the forehardened photographic elements of this invention have a higher 
covering power than comparable forehardened photographic elements 
containing nontabular silver halide grains of the same average diameter, 
based on projected area. Further, the photographic elements according to 
the present invention also exhibit a higher covering power than otherwise 
comparable photographic elements employing tabular silver halide grains of 
greater average tabular grain thickness, whether of the same average 
diameter or higher average aspect ratio. Although high covering power has 
heretofore been attained in the art by employing smaller average silver 
halide grains, such grain sizes have restricted photographic speed. The 
present invention provides for the first time the opportunity to provide 
higher speed forehardened photographic elements without incurring a 
substantial reduction in covering power. 
Since the photographic elements of this invention can contain other, 
conventional emulsion layers in addition to the required thin tabular 
grain silver halide emulsions, the overall covering power for the 
photographic element (as opposed to individual emulsion layers) can vary 
widely. However, in preferred photographic elements according to the 
invention, particularly those in which all of the emulsion layers present 
contain thin tabular grains having a thickness of less than 0.2 micron, 
the photographic elements exhibit a covering power of at least 80, 
preferably at least 100, and optimally at least 110 when developed in less 
than 1 minute, particularly at higher than ambient temperatures (e.g., 
25.degree. to 50.degree. C.). 
The thin tabular grain silver halide emulsion layers and other layers of 
the photographic elements can contain various hardenable colloids alone or 
in combination as vehicles. Suitable hydrophilic colloids include 
substances such as proteins, protein derivatives, cellulose 
derivatives--e.g., cellulose esters, gelatin--e.g., alkali-treated gelatin 
(cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin), 
gelatin derivatives--e.g., acetylated gelatin, phthalated gelatin and the 
like, polysaccharides such as dextran, gum arabic, zein, casein, pectin, 
collagen derivatives, agar-agar, arrowroot, albumin and the like as 
described in Yutzy et al U.S. Pat. Nos. 2,614,928 and '929, Lowe et al 
U.S. Pat. Nos. 2,691,582, 2,614,930, '931, 2,327,808 and 2,448,534, Gates 
et al U.S. Pat. Nos. 2,787,545 and 2,956,880, Himmelmann et al U.S. Pat. 
No. 3,061,436, Farrell et al U.S. Pat. No. 2,816,027, Ryan U.S. Pat. Nos. 
3,132,945, 3,138,461 and 3,186,846, Dersch et al U.K. Pat. No. 1,167,159 
and U.S. Pat. Nos. 2,960,405 and 3,436,220, Geary U.S. Pat. No. 3,486,896, 
Gazzard U.K. Pat. No. 793,549, Gates et al U.S. Pat. No. 2,992,213, 
3,157,506, 3,184,312 and 3,539,353, Miller et al U.S. Pat. No. 3,227,571, 
Boyer et al U.S. Pat. No. 3,532,502, Malan U.S. Pat. No. 3,551,151, Lohmer 
et al U.S. Pat. No. 4,018,609, Luciani et al U.K. Pat. No. 1,186,790, Hori 
et al U.K. Pat. No. 1,489,080 and Hori et al Belgian Pat. No. 856,631, 
U.K. Pat. No. 1,490,644, U.K. Pat. No. 1,483,551, Arase et al U.K. Pat. 
No. 1,459,906, Salo U.S. Pat. Nos. 2,110,491 and 2,311,086, Fallesen U.S. 
Pat. No. 2,343,650, Yutzy U.S. Pat. No. 2,322,085, Lowe U.S. Pat. No. 
2,563,791, Talbot et al U.S. Pat. No. 2,725,293, Hilborn U.S. Pat. No. 
2,748,022, DePauw et al U.S. Pat. No. 2,956,883, Ritchie U.K. Pat. No. 
2,095, DeStubner U.S. Pat. No. 1,752,069, Sheppard et al U.S. Pat. No. 
2,127,573, Lierg U.S. Pat. No. 2,256,720, Gaspar U.S. Pat. No. 2,361,936, 
Farmer U.K. Pat. No. 15,727, Stevens U.K. Pat. No. 1,062,116 and Yamamoto 
et al U.S. Pat. No. 3,923,517. Gelatin and gelatin derivatives are 
preferred vehicles. 
The emulsion layers and other layers of the photographic elements, such as 
overcoat layers, interlayers and subbing layers can also contain alone or 
in combination with hydrophilic water permeable colloids as vehicles or 
vehicle extenders (e.g., in the form of latices), synthetic polymeric 
peptizers, carriers and/or binders such as poly(vinyl lactams), acrylamide 
polymers, polyvinyl alcohol and its derivatives, polyvinyl acetals, 
polymers of alkyl and sulfoalkyl acrylates and methacrylates, hydrolyzed 
polyvinyl acetates, polyamides, polyvinyl pyridine, acrylic acid polymers, 
maleic anhydride copolymers, polyalkylene oxides, methacrylamide 
copolymers, polyvinyl oxazolidinones, maleic acid copolymers, vinylamine 
copolymers, methacrylic acid copolymers, acryloyloxyalkylsulfonic acid 
copolymers, sulfoalkylacrylamide copolymers, polyalkyleneimine copolymers, 
polyamines, N,N-dialkylaminoalkyl acrylates, vinyl imidazole copolymers, 
vinyl sulfide copolymers, halogenated styrene polymers, amineacrylamide 
polymers, polypeptides and the like as described in Hollister et al U.S. 
Pat. Nos. 3,679,425, 3,706,564 and 3,813,251, Lowe U.S. Pat. Nos. 
2,253,078, 2,276,322, '323, 2,281,703, 2,311,058 and 2,414,207, Lowe et al 
U.S. Pat. Nos. 2,484,456, 2,541,474 and 2,632,704, Perry et al U.S. Pat. 
No. 3,425,836, Smith et al U.S. Pat. Nos. 3,415,653 and 3,615,624, Smith 
U.S. Pat. No. 3,488,708, Whiteley et al U.S. Pat. Nos. 3,392,025 and 
3,511,818, Fitzgerald U.S. Pat. Nos. 3,681,079, 3,721,565, 3,852,073, 
3,861,918 and 3,925,083, Fitzgerald et al U.S. Pat. No. 3,879,205, Nottorf 
U.S. Pat. No. 3,142,568, Houck et al U.S. Pat. Nos. 3,062,674 and 
3,220,844, Dann et al U.S. Pat. No. 2,882,161, Schupp U.S. Pat. No. 
2,579,016, Weaver U.S. Pat. No. 2,829,053, Alles et al U.S. Pat. No. 
2,698,240, Priest et al U.S. Pat. No. 3,003,879, Merrill et al U.S. Pat. 
No. 3,419,397, Stonham U.S. Pat. No. 3,284,207, Lohmer et al U.S. Pat. No. 
3,167,430, Williams U.S. Pat. No. 2,957,767, Dawson et al U.S. Pat. No. 
2,893,867, Smith et al U.S. Pat. Nos. 2,860,986 and 2,904,539, Ponticello 
et al U.S. Pat. Nos. 3,929,482 and 3,860,428, Ponticello U.S. Pat. No. 
3,939,130, Dykstra U.S. Pat. No. 3,411,911 and Dykstra et al Canadian Pat. 
No. 774,054, Ream et al U.S. Pat. No. 3,287,289, Smith U.K. Pat. No. 
1,466,600, Stevens U.K. Pat. No. 1,062,116, Fordyce U.S. Pat. No. 
2,211,323, Martinez U.S. Pat. No. 2,284,877, Watkins U.S. Pat. No. 
2,420,455, Jones U.S. Pat. No. 2,533,166, Bolton U.S. Pat. No. 2,495,918, 
Graves U.S. Pat. No. 2,289,775, Yackel U.S. Pat. No. 2,565,418, Unruh et 
al U.S. Pat. Nos. 2,865,893 and 2,875,059, Rees et al U.S. Pat. No. 
3,536,491, Broadhead et al U.K. Pat. No. 1,348,815, Taylor et al U.S. Pat. 
No. 3,479,186, Merrill et al U.S. Pat. No. 3,520,857, Bacon et al U.S. 
Pat. No. 3,690,888, Bowman U.S. Pat. No. 3,748,143, Dickinson et al U.K. 
Pat. Nos. 808,227 and '228, Wood U.K. Pat. No. 822,192 and Iguchi et al 
U.K. Pat. No. 1,398,055. 
The layers of the photographic element containing crosslinkable 
colloids--e.g., the gelatin or gelatin derivative containing layers--can 
be forehardened by various organic and inorganic hardeners, such as those 
described in T. H. James, The Theory of the Photographic Process, 4th Ed., 
MacMillan, 1977, pp. 77-87. The forehardeners can be used alone or in 
combination and in free or in blocked form. 
Typical useful forehardeners include formaldehyde and free dialdehydes, 
such as succinaldehyde and glutaraldehyde, as illustrated by Allen et al 
U.S. Pat. No. 3,232,764; blocked dialdehydes, as illustrated by Kaszuba 
U.S. Pat. No. 2,586,168, Jeffreys U.S. Pat. No. 2,870,013, and Yamamoto et 
al U.S. Pat. No. 3,819,608; .alpha.-diketones, as illustrated by Allen et 
al U.S. Pat. No. 2,725,305; active esters of the type described by Burness 
et al U.S. Pat. No. 3,542,558; sulfonate esters, as illustrated by Allen 
et al U.S. Pat. Nos. 2,725,305 and 2,726,162; active halogen compounds, as 
illustrated by Burness U.S. Pat. No. 3,106,468, Silverman et al U.S. Pat. 
No. 3,839,042, Ballantine et al U.S. Pat. No. 3,951,940 and Himmelmann et 
al U.S. Pat. No. 3,174,861; s-triazines and diazines, as illustrated by 
Yamamoto et al U.S. Pat. No. 3,325,287, Anderau et al U.S. Pat. No. 
3,288,775 and Stauner et al U.S. Pat. No. 3,992,366; epoxides, as 
illustrated by Allen et al U.S. Pat. No. 3,047,394, Burness U.S. Pat. No. 
3,189,459 and Birr et al German Pat. No. 1,085,663; aziridines, as 
illustrated by Allen et al U.S. Pat. No. 2,950,197, Burness et al U.S. 
Pat. No. 3,271,175 and Sato et al U.S. Pat. No. 3,575,705; active olefins 
having two or more active vinyl groups (e.g. vinylsulfonyl groups), as 
illustrated by Burness et al U.S. Pat. Nos. 3,490,911, 3,539,644 and 
3,841,872 (Reissue 29,305), Cohen U.S. Pat. No. 3,640,720, Kleist et al 
German Pat. No. 872,153 and Allen U.S. Pat. No. 2,992,109; blocked active 
olefins, as illustrated by Burness et al U.S. Pat. No. 3,360,372 and 
Wilson U.S. Pat. No. 3,345,177; carbodiimides, as illustrated by Blout et 
al German Pat. No. 1,148,446; isoxazolium salts unsubstituted in the 
3-position, as illustrated by Burness et al U.S. Pat. No. 3,321,313; 
esters of 2-alkoxy-N-carboxydihydroquinoline, as illustrated by 
Bergthaller et al U.S. Pat. No. 4,013,468; N-carbamoyl and 
N-carbamoyloxypyridinium salts, as illustrated by Himmelmann U.S. Pat. No. 
3,880,665; hardeners of mixed function, such as halogen-substituted 
aldehyde acids (e.g., mucochloric and mucobromic acids), as illustrated by 
White U.S. Pat. No. 2,080,019, 'onium substituted acroleins, as 
illustrated by Tschopp et al U.S. Pat. No. 3,792,021, and vinyl sulfones 
containing other hardening functional groups, as illustrated by Sera et al 
U.S. Pat. No. 4,028,320; and polymeric hardeners, such as dialdehyde 
starches, as illustrated by Jeffreys et al U.S. Pat. No. 3,057,723, and 
copoly(acrolein-methacrylic acid), as illustrated by Himmelman et al U.S. 
Pat. No. 3,396,029. 
The use of forehardeners in combination is illustrated by Sieg et al U.S. 
Pat. No. 3,497,358, Dallon et al U.S. Pat. Nos. 3,832,181 and 3,840,370 
and Yamamoto et al U.S. Pat. No. 3,898,089. Hardening accelerators can be 
used, as illustrated by Sheppard et al U.S. Pat. No. 2,165,421, Kleist 
German Pat. No. 881,444, Riebel et al U.S. Pat. No. 3,628,961 and Ugi et 
al U.S. Pat. No. 3,901,708. The patents illustrative of hardeners and 
hardener combinations are here incorporated by reference. 
The tabular grains can be of any silver halide crystal composition known to 
be useful in photography. In a preferred form offering a broad range of 
observed advantages the present invention employs thin tabular grain 
silver bromoiodide emulsions. High aspect ratio silver bromoiodide 
emulsions and their preparation is the subject of Wilgus and Haefner, 
cited above and here incorporated by reference. Generally similar 
procedures can be used to form thin, high aspect ratio tabular grain 
silver bromoiodide emulsions for use in the radiographic elements of this 
invention. Intermediate, as opposed to high, aspect ratios can be achieved 
merely by terminating precipitation earlier. Obtaining thin grains at the 
outset of precipitation, as described below, will result in the tabular 
grain emulsions having thin tabular grains. 
Thin tabular grain silver bromoiodide emulsions can be prepared by a 
precipitation process similar to that which forms a part of the Wilgus and 
Haefner invention as follows: Into a conventional reaction vessel for 
silver halide precipitation equipped with an efficient stirring mechanism 
is introduced a dispersing medium. Typically the dispersing medium 
initially introduced into the reaction vessel is at least about 10 
percent, preferably 20 to 80 percent, by weight based on total weight of 
the dispersing medium present in the silver bromoiodide emulsion at the 
conclusion of grain precipitation. Since dispersing medium can be removed 
from the reaction vessel by ultrafiltration during silver bromoiodide 
grain precipitation, as taught by Mignot U.S. Pat. No. 4,334,012, here 
incorporated by reference, it is appreciated that the volume of dispersing 
medium initially present in the reaction vessel can equal or even exceed 
the volume of the silver bomoiodide emulsion present in the reaction 
vessel at the conclusion of grain precipitation. The dispersing medium 
initially introduced into the reaction vessel is preferably water or a 
dispersion of peptizer in water, optionally containing other ingredients, 
such as one or more silver halide ripening agents and/or metal dopants, 
more specifically described below. Where a peptizer is initially present, 
it is preferably employed in a concentration of at least 10 percent, most 
preferably at least 20 percent, of the total peptizer present at the 
completion of silver bromoiodide precipitation. Additional dispersing 
medium is added to the reaction vessel with the silver and halide salts 
and can also be introduced through a separate jet. It is common practice 
to adjust the proportion of dispersing medium, particularly to increase 
the proportion of peptizer, after the completion of the salt 
introductions. 
A minor portion, typically less than 10 percent, of the bromide salt 
employed in forming the silver bromoiodide grains is initially present in 
the reaction vessel to adjust the bromide ion concentration of the 
dispersing medium at the outset of silver bromoiodide precipitation. Also, 
the dispersing medium in the reaction vessel is initially substantially 
free of iodide ions, since the presence of iodide ions prior to concurrent 
introduction of silver and bromide salts favors the formation of thick and 
nontabular grains. As employed herein, the term "substantially free of 
iodide ions" as applied to the contents of the reaction vessel means that 
there are insufficient iodide ion present as compared to bromide ions to 
precipitate as a separate silver iodide phase. It is preferred to maintain 
the iodide concentration in the reaction vessel prior to silver salt 
introduction at less than 0.5 mole percent of the total halide ion 
concentration present. 
If the pBr of the dispersing medium is initially too high, the tabular 
silver bromoiodide grains produced will be comparatively thick and 
therefore of low aspect ratios. It is contemplated to maintain the pBr of 
the reaction vessel initially at or below 1.6, preferably below 1.5. On 
the other hand, if the pBr is too low, the formation of nontabular silver 
bromoiodide grains is favored. Therefore, it is contemplated to maintain 
the pBr of the reaction vessel at or above 0.6, preferably above 1.1. (As 
herein employed, pBr is defined as the negative logarithm of bromide ion 
concentration. Both pH and pAg are similarly defined for hydrogen and 
silver ion concentrations, respectively.) 
During precipitation silver, bromide, and iodide salts are added to the 
reaction vessel by techniques well known in the precipitation of silver 
bromoiodide grains. Typically an aqueous silver salt solution of a soluble 
silver salt, such as silver nitrate, is introduced into the reaction 
vessel concurrently with the introduction of the bromide and iodide salts. 
The bromide and iodide salts are also typically introduced as aqueous salt 
solutions, such as aqueous solutions of one or more soluble ammonium, 
alkali metal (e.g., sodium or potassium), or alkaline earth metal (e.g., 
magnesium or calcium) halide salts. The silver salt is at least initially 
introduced into the reaction vessel separately from the iodide salt. The 
iodide and bromide salts are added to the reaction vessel separately or as 
a mixture. 
With the introduction of silver salt into the reaction vessel the 
nucleation stage of grain formation is initiated. A population of grain 
nuclei are formed which are capable of serving as precipitation sites for 
silver bromide and silver iodide as the introduction of silver, bromide, 
and iodide salts continues. The precipitation of silver bromide and silver 
iodide onto existing grain nuclei constitutes the growth stage of grain 
formation. The aspect ratios of the tabular grains formed according to 
this invention are less affected by iodide and bromide concentrations 
during the growth stage than during the nucleation stage. It is therefore 
possible during the growth stage to increase the permissible latitude of 
pBr during concurrent introduction of silver, bromide, and iodide salts 
above 0.6, preferably in the range of from about 0.6 to 2.2, most 
preferably from about 0.8 to about 1.5. it is, of course, possible and, in 
fact, preferred to maintain the pBr within the reaction vessel throughout 
silver and halide salt introduction within the initial limits, described 
above prior to silver salt introduction. This is particularly preferred 
where a substantial rate of grain nuclei formation continues throughout 
the introduction of silver, bromide, and iodide salts, such as in the 
preparation of highly polydispersed emulsions. Raising pBr values above 
2.2 during tabular grain growth results in thickening of the grains, but 
can be tolerated in many instances while still realizing thin tabular 
silver bromoiodide grains. 
As an alternative to the introduction of silver, bromide, and iodide salts 
as aqueous solutions, it is specifically contemplated to introduce the 
silver, bromide, and iodide salts, initially or in the growth stage, in 
the form of fine silver halide grains suspended in dispersing medium. The 
grains are sized so that they are readily Ostwald ripened onto larger 
grain nuclei, if any are present, once introduced into the reaction 
vessel. The maximum useful grain sizes will depend on the specific 
conditions within the reaction vessel, such as temperature and the 
presence of solubilizing and ripening agents. Silver bromide, silver 
iodide, and/or silver bromoiodide grains can be introduced. (Since bromide 
and/or iodide are precipitated in preference to chloride, it is also 
possible to employ silver chlorobromide and silver chlorobromoiodide 
grains.) The silver halide grains are preferably very fine--e.g., less 
than 0.1 micron in mean diameter. 
Subject to the pBr requirements set forth above, the concentrations and 
rates of silver, bromide, and iodide salt introductions can take any 
convenient conventional form. The silver and halide salts are preferably 
introduced in concentrations of from 0.1 to 0.5 moles per liter, although 
broader conventional concentration ranges, such as from 0.01 mole per 
liter to saturation, for example, are contemplated. Specifically preferred 
precipitation techniques are those which achieve shortened precipitation 
times by increasing the rate of silver and halide salt introduction during 
the run. The rate of silver and halide salt introduction can be increased 
either by increasing the rate at which the dispersing medium and the 
silver and halide salts are introduced or by increasing the concentration 
of the silver and halide salts within the dispersing medium being 
introduced. It is specifically preferred to increase the rate of silver 
and halide salt introduction, but to maintain the rate of introduction 
below the threshold level at which the formation of new grain nuclei is 
favored--i.e., to avoid renucleation, as taught by Irie U.S. Pat. No. 
3,650,757, Kurz U.S. Pat. No. 3,672,900, Saito U.S. Pat. No. 4,242,445, 
Wilgus German OLS No. 2,107,118,Teitscheid et al European Patent 
Application No. 80102242, and Wey "Growth Mechanism of AgBr Crystals in 
Gelatin Solution", Photographic Science and Engineering, Vol. 21, No. 1, 
January/February 1977, p. 14, et. seq. By avoiding the formation of 
additional grain nuclei after passing into the growth stage of 
precipitation, relatively monodispersed thin tabular silver bromoiodide 
grain populations can be obtained. Emulsions having coefficients of 
variation of less than about 30 percent can be prepared. (As employed 
herein the coefficient of variation is defined as 100 times the standard 
deviation of the grain diameter divided by the average grain diameter). By 
intentionally favoring renucleation during the growth stage of 
precipitation, it is, of course, possible to produce polydispersed 
emulsions of substantially higher coefficients of variation. 
The concentration of iodide in the silver bromoiodide emulsions of this 
invention can be controlled by the introduction of iodide salts. Any 
conventional iodide concentration can be employed. Even very small amounts 
of iodide--e.g., as low as 0.05 mole percent--are recognized in the art to 
be beneficial. (Except as otherwise indicated, all references to halide 
percentages are based on silver present in the corresponding emulsion, 
grain, or grain region being discussed; e.g., a grain consisting of silver 
bromoiodide containing 40 mole percent iodide also contains 60 mole 
percent bromide.) In one preferred form the emulsions of the present 
invention incorporate at least about 0.1 mole percent iodide. Silver ioide 
can be incorporated into the tabular silver bromoiodide grains up to its 
solubility limit in silver bromide at the temperature of grain formation. 
Thus, silver iodide concentrations of up to about 40 mole percent in the 
tabular silver bromoiodide grains can be achieved at precipitation 
temperatures of 90.degree. C. In practice precipitation temperatures can 
range down to near ambient room temperatures--e.g., about 30.degree. C. It 
is generally preferred that precipitation be undertaken at temperatures in 
the range of from 40.degree. to 80.degree. C. For most photographic 
applications it is preferred to limit maximum iodide concentrations to 
about 20 mole percent, with optimum iodide concentrations being up to 
about 15 mole percent. In radiographic elements iodide is preferably 
present in concentrations up to 6 mole percent. 
The relative proportion of iodide and bromide salts introduced into the 
reaction vessel during precipitation can be maintained in a fixed ratio to 
form a substantially uniform iodide profile in the tabular silver 
bromoiodide grains or varied to achieve differing photographic effects. 
Solberg et al, cited above, has recognized that advantages in photographic 
speed and/or granularity can result from increasing the proportion of 
iodide in laterally displaced, preferably annular, regions of tabular 
grain silver bromoiodide emulsions as compared to central regions of the 
tabular grains. Solberg et al teaches iodide concentrations in the central 
regions of tabular grains of from 0 to 5 mole percent, with at least one 
mole percent higher iodide concentrations in the laterally surrounding 
annular regions up to the solubility limit of silver iodide in silver 
bromide, preferably up to about 20 mole percent and optimally up to about 
15 mole percent. The teachings of Solberg et al are directly applicable to 
this invention. The tabular silver bromoiodide grains of the present 
invention can exhibit substantially uniform or graded iodide concentration 
profiles and the gradation can be controlled, as desired, to favor higher 
iodide concentrations internally or at or near the surfaces of the tabular 
silver bromoiodide grains. 
Although the preparation of the thin tabular grain silver bromoiodide 
emulsions has been described by reference to the process of Wilgus and 
Haefner, which produces neutral or nonammoniacal emulsions, the emulsions 
of the present invention and their utility are not limited by any 
particular process for their preparation. A process of preparing high 
aspect ratio tabular grain silver bromoiodide emulsions discovered 
subsequent to that of Wilgus and Haefner is described by Daubendiek and 
Strong, cited above and here incorporated by reference. Daubendiek and 
Strong teaches an improvement over the processes of Maternaghan, cited 
above, wherein in a preferred form the silver iodide concentration in the 
reaction vessel is reduced below 0.05 mole per liter and the maximum size 
of the silver iodide grains initially present in the reaction vessel is 
reduced below 0.05 micron. Again, merely by terminating precipitaton 
sooner, thin, intermediate aspect ratio tabular grain bromoiodide 
emulsions can be prepared. 
Thin, high and intermediate aspect ratio tabular grain silver bromide 
emulsions lacking iodide can be prepared by the process described above 
similar to the process of Wilgus and Haefner further modified to exclude 
iodide. Thin tabular silver bromide emulsions containing square and 
rectangular grains can be prepared similarly as taught by Mignot U.S. Ser. 
No. 320,912, cited above. In this process cubic seed grains having an edge 
length of less than 0.15 micron are employed. While maintaining the pAg of 
the seed grain emulsion in the range of from 5.0 to 8.0, the emulsion is 
ripened in the substantial absence of nonhalide silver ion complexing 
agents to produce tabular silver bromide grains having the desired average 
aspect ratio. Still other preparations of thin tabular grain silver 
bromide emulsions lacking iodide are illustrated in the examples. 
To illustrate other thin tabular grain silver halide emulsions which can be 
prepared merely by terminating precipitation when the desired aspect 
ratios are achieved, attention is directed to the following: 
Maskasky, titled SILVER CHLORIDE EMULSIONS OF MODIFIED CRYSTAL HABIT AND 
PROCESSES FOR THEIR PREATION, cited above and here incorporated by 
reference, discloses a process of preparing tabular grains of at least 50 
mole percent chloride having opposed crystal faces lying in {111} crystal 
planes and at least one peripheral edge lying parallel to a &lt;211&gt; 
crystallographic vector in the plane of one of the major surfaces. Such 
tabular grain emulsions can be prepared by reacting aqueous silver and 
chloride-containing halide salt solutions in the presence of a crystal 
habit modifying amount of an aminoazaindene and a peptizer having a 
thioether linkage. 
Wey and Wilgus, cited above and here incorporated by reference, discloses 
tabular grain emulsions wherein the silver halide grains contain chloride 
and bromide in at least annular grain regions and preferably throughout. 
The tabular grain regions containing silver chloride and bromide are 
formed by maintaining a molar ratio of chloride and bromide ions of from 
1.6:1 to about 260:1 and the total concentration of halide ions in the 
reaction vessel in the range of from 0.10 to 0.90 normal during 
introduction of silver, chloride, bromide, and, optionally, iodide salts 
into the reaction vessel. The molar ratio of silver chloride to silver 
bromide in the tabular grains can range from 1:99 to 2:3. 
Modifying compounds can be present during tabular grain precipitation. Such 
compounds can be initially in the reaction vessel or can be added along 
with one or more of the salts according to conventional procedures. 
Modifying compounds, such as compounds of copper, thallium, lead, bismuth, 
cadmium, zinc, middle chalcogens (i.e., sulfur, selenium, and tellurium), 
gold, and Group VIII noble metals, can be present during silver halide 
precipitation, as illustrated by Arnold et al U.S. Pat. No. 1,195,432, 
Hochstetter U.S. Pat. No. 1,951,933, Trivelli et al U.S. Pat. No. 
2,448,060, Overman U.S. Pat. No. 2,628,167, Mueller et al U.S. Pat. No. 
2,950,972, Sidebotham U.S. Pat. No. 3,488,709, Rosecrants et al U.S. Pat. 
No. 3,737,313, Berry et al U.S. Pat. No. 3,772,031, Atwell U.S. Pat. No. 
4,269,927, and Research Disclosure, Vol. 134, June 1975, Item 13452. 
Research Disclosure and its predecessor, Product Licensing Index, are 
publications of Industrial Opportunities Ltd.; Homewell, Havant; 
Hampshire, P09 1EF, United Kingdom. The tabular grain emulsions can be 
internally reduction sensitized during precipitation, as illustrated by 
Moisar et al, Journal of Photographic Science, Vol. 25, 1977, pp. 19-27. 
The individual silver and halide salts can be added to the reaction vessel 
through surface or subsurface delivery tubes by gravity feed or by 
delivery apparatus for maintaining control of the rate of delivery and the 
pH, pBr, and/or pAg of the reaction vessel contents, as illustrated by 
Culhane et al U.S. Pat. No. 3,821,002, Oliver U.S. Pat. No. 3,031,304 and 
Claes et al, Photographische Korrespondenz, Band 102, Number 10, 1967, p. 
162. In order to obtain rapid distribution of the reactants within the 
reaction vessel, specially constructed mixing devices can be employed, as 
illustrated by Audran U.S. Pat. No. 2,996,287, McCrossen et al U.S. Pat. 
No. 3,342,605, Frame et al U.S. Pat. No. 3,415,650, Porter et al U.S. Pat. 
No. 3,785,777, Finnicum et al U.S. Pat. No. 4,147,551, Verhille et al U.S. 
Pat. No. 4,171,224, Calamur U.K. Patent Application No. 2,022,431A, Saito 
et al German OLS 2,555,364 and 2,556,885, and Research Disclosure, Volume 
166, February 1978, Item 16662. 
In forming the tabular grain emulsions peptizer concentrations of from 0.2 
to about 10 percent by weight, based on the total weight of emulsion 
components in the reaction vessel, can be employed; it is preferred to 
keep the concentration of the peptizer in the reaction vessel prior to and 
during silver bromoiodide formation below about 6 percent by weight, based 
on the total weight. It is common practice to maintain the concentration 
of the peptizer in the reaction vessel in the range of below about 6 
percent, based on the total weight, prior to and during silver halide 
formation and to adjust the emulsion vehicle concentration upwardly for 
optimum coating characteristics by delayed, supplemental vehicle 
additions. It is contemplated that the emulsion as initially formed will 
contain from about 5 to 50 grams of peptizer per mole of silver halide, 
preferably about 10 to 30 grams of peptizer per mole of silver halide. 
Additional vehicle can be added later to bring the concentration up to as 
high as 1000 grams per mole of silver halide. Preferably the concentration 
of vehicle in the finished emulsion is above 50 grams per mole of silver 
halide. When coated and dried in forming a photographic element the 
vehicle preferably forms about 30 to 70 percent by weight of the emulsion 
layer. 
It is specifically contemplated that grain ripening can occur during the 
preparation of silver halide emulsions according to the present invention, 
and it is preferred that grain ripening occur within the reaction vessel 
during at least silver bromoiodide grain formation. Known silver halide 
solvents are useful in promoting ripening. For example, an excess of 
bromide ions, when present in the reaction vessel, is known to promote 
ripening. It is therefore apparent that the bromide salt solution run into 
the reaction vessel can itself promote ripening. Other ripening agents can 
also be employed and can be entirely contained within the dispersing 
medium in the reaction vessel before silver and halide salt addition, or 
they can be introduced into the reaction vessel along with one or more of 
the halide salt, silver salt, or peptizer. In still another variant the 
ripening agent can be introduced independently during halide and silver 
salt additions. Although ammonia is a known ripening agent, it is not a 
preferred ripening agent for the silver bromoiodide emulsions of this 
invention exhibiting the highest realized speed-granularity relationships. 
Among preferred ripening agents are those containing sulfur. Thiocyanate 
salts can be used, such as alkali metal, most commonly sodium and 
potassium, and ammonium thiocyanate salts. While any conventional quantity 
of the thiocyanate salts can be introduced, preferred concentrations are 
generally from about 0.1 to 20 grams of thiocyanate salt per mole of 
silver halide, based on the weight of silver. Illustrative prior teachings 
of employing thiocyanate ripening agents are found in Nietz et al, U.S. 
Pat. No. 2,222,264, cited above; Lowe et al U.S. Pat. No. 2,448,534 and 
Illingsworth U.S. Pat. No. 3,320,069; the disclosures of which are here 
incorporated by reference. Alternatively, conventional thioether ripening 
agents, such as those disclosed in McBride U.S. Pat. No. 3,271,157, Jones 
U.S. Pat. No. 3,574,628, and Rosecrants et al U.S. Pat. No. 3,737,313, 
here incorporated by reference, can be employed. 
The thin tabular grain emulsions employed in the present invention are 
preferably washed to remove soluble salts. The soluble salts can be 
removed by decantation, filtration, and/or chill setting and leaching, as 
illustrated by Craft U.S. Pat. No. 2,316,845 and McFall et al U.S. Pat. 
No. 3,396,027; by coagulation washing, as illustrated by Hewitson et al 
U.S. Pat. No. 2,618,556, Yutzy et al U.S. Pat. No. 2,614,928, Yackel U.S. 
Pat. No. 2,565,418, Hart et al U.S. Pat. No. 3,241,969, Waller et al U.S. 
Pat. No. 2,489,341, Klinger U.K. Pat. No. 1,305,409 and Dersch et al U.K. 
Pat. No. 1,167,159; by centrifugation and decantation of a coagulated 
emulsion, as illustrated by Murray U.S. Pat. No. 2,463,794, Ujihara et al 
U.S. Pat. No. 3,707,378, Audran U.S. Pat. No. 2,996,287 and Timson U.S. 
Pat. No. 3,498,454; by employing hydrocyclones alone or in combination 
with centrifuges, as illustrated by U.K. Pat. No. 1,336,692, Claes U.K. 
Pat. No. 1,356,573 and Ushomirskii et al Soviet Chemical Industry, Vol. 6, 
No. 3, 1974, pp. 181-185; by diafiltration with a semipermeable membrane, 
as illustrated by Research Disclosure, Vol. 102, October 1972, Item 10208, 
Hagemaier et al Research Disclosure, Vol. 131, March 1975, Item 13122, 
Bonnet Research Disclosure, Vol. 135, July 1975, Item 13577, Berg et al 
German OLS 2,436,461, Bolton U.S. Pat. No. 2,495,918, and Mignot U.S. Pat. 
No. 4,334,012, cited above, or by employing an ion exchange resin, as 
illustrated by Maley U.S. Pat. No. 3,782,953 and Noble U.S. Pat. No. 
2,827,428. The emulsions, with or without sensitizers, can be dried and 
stored prior to use as illustrated by Research Disclosure, Vol. 101, 
September 1972, Item 10152. In the present invention washing is 
particularly advantageous in terminating ripening of the tabular grains 
after the completion of precipitation to avoid increasing their thickness 
and reducing their aspect ratio. 
High aspect ratio tabular grain emulsions useful in the practice of this 
invention can have extremely high average aspect ratios. Tabular grain 
average aspect ratios can be increased by increasing average grain 
diameters. This can produce sharpness advantages, but maximum average 
grain diameters are generally limited by granularity requirements for a 
specific photographic application. Tabular grain average aspect ratios can 
also or alternatively be increased by decreasing average grain 
thicknesses. When silver coverages are held constant, decreasing the 
thickness of tabular grains generally improves granularity as a direct 
function of increasing aspect ratio. Hence the maximum average aspect 
ratios of the tabular grain emulsions of this invention are a function of 
the maximum average grain diameters acceptable for the specific 
photographic application and the minimum attainable tabular grain 
thicknesses which can be produced. Maximum average aspect ratios have been 
observed to vary, depending upon the precipitation technique employed and 
the tabular grain halide composition. The highest observed average aspect 
ratios, 500:1, for tabular grains with photographically useful average 
grain diameters, have been achieved by Ostwald ripening preparations of 
silver bromide grains, with aspect ratios of 100:1, 200:1, or even higher 
being obtainable by double-jet precipitation procedures. The presence of 
iodide generally decreases the maximum average aspect ratios realized, but 
the preparation of silver bromoiodide tabular grain emulsions having 
average aspect ratios of 100:1 or even 200:1 or more is feasible. Average 
aspect ratios as high as 50:1 or even 100:1 for silver chloride tabular 
grains, optionally containing bromide and/or iodide, can be prepared as 
taught by Maskasky, cited above. It is contemplated that in all instances 
the average diameter of the thin tabular grains will be less than 30 
microns, preferably less than 15 microns, and optimally no greater than 10 
microns. 
The present invention is equally applicable to photographic elements 
intended to form negative or positive images. For example, the 
photographic elements can be of a type which form either surface or 
internal latent images on exposure and which produce negative images on 
processing. Alternatively, the photographic elements can be of a type that 
produce direct positive images in response to a single development step. 
When the tabular and other imaging silver halide grains present in the 
photographic element are intended to form direct positive images, they can 
be surface fogged and employed in combination with an organic electron 
acceptor, as taught, for example, by Kendall et al U.S. Pat. No. 
2,541,472, Shouwenaars U.K. Pat. No. 723,019, Illingsworth U.S. Pat. Nos. 
3,501,305, '306, and '307, Research Disclosure, Vol. 134, June 1975, Item 
13452, Kurz U.S. Pat. No. 3,672,900, Judd et al U.S. Pat. No. 3,600,180, 
and Taber et al U.S. Pat. No. 3,647,643. The organic electron acceptor can 
be employed in combination with a spectrally sensitizing dye or can itself 
be a spectrally sensitizing dye, as illustrated by Illingsworth et al U.S. 
Pat. No. 3,501,310. If internally sensitive emulsions are employed, 
surface fogging and organic electron acceptors can be employed in 
combination, as illustrated by Lincoln et al U.S. Pat. No. 3,501,311, but 
neither surface fogging nor organic electron acceptors are required to 
produce direct positive images. Direct positive images can be formed by 
development of internally sensitive emulsions in the presence of 
nucleating agents, which can be contained in either the developer or the 
photographic element, as illustrated by Research Disclosure, Vol. 151, 
November 1976, Item 15162. Preferred nucleating agents are those adsorbed 
directly to the surfaces of the silver halide grains, as illustrated by 
Lincoln et al U.S. Pat. Nos. 3,615,615 and 3,759,901, Spence et al U.S. 
Pat. No. 3,718,470, Kurtz et al U.S. Pat. Nos. 3,719,494 and 3,734,738, 
Leone et al U.S. Pat. Nos. 4,030,925 and 4,080,207, Adachi et al U.S. Pat. 
No. 4,115,122, von Konig et al U.S. Pat. No. 4,139,387, and U.K. Pat. Nos. 
2,011,391 and 2,012,443. Evans, Daubendiek, and Raleigh U.S. Ser. No. 
431,912, titled DIRECT REVERSAL EMULSIONS AND PHOTOGRAPHIC ELEMENTS USEFUL 
IN IMAGE TRANSFER FILM UNITS, filed concurrently herewith and commonly 
assigned, which is a continuation-in-part of U.S. Ser. No. 320,891, filed 
Nov. 12, 1981, both here incorporated by reference, discloses internal 
latent image-forming high aspect ratio thin tabular grain emulsions 
containing nucleating agents. Similar emulsions, but containing thin 
tabular grains of intermediate aspect ratios, are also useful in the 
practice of this invention. 
In addition to the specific features described above the photographic 
elements of this invention can employ conventional features, such as those 
of the paragraphs cited below in Research Disclosure, Item 17643, 
previously cited, here incorporated by reference. The emulsions can be 
chemically sensitized, as described in Paragraph III, and/or spectrally 
sensitized or desensitized, as described in Paragraph IV. Preferred 
chemical and spectral sensitization of thin tabular grain emulsions 
according to this invention is described by Kofron et al, cited above. The 
photographic elements can contain brighteners, antifoggants, stabilizers, 
scattering or absorbing materials, coating aids, plasticizers, lubricants, 
and matting agents, as described in Research Disclosure, Item 17643, cited 
above, Paragraphs V, VI, VIII, XI, XII, and XVI. Methods of addition and 
coating and drying procedures can be employed, as described in Paragraphs 
XIV and XV. Conventional photographic supports can be employed, as 
described in Paragraph XVII. Other conventional features will readily be 
suggested to those skilled in the art. 
The invention is particularly applicable to radiographic elements. The 
preferred radiographic elements of this invention are those produced by 
fully forehardening the radiographic elements containing at least one 
thin, high or intermediate aspect ratio tabular grain emulsion layer 
disclosed by Abbott and Jones, cited above and here incorporated by 
reference. Abbott and Jones disclose the use of two image-forming layer 
units located on opposed major surfaces of the support. The interposed 
support is capable of transmitting radiation to which at least one and, 
typically, both of the image-forming layer units are responsive. That is, 
the support is specularly transmissive to exposing radiation. The support 
is substantially colorless and transparent, even though it can be tinted. 
The two image-forming layer units each contain at least one 
radiation-sensitive emulsion containing thin tabular silver halide grains 
having an intermediate average aspect ratio of the type more specifically 
described above. 
To achieve both the advantages in covering power of the present invention 
and the crossover advantages disclosed by Abbott et al the tabular silver 
halide grains have adsorbed to their surfaces spectral sensitizing dye. It 
is specifically contemplated to employ spectral sensitizing dyes that 
exhibit absorption maxima in the blue and minus blue--i.e., green and red, 
portions of the visible spectrum. In addition, for specialized 
applications, spectral sensitizing dyes can be employed which improve 
spectral response beyond the visible spectrum. For example, the use of 
infrared absorbing spectral sensitizers is specifically contemplated. 
The thin tabular grain silver halide emulsions can be spectrally sensitized 
with dyes from a variety of classes, including the polymethine dye class, 
which includes the cyanines, merocyanines, complex cyanines and 
merocyanines (i.e., tri-, tetra- and poly-nuclear cyanines and 
merocyanines), oxonols, hemioxonols, styryls, merostyryls and 
streptocyanines. 
The cyanine spectral sensitizing dyes include, joined by a methine linkage, 
two basic heterocyclic nuclei, such as those derived from quinolinium, 
pyridinium, isoquinolinium, 3H-indolium, benz[e]indolium, oxazolium, 
oxazolinium, thiazolium, thiazolinium, selenazolium, selenazolinium, 
imidazolium, imidazolinium, benzoxazolium, benzothiazolium, 
benzoselenazolium, benzimidazolium, naphthoxazolium, naphthothiazolium, 
naphthoselenazolium, dihydronaphthothiazolium, pyrylium and 
imidazopyrazinium quaternary salts. 
The merocyanine spectral sensitizing dyes include, joined by a methine 
linkage, a basic heterocyclic nucleus of the cyanine dye type and an 
acidic nucleus, such as can be derived from barbituric acid, 
2-thiobarbituric acid, rhodanine, hydantoin, 2-thiohydantoin, 
4-thiohydantoin, 2-pyrazolin-5-one, 2-isoxazolin-5-one, indan-1,3-dione, 
cyclohexane-1,3-dione, 1,3-dioxane-4,6-dione, pyrazolin-3,5-dione, 
pentane-2,4-dione, alkylsulfonylacetonitrile, malononitrile, 
isoquinolin-4-one, and chroman-2,4-dione. 
One or more spectral sensitizing dyes may be used. Dyes with sensitizing 
maxima at wavelengths throughout the visible spectrum and with a great 
variety of spectral sensitivity curve shapes are known. The choice and 
relative proportions of dyes depends upon the region of the spectrum to 
which sensitivity is desired and upon the shape of the spectral 
sensitivity curve desired. Dyes with overlapping spectral sensitivity 
curves will often yield in combination a curve in which the sensitivity at 
each wavelength in the area of overlap is approximately equal to the sum 
of the sensitivities of the individual dyes. Thus, it is possible to use 
combinations of dyes with different maxima to achieve a spectral 
sensitivity curve with a maximum intermediate to the sensitizing maxima of 
the individual dyes. 
Combinations of spectral sensitizing dyes can be used which result in 
supersensitization--that is, spectral sensitization that is greater in 
some spectral region than that from any concentration of one of the dyes 
alone or that which would result from the additive effect of the dyes. 
Supersensitization can be achieved with selected combinations of spectral 
sensitizing dyes and other addenda, such as stabilizers and antifoggants, 
development accelerators or inhibitors, coating aids, brighteners and 
antistatic agents. Any one of several mechanisms as well as compounds 
which can be responsible for supersensitization are discussed by Gilman, 
"Review of the Mechanisms of Supersensitization", Photographic Science and 
Engineering, Vol. 18, 1974, pp. 418-430. 
Spectral sensitizing dyes also affect the emulsions in other ways. Spectral 
sensitizing dyes can also function as antifoggants or stabilizers, 
development accelerators or inhibitors, and halogen acceptors or electron 
acceptors, as disclosed in Brooker et al U.S. Pat. No. 2,131,038 and Shiba 
et al U.S. Pat. No. 3,930,860. 
Sensitizing action can be correlated to the position of molecular energy 
levels of a dye with respect to ground state and conduction band energy 
levels of the silver halide crystals. These energy levels can in turn be 
correlated to polarographic oxidation and reduction potentials, as 
discussed in Photographic Science and Engineering, Vol. 18, 1974, pp. 
49-53 (Sturmer et al), pp. 175-178 (Leubner) and pp. 475-485 (Gilman). 
Oxidation and reduction potentials can be measured as described by R. F. 
Large in Photographic Sensitivity, Academic Press, 1973, Chapter 15. 
The chemistry of cyanine and related dyes is illustrated by Weissberger and 
Taylor, Special Topics of Heterocyclic Chemistry, John Wiley and Sons, New 
York, 1977, Chapter VIII; Venkataraman, The Chemistry of Synthetic Dyes, 
Academic Press, New York, 1971, Chapter V; James, The Theory of the 
Photographic Process, 4th Ed., Macmillan, 1977, Chapter 8, and F. M. 
Hamer, Cyanine Dyes and Related Compounds, John Wiley and Sons, 1964. 
In a preferred form of this invention the tabular silver halide grains have 
adsorbed to their surfaces spectral sensitizing dye which exhibits a shift 
in hue as a function of adsorption. Any conventional spectral sensitizing 
dye known to exhibit a bathochromic or hypsochromic increase in light 
absorption as a function of adsorption to the surface of silver halide 
grains can be employed in the practice of this invention. Dyes satisfying 
such criteria are well known in the art, as illustrated by T. H. James, 
The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, Chapter 
8 (particularly, F. Induced Color Shifts in Cyanine and Merocyanine Dyes) 
and Chapter 9 (particularly, H. Relations Between Dye Structure and 
Surface Aggregation) and F. M. Hamer, Cyanine Dyes and Related Compounds, 
John Wiley and Sons, 1964, Chapter XVII (particularly, F. Polymerization 
and Sensitization of the Second Type). Merocyanine, hemicyanine, styryl, 
and oxonol spectral sensitizing dyes which produce H aggregates 
(hypsochromic shifting) are known to the art, although J aggregates 
(bathochromic shifting) is not common for dyes of these classes. Preferred 
spectral sensitizing dyes are cyanine dyes which exhibit either H or J 
aggregation. 
In a specifically preferred form the spectral sensitizing dyes are 
carbocyanine dyes which exhibit J aggregation. Such dyes are characterized 
by two or more basic heterocyclic nuclei joined by a linkage of three 
methine groups. The heterocyclic nuclei preferably include fused benzene 
rings to enhance J aggregation. Preferred heterocyclic nuclei for 
promoting J aggregation are quinolinium, benzoxazolium, benzothiazolium, 
benzoselenazolium, benzimidazolium, naphthoxazolium, naphthothiazolium, 
and naphthoselenazolium quaternary salts. 
Although native blue sensitivity of silver bromide or bromoiodide is 
usually relied upon in the art in emulsion layers intended to record 
exposure to blue light, significant advantages can be obtained by the use 
of spectral sensitizers, even where their principal absorption is in the 
spectral region to which the emulsions possess native sensitivity. For 
example, it is specifically recognized that advantages can be realized 
from the use of blue spectral sensitizing dyes. 
Useful blue spectral sensitizing dyes for thin tabular grain silver bromide 
and silver bromoiodide emulsions can be selected from any of the dye 
classes known to yield spectral sensitizers. Polymethine dyes, such as 
cyanines, merocyanines, hemicyanines, hemioxonols, and merostyryls, are 
preferred blue spectral sensitizers. Generally useful blue spectral 
sensitizers can be selected from among these dye classes by their 
absorption characteristics--i.e., hue. There are, however, general 
structural correlations that can serve as a guide in selecting useful blue 
sensitizers. Generally the shorter the methine chain, the shorter the 
wavelength of the sensitizing maximum. Nuclei also influence absorption. 
The addition of fused rings to nuclei tends to favor longer wavelengths of 
absorption. Substituents can also alter absorption characteristics. 
Among useful spectral sensitizing dyes for sensitizing silver halide 
emulsions are those found in U.K. Pat. No. 742,112, Brooker U.S. Pat. Nos. 
1,846,300, '301, '302, '303, '304, 2,078,233 and 2,089,729, Brooker et al 
U.S. Pat. Nos. 2,165,338, 2,213,238, 2,231,658, 2,493,747, '748, 
2,526,632, 2,739,964 (Reissue 24,292), 2,778,823, 2,917,516, 3,352,857, 
3,411,916 and 3,431,111, Wilmanns et al U.S. Pat. No. 2,295,276, Sprague 
U.S. Pat. Nos. 2,481,698 and 2,503,776, Carroll et al U.S. Pat. Nos. 
2,688,545 and 2,704,714, Larive et al U.S. Pat. No. 2,921,067, Jones U.S. 
Pat. No. 2,945,763, Nys et al U.S. Pat. No. 3,282,933, Schwan et al U.S. 
Pat. No. 3,397,060, Riester U.S. Pat. No. 3,660,102, Kampfer et al U.S. 
Pat. No. 3,660,103, Taber et al U.S. Pat. Nos. 3,335,010, 3,352,680 and 
3,384,486, Lincoln et al U.S. Pat. No. 3,397,981, Fumia et al U.S. Pat. 
Nos. 3,482,978 and 3,623,881, Spence et al U.S. Pat. No. 3,718,470 and Mee 
U.S. Pat. No. 4,025,349. Examples of useful dye combinations, including 
supersensitizing dye combinations, are found in Motter U.S. Pat. No. 
3,506,443 and Schwan et al U.S. Pat. No. 3,672,898. As examples of 
supersensitizing combinations of spectral sensitizing dyes and nonlight 
absorbing addenda, it is specifically contemplated to employ thiocyanates 
during spectral sensitization, as taught by Leermakers U.S. Pat. No. 
2,221,805; bis-triazinylaminostilbenes, as taught by McFall et al U.S. 
Pat. No. 2,933,390; sulfonated aromatic compounds, as taught by Jones et 
al U.S. Pat. No. 2,937,089; mercapto-substituted heterocycles, as taught 
by Riester U.S. Pat. No. 3,457,078; iodide, as taught by U.K. Pat. No. 
1,413,826; and still other compounds, such as those disclosed by Gilman, 
"Review of the Mechanisms of Supersensitization", cited above. 
Conventional amounts of dyes can be employed in spectrally sensitizing the 
emulsion layers containing nontabular or thick tabular silver halide 
grains. To realize the full advantages of thin tabular grain emulsions it 
is preferred to adsorb spectral sensitizing dye to the tabular grain 
surfaces in a substantially optimum amount--that is, in an amount 
sufficient to realize at least 60 percent of the maximum photographic 
speed attainable from the grains under contemplated conditions of 
exposure. The quantity of dye employed will vary with the specific dye or 
dye combination chosen as well as the size and aspect ratio of the grains. 
It is known in the photographic art that optimum spectral sensitization is 
obtained with organic dyes at about 25 to 100 percent or more of monolayer 
coverage of the total available surface area of surface sensitive silver 
halide grains, as disclosed, for example, in West et al, "The Adsorption 
of Sensitizing Dyes in Photographic Emulsions", Journal of Phys. Chem., 
Vol. 56, p. 1065, 1952; Spence et al, "Desensitization of Sensitizing 
Dyes", Journal of Physical and Colloid Chemistry, Vol. 56, No. 6, June 
1948, pp. 1090-1103; and Gilman et al U.S. Pat. No. 3,979,213. Optimum dye 
concentration levels can be chosen by procedures taught by Mees, Theory of 
the Photographic Process, 1942, Macmillan, pp. 1067-1069. 
Spectral sensitization can be undertaken at any stage of emulsion 
preparation heretofore known to be useful. Most commonly spectral 
sensitization is undertaken in the art subsequent to the completion of 
chemical sensitization. However, it is specifically recognized that 
spectral sensitization can be undertaken alternatively concurrently with 
chemical sensitization, can entirely precede chemical sensitization, and 
can even commence prior to the completion of silver halide grain 
precipitation, as taught by Philippaerts et al U.S. Pat. No. 3,628,960, 
and Locker et al U.S. Pat. No. 4,225,666. As taught by Locker et al, it is 
specifically contemplated to distribute introduction of the spectral 
sensitizing dye into the emulsion so that a portion of the spectral 
sensitizing dye is present prior to chemical sensitization and a remaining 
portion is introduced after chemical sensitization. Unlike Locker et al, 
it is specifically contemplated that the spectral sensitizing dye can be 
added to the emulsion after 80 percent of the silver halide has been 
precipitated. Sensitization can be enhanced by pAg adjustment, including 
cycling, during chemical and/or spectral sensitization. A specific example 
of pAg adjustment is provided by Research Disclosure, Vol. 181, May 1979, 
Item 18155. 
In one preferred form, spectral sensitizers can be incorporated in the 
emulsions of the present invention prior to chemical sensitization. 
Similar results have also been achieved in some instances by introducing 
other adsorbable materials, such as finish modifiers, into the emulsions 
prior to chemical sensitization. 
Independent of the prior incorporation of adsorbable materials, it is 
preferred to employ thiocyanates during chemical sensitization in 
concentrations of from about 2.times.10.sup.-3 to 2 mole percent, based on 
silver, as taught by Damschroder U.S. Pat. No. 2,642,361, cited above. 
Other ripening agents can be used during chemical sensitization. 
In still a third approach, which can be practiced in combination with one 
or both of the above approaches or separately thereof, it is preferred to 
adjust the concentration of silver and/or halide salts present immediately 
prior to or during chemical sensitization. Soluble silver salts, such as 
silver acetate, silver trifluoroacetate, and silver nitrate, can be 
introduced as well as silver salts capable of precipitating onto the grain 
surfaces, such as silver thiocyanate, silver phosphate, silver carbonate, 
and the like. Fine silver halide (i.e., silver bromide, iodide, and/or 
chloride) grains capable of Ostwald ripening onto the tabular grain 
surfaces can be introduced. For example, a Lippmann emulsion can be 
introduced during chemical sensitization. Maskasky, titled CONTROLLED SITE 
EPITAXIAL SENSITIZATION, cited above and here incorporated by reference, 
discloses the chemical sensitization of spectrally sensitized thin tabular 
grain emulsions at one or more ordered discrete sites of the tabular 
grains. It is believed that the preferential adsorption of spectral 
sensitizing dye on the crystallographic surfaces forming the major faces 
of the tabular grains allows chemical sensitization to occur selectively 
at unlike crystallographic surfaces of the tabular grains. 
The preferred chemical sensitizers for the highest attained 
speed-granularity relationships are gold and sulfur sensitizers, gold and 
selenium sensitizers, and gold, sulfur, and selenium sensitizers. Thus, in 
a preferred form of the invention, thin tabular grain silver bromide or, 
most preferably, silver bromoiodide emulsions contain a middle chalcogen, 
such as sulfur and/or selenium, which may not be detectable, and gold, 
which is detectable. The emulsions also usually contain detectable levels 
of thiocyanate, although the concentration of the thiocyanate in the final 
emulsions can be greatly reduced by known emulsion washing techniques. In 
various of the preferred forms indicated above the tabular silver bromide 
or silver bromoiodide grains can have another silver salt at their 
surface, such as silver thiocyanate or another silver halide of differing 
halide content (e.g., silver chloride or silver bromide), although the 
other silver salt may be present below detectable levels. 
Although not required to realize all of their advantages, the emulsions 
employed in the present invention are preferably, in accordance with 
prevailing manufacturing practices, substantially optimally chemically and 
spectrally sensitized. That is, they preferably achieve speeds of at least 
60 percent of the maximum log speed attainable from the grains in the 
spectral region of sensitization under the contemplated conditions of use 
and processing. Log speed is herein defined as 100 (1-log E), where E is 
measured in meter-candle-seconds at a density of 0.1 above fog. Once the 
silver halide grains of an emulsion have been characterized, it is 
possible to estimate from further product analysis and performance 
evaluation whether an emulsion layer of a product appears to be 
substantially optimally chemically and spectrally sensitized in relation 
to comparable commercial offerings of other manufacturers. 
In addition to the features specifically described above the radiographic 
elements of this invention can include additional features of a 
conventional nature in radiographic elements. Exemplary features of this 
type are disclosed, for example, in Research Disclosure, Vol. 184, August 
1979, Item 18431. For example, the emulsions can contain stabilizers, 
antifoggants, and antikink agents, as set forth in Paragraph II, A through 
K. The radiographic element can contain antistatic agents and/or layers, 
as set forth in Paragraph III. The radiographic elements can contain 
overcoat layers, as set out in Paragraph IV. The crossover advantages of 
Abbott et al can be further improved by employing conventional crossover 
exposure control approaches, as disclosed in Item 18431, Paragraph V. 
Preferred radiographic elements are of the type disclosed by Abbott and 
Jones, cited above. That is, at least one thin tabular grain emulsion 
layer is incorporated in each of two imaging units located on opposite 
major surfaces of a support capable of permitting substantially specular 
transmission of imaging radiation. Such radiographic supports are most 
preferably polyester film supports. Poly(ethylene terephthalate) film 
supports are specifically preferred. Such supports as well as their 
preparation are disclosed in Scarlett U.S. Pat. No. 2,823,421, Alles U.S. 
Pat. No. 2,779,684, and Arvidson and Stottlemyer U.S. Pat. No. 3,939,000. 
Medical radiographic elements are usually blue tinted. Generally the 
tinting dyes are added directly to the molten polyester prior to extrusion 
and therefore must be thermally stable. Preferred tinting dyes are 
anthraquinone dyes, such as those disclosed by Hunter U.S. Pat. No. 
3,488,195, Hibino et at U.S. Pat. No. 3,849,139, Arai et al U.S. Pat. Nos. 
3,918,976 and 3,933,502, Okuyama et al U.S. Pat. No. 3,948,664, and U.K. 
Pat. Nos. 1,250,983 and 1,372,668. 
The preferred spectral sensitizing dyes are chosen to exhibit an absorption 
peak shift in their adsorbed state, usually in the H or J band, to a 
region of the spectrum corresponding to the wavelength of electromagnetic 
radiation to which the element is intended to be imagewise exposed. The 
electromagnetic radiation producing imagewise exposure is typically 
emitted from phosphors of intensifying screens. A separate intensifying 
screen exposes each of the two imaging units located on opposite sides of 
the support. The intensifying screens can emit light in the ultraviolet, 
blue, green, or red portions of the spectrum, depending upon the specific 
phosphors chosen for incorporation therein. In a specifically preferred 
form of the invention the spectral sensitizing dye is a carbocyanine dye 
exhibiting a J band absorption when adsorbed to the tabular grains in a 
spectral region corresponding to peak emission by the intensifying screen, 
usually the green region of the spectrum. 
The intensifying screens can themselves form a part of the radiographic 
elements, but usually they are separate elements which are reused to 
provide exposures of successive radiographic elements. Intensifying 
screens are well known in the radiographic art. Conventional intensifying 
screens and their components are disclosed by Research Disclosure, Vol. 
18431, cited above, Paragraph IX, and by Rosecrants U.S. Pat. No. 
3,737,313, the disclosures of which are here incorporated by reference. 
To obtain a viewable silver image the photographic or, in preferred 
applications, radiographic elements are processed in an aqueous alkaline 
developer or, where the developing agent is incorporated in the 
photographic element, in an aqueous alkaline activator solution. 
Development which favors the highest attainable covering power is 
preferred. As pointed out by James, The Theory of the Photographic 
Process, cited above, pp. 40-4,405, 489, and 490, as well as Farnell and 
Solman, also cited above, the highest levels of covering power result from 
obtaining the most filamentary developed silver. Direct or chemical 
development produces comparatively higher covering power than physical 
development and is therefore preferred. Where silver halide grains are 
employed that form predominantly surface latent images, it is preferred to 
employ developers which contain low levels of silver halide 
solvents--i.e., surface developers. It is recognized that covering power 
is increased by developing over a short time period--that is, at a 
comparatively high rate. The exposed photographic elements of this 
invention when developed in less than 1 minute and preferably less than 30 
seconds to produce a viewable silver image exhibit increased covering 
power; however, covering power is substantially reduced and bears little 
relation to grain aspect ratio when development is conducted over eight 
minutes. To achieve rapid development, it is preferred to employ 
comparatively vigorous developing agents. Preferred developing agents are 
hydroquinones employed alone or, preferably, in combination with secondary 
developing agents, such as pyrazolidones, particularly 3-pyrazolidones 
such as disclosed by Kendall U.S. Pat. No. 2,289,367, Allen U.S. Pat. No. 
2,772,282, Stewart et al U.K. Pat. No. 1,023,701, and DeMarle et al U.S. 
Pat. Nos. 3,221,023 and 3,241,967, and aminophenols, such as 
p-methylaminophenol sulfate. 
Processing techniques of the type illustrated by Research Disclosure, Item 
17643, cited above, Paragraph XIX, can be employed. Roller transport 
processing of radiographic elements is particularly preferred, as 
illustrated by Russell et al U.S. Pat. Nos. 3,025,779 and 3,515,556, 
Masseth U.S. Pat. No. 3,573,914, Taber et al U.S. Pat. No. 3,647,459, and 
Rees et al U.K. Pat. No. 1,269,268. While the photographic elements of 
this invention are forehardened, they can be used with conventional 
developers containing prehardeners without any loss in covering power. 
Since the elements are normally fully forehardened, it is, of course, 
preferred to entirely eliminate hardeners from the processing solutions. 
Following development the photographic elements can be fixed to remove 
residual silver halide by any convenient conventional technique. 
EXAMPLES 
The invention can be better appreciated by reference to the following 
illustrative examples. In each of the examples the contents of the reacton 
vessel were stirred vigorously throughout silver and halide salt 
introductions; the term "percent" means percent by weight, unless 
otherwise indicated; and the term "M" stands for molar concentration, 
unless otherwise indicated. All solutions, unless otherwise indicated, are 
aqueous solutions.

EXAMPLES 1 THROUGH 15 
For the purpose of comparing covering power as a function of tabular grain 
aspect ratio, three tabular silver bromide emulsions according to the 
present invention and a tabular silver bromoiodide prepared according to 
the teachings of Maternaghan U.S. Pat. No. 4,150,994 having a lower aspect 
ratio were prepared. The tabular grain characteristics of the emulsions 
are set forth below in Table I. 
TABLE I 
______________________________________ 
Average Percent of 
Aspect Diameter Thickness Projected 
Emulsion 
Ratio (.mu.m) (.mu.m) Area 
______________________________________ 
Control 
3.3:1 1.4 0.42 
Emulsion 
Example 
12:1 2.7 0.22 &gt;80 
Emulsion 
Example 
14:1 2.3 0.16 &gt;90 
Emulsion 
B 
Example 
25:1 2.5 0.10 &gt;90 
Emulsion 
C 
______________________________________ 
Example emulsions A, B, and C were high aspect ratio tabular grain 
emulsions within the definition limits of this patent application. 
Although some tabular grains of less than 0.6 micron in diameter were 
included in computing the tabular grain average diameters and percent 
projected area in these and other example emulsions, except where their 
exclusion is specifically noted, insufficient small diameter grains were 
present to alter significantly the numbers reported. To obtain a 
representative average aspect ratio for the grains of the control emulsion 
the average grain diameter was compared to the average grain thickness. 
Although not measured, the projected area that could be attributed to the 
few tabular grains meeting the less than 0.3 micron thickness and at least 
0.6 micron diameter criteria in the control emulsion was estimated by 
visual inspection to account for very little, if any, of the total 
projected area of the total grain population of the control emulsion. 
The emulsions were each chemically sensitized with sulfur and gold and 
sensitized to the green portion of the spectrum with 600 mg/Ag mole of 
anhydro-5,5'-dichloro-9-ethyl-3,3'-di(3-sulfopropyl)-oxacarbocyanine, 
sodium salt and 400 mg/Ag mole of potassium iodide. 
The emulsions were then divided into separate samples for hardening. Three 
samples of each emulsion received 0.5, 1.5, and 4.5 percent by weight, 
based on the weight of gelatin, respectively, of the hardener 
bis(vinylsulfonylmethyl) ether (BVSME). Three samples of each emulsion 
received 0.24, 0.75, and 2.5 percent by weight, based on the weight of 
gelatin, respectively, of the hardener formaldehyde (HCHO). Three samples 
of each emulsion received 0.24, 0.75, and 2.5 percent by weight, based on 
the weight of gelatin, respectively, of the hardener mucochloric acid 
(MA). Immediately after receipt of the hardener each sample was 
identically coated on separate, identical poly(ethylene terephthalate) 
transparent film supports. The emulsion samples were each coated at 2.15 g 
silver per m.sup.2 and 2.87 g gelatin per m.sup.2. Each sample was 
overcoated with 0.88 g gelatin per m.sup.2. 
The unprocessed coated samples were measured for percent swell 7 days after 
coating, which included 3 days incubation at 38.degree. C. at 50 percent 
relative humidity. Emulsion layer thickness was initially measured, and 
each sample was then immersed in distilled water at 21.degree. C. for 3 
minutes. The change in the emulsion layer thickness was then measured. 
Only a portion of each sample was required to perform the swell measurement 
procedure described above. A remaining portion of each sample was exposed 
to obtain a maximum density and processed in a conventional radiographic 
element processor, commercially available under the trademark Kodak RP 
X-Omat Film Processor M6A-N. Development time was 21 seconds at 35.degree. 
C. Instead of using the standard developer for this processor, which 
contains glutaraldehyde as a prehardener, a similar developer of the type 
disclosed by Example 1 of Barnes et al U.S. Pat. No. 3,545,971 was 
employed, but the glutaraldehyde prehardener was omitted, and the 
developer contained no hardener. 
By plotting covering power versus percent swell using three samples 
hardened to differing degrees with the same hardener, the covering power 
of each emulsion with each hardener at 199 percent swell (except as 
indicated) was determined. The results are set forth below in Table II. 
TABLE II 
______________________________________ 
Average 
Sample Aspect Ratio Hardener Covering Power 
______________________________________ 
Control-1 
3:1 BVSME 60 
Control-2 
3:1 MA 69 (at 150% 
swell) 
Control-3 
3:1 HCHO 68 (at 115% 
swell) 
Example-1 
12:1 BVSME 79 
Example-2 
12:1 MA 78 
Example-3 
12:1 HCHO 75 
Example-4 
14:1 BVSME 98 
Example-5 
14:1 MA 97 
Example-6 
14:1 HCHO 94 
Example-7 
25:1 BVSME 115 
Example-8 
25:1 MA 122 
Example-9 
25:1 HCHO 114 
______________________________________ 
From Table II it is apparent that at the same level of hardening the 
photographic elements prepared with the emulsions of the present invention 
exhibited higher covering power and that the covering power increase was 
produced by the higher aspect ratios of the tabular silver bromide 
emulsions. 
The results in Table III are similar to those reported in Table II, but 
with the difference that the covering power was measured at 99 percent 
swell (except as otherwise indicated). 
TABLE III 
______________________________________ 
Average 
Sample Aspect Ratio Hardener Covering Power 
______________________________________ 
Control-1 
3:1 BVSME 48 
Control-2 
3:1 MA 69 (at 150% 
swell) 
Control-3 
3:1 HCHO 68 (at 115% 
swell) 
Example-10 
12:1 BVSME 80 
Example-11 
12:1 HCHO 76 
Example-12 
14:1 BVSME 95 
Example-13 
14:1 HCHO 92 
Example-14 
25:1 BVSME 110 
Example-15 
25:1 HCHO 115 
______________________________________ 
Because mucochloric acid is a weaker hardener, the concentrations employed 
were insufficient to reduce percent swell below 100 percent, and 
accordingly covering power at that swell level cannot be reported. It is 
believed that the swell could have been reduced below 100 percent with 
mucochloric acid, if higher concentrations had been employed. 
Appendix 
The following preparative details form no part of this invention: 
A. Example Emulsion A 
To a 17.5 liter aqueous bone gelatin, 0.14 molar potassium bromide solution 
(1.5% gelatin, Solution A) at 55.degree. C. and pBr 0.85 were added by 
double-jet addition over an 8 minute period (consuming 1.05% of the total 
silver used) an aqueous solution of potassium bromide (1.15 molar, 
Solution B-1) and an aqueous solution of silver nitrate (1.00 molar, 
Solution C-1). After the initial 8 minutes, Solutions B-1 and C-1 were 
halted. 
Aqueous solutions of potassium bromide (2.29 molar, Solution B-2) and 
silver nitrate (2.0 molar, Solution C-2) were added next to the reaction 
vessel by the double-jet technique at pBr 0.85 and 55.degree. C. using an 
accelerated flow rate (4.2X from start to finish) until Solution C-2 was 
exhausted (approximately 20 minutes; consuming 14.1% of the total silver 
used). Solution B-2 was halted. 
An aqueous solution of silver nitrate (2.0 molar, Solution C-3) was added 
to the reaction vessel for approximately 12.3 minutes until pBr 2.39 at 
55.degree. C. was attained, consuming 10.4% of the total silver used. The 
emulsion was held at pBr 2.39 at 55.degree. C. with stirring for 15 
minutes. 
Solution C-3 and an aqueous solution of potassium bromide (2.0 molar, 
Solution B-3) were added next by double-jet addition to the reaction 
vessel at a constant flow rate over approximately an 88 minute period 
(consuming 74.5% of the total silver used) while maintaining pBr 2.39 at 
55.degree. C. Solutions B-3 and C-3 were halted. A total of 41.1 moles of 
silver were used to prepare this emulsion. 
Finally the emulsion was cooled to 35.degree. C. and coagulation washed as 
described in Yutzy and Russell U.S. Pat. No. 2,614,929. 
B. Example Emulsion B 
To an aqueous 0.14 molar potassium bromide solution of bone gelatin (1.5% 
gelatin, Solution A) at pBr 0.85 and 55.degree. C. were added with 
stirring by double-jet addition at constant flow rate over an 8 minute 
period (consuming 3.22% of the total silver used) an aqueous solution of 
potassium bromide (1.15 molar, Solution B-1) and silver nitrate (1.0 
molar, Solution C-1). After the initial 8 minute period Solutions B-1 and 
C-1 were halted. 
Aqueous solutions of potassium bromide (3.95 molar, Solution B-2) and 
silver nitrate (2.0 molar, Solution C-2) were added next at pBr 0.85 and 
55.degree. C. utilizing an accelerated double-jet flow rate (4.2X from 
start to finish) until Solution C-2 was exhausted (approximately 20 
minutes; consuming 28.2% of the total silver used). Solution B-2 was 
halted. 
An aqueous solution of silver nitrate (2.0 molar, Solution C-3) was added 
at constant flow rate for approximately 2.5 minutes until pBr 2.43 at 
55.degree. C. was attained, consuming 4.18% of the total silver used. The 
emulsion was held with stirring for 15 minutes at 55.degree. C. 
Solution C-3 and an aqueous solution of potassium bromide (2.0 molar, 
Solution B-3) were added next at pBr 2.43 and 55.degree. C. utilizing an 
accelerated flow rate technique (1.4X from start to finish) for 31.1 
minutes (consuming 64.4% of the total silver used). Solutions B-3 and C-3 
were halted. 29.5 Moles of silver were used to prepare the emulsion. 
Finally, the emulsion was cooled to 35.degree. C. and coagulation washed as 
described for Example 1. 
C. Example Emulsion C 
To an aqueous bone gelatin, 0.14 molar potassium bromide solution (1.5% 
gelatin, Solution A) at pBr 0.85 and 55.degree. C. were added by 
double-jet addition with stirring at constant flow rate over an 8 minute 
period (consuming 4.76% of the total silver used) an aqueous solution of 
potassium bromide (1.15 molar, Solution B-1) and an aqueous solution of 
silver nitrate (1.0 molar, Solution C-1). After the initial 8 minutes, 
Solutions B-1 and C-1 were halted. 
Aqueous solutions of potassium bromide (2.29 molar, Solution B-2) and 
silver nitrate (2.0 molar, Solution C-2) were added next at pBr 0.85 and 
55.degree. C. by double-jet addition utilizing accelerated flow (4.2X from 
start to finish) until Solution C-2 was exhausted (approximately 20 
minutes; consuming 59.5% of the total silver used). Solution B-2 was 
halted. Halide salts Solutions B-1 and B-2 were each added at three points 
to the surface of Solution A in the procedure described above. 
An aqueous solution of silver nitrate (2.0 molar, Solution C-3) was added 
for approximately 10 minutes at a constant flow rate to the reaction 
vessel until pBr 2.85 at 55.degree. C. was attained, consuming 35.7% of 
the total silver used. A total of 23.5 moles of silver were used to 
prepare this emulsion. 
Finally, the emulsion was cooled to 35.degree. C. and coagulation washed as 
described for Example 1. 
D. Control Emulsion--This emulsion was precipitated as described in 
Maternaghan U.S. Pat. No. 4,184,877. 
To a 5 percent solution of gelatin in 17.5 liters of water at 65.degree. C. 
were added with stirring and by double-jet 4.7 M ammonium iodide and 4.7 M 
silver nitrate solutions at a constant equal flow rate over a 3 minute 
period while maintaining a pI of 2.1 (consuming approximately 22 percent 
of the silver used in the seed grain preparation). The flow of both 
solutions was then adjusted to a rate consuming approximately 78 percent 
of the total silver used in the seed grain preparation over a period of 15 
minutes. The run of the ammonium iodide solution was then stopped, and the 
addition of the silver nitrate solution continued to a pI of 5.0. A total 
of approximately 56 moles of silver was used in the preparation of the 
seed grains. The emulsion was cooled to 30.degree. C. and used as a seed 
grain for further precipitation as described hereinafter. 
A 15.0 liter 5 percent gelatin solution containing 4.1 moles of the 0.24 
.mu.m AgI emulsion (as prepared above) was heated to 65.degree. C. A 4.7 M 
ammonium bromide solution and a 4.7 M silver nitrate solution were added 
by double-jet at an equal constant flow rate over a period of 7.1 minutes 
while maintaining a pBr of 4.7 (consuming 40.2 percent of the total silver 
used in the precipitation on the seed grains). Addition of the ammonium 
bromide solution alone was then continued until a pBr of approximately 0.9 
was attained at which time it was stopped. A 2.7 liter solution of 11.7 M 
ammonium hydroxide was then added, and the emulsion was held for 10 
minutes. The pH was adjusted to 5.0 with sulfuric acid, and the double-jet 
introduction of the ammonium bromide and silver nitrate solution was 
resumed for 14 minutes maintaining a pBr of approximately 0.9 and at a 
rate consuming 56.8% of the total silver consumed. The pBr was then 
adjusted to 3.3 and the emulsion cooled to 30.degree. C. A total of 
approximately 87 moles of silver was used. The emulsion was coagulation 
washed as described in Example 1. 
E. Example Emulsions A, B, and C prepared as described above were each 
optimally chemically sensitized with 5 mg/Ag mole of potassium 
tetrachloroaurate, 150 mg/Ag mole of sodium thiocyanate, and 10 mg/Ag mole 
of sodium thiosulfate at 70.degree. C. The Control Emusion was optimally 
chemically sensitized according to the teaching of Maternaghan with 0.6 
mg/Ag mole of potassium tetrachloroaurate and 4.2 mg/Ag mole of sodium 
thiosulfate at 70.degree. C. 
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.