Here are some excerpts from a couple of patents:
US 6,248,507:
To facilitate Ostwald ripening it is contemplated to employ fine grain silver iodide emulsions having a mean grain size of less than 0.1 micrometer. The small sizes of the silver iodide grains are chosen to maximize available grain surface area per unit volume and to improve the distribution of the silver iodide at the time emulsions are blended. In a preferred form the silver iodide grain emulsion is a Lippmann emulsion. Lippmann emulsions with mean grain sizes down to about 30 angstroms have been reported, although the typical mean grain size of Lippmann emulsions is about 0.05 micrometer. While the iodochloride prior art referenced above typically states that any iodide ion source may be used in the preparation thereof, it is a critical feature of the invention that fine silver iodide grains be used as the iodide source for~the preparation of the region of the grains containing a maximum iodide concentration.
US 4,082,553:
Nicholas et al points out that iodide-free Lippmann emulsions have been used as overcoats to inhibit release of iodide to the developer solution. Nicholas et al notes, however, that these Lippmann emulsions in turn produce disadvantages by silver plating out on transport rollers during processing. To obviate this, Nicholas et al teaches the coating of the Lippmann emulsion layer with a silver precipitating agent, such as metal sulfides, selenides, polysulfides and polyselenides, thiourea; heavy metals and heavy metal salts; fogged silver halide and Carey Lea silver.
US 6,472,137:
In another preferred embodiment addition of iodide to emulsion grains rich in silver bromide is performed by adding fine preformed grains of silver iodide, whether or not including bromide and/or chloride in minor amounts, said grains having a grain diameter of not more than 100 nm, and more preferably, not more than 50 nm. Such fine grains are so-called "Lippmann" emulsions. Addition of iodide making use from such fine grains rich in silver iodide has been described for the preparation of {111} tabular grains in JP-A's 04-251241 and 08-029904 and in EP-A's 0 662 632 and 0 658 805, wherein an outermost phase rich in silver iodide has been added to {111} tabular grains rich in silver bromide (optionally comprising up to less than 10 mole % of silver chloride). Addition of said fine AgI-Lippmann emulsions to the surface of the silver halide crystals in order to get a global iodide content of less than 1 mole % in the grain may advantageously proceed as disclosed in EP-A 0 475 191, wherein an excellent speed/fog ratio and a high covering power are attained.
US 5,879,873:
A conventional growth step for the precipitation of high bromide grains with {100} crystal faces can then be effected, but with vAg preferably maintained in the range of from 150 to 220 mV to minimize thickening of the tabular grains. Typically growth is achieved by the addition of jets, at a constant or accelerated flow rate, of silver nitrate and alkaline metal halides or, again, a fine-grain emulsion of the Lippmann type.
US 6,518,009:
A method of imaging employing sensitized high chloride silver halide emulsions which exhibit improved speed and high intensity reciprocity performance for use with short duration, high intensity exposure optical and digital exposure systems. The method comprising providing a photographic element comprising at least one high chloride silver halide emulsion layer, exposing said element utilizing a high intensity actinic radiation exposure for an exposure time of less than 1/100 second, and developing said element to produce a photographic image, wherein the high chloride silver halide emulsion layer is comprised of silver halide emulsion grains containing at least 90 mole percent chloride, based on silver, obtained by providing a high chloride host emulsion, bringing a Lippmann emulsion comprising primarily fine silver bromide grains doped with iridium into contact with said high chloride host emulsion, and subsequently chemically sensitizing the high chloride emulsion.
