| 摘要 |
A method of preparing photographic silver iodobromide or iodochloride emulsions the silver halide crystals of which are of the twinned octahedral or cubic type, which emulsions have improved homogeneity of iodide content and controlled size distribution of the crystal grains, comprising the following steps: (a) forming a colloid dispersing medium silver halide crystals containing at least 90 mole % iodide these said crystals being predominantly of the hexagonal lattice structure, (b) mixing in the dispersing medium containing the said silver halide crystals an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium halide so forming twinned silver halide crystals containing iodide and the halides being added, the rate of addition of silver salt and alkali metal or ammonium bromide being so controlled that the silver halide crystals formed in this step are predominantly twinned and optionally (c) then causing the twinned crystals to increase in size by adding to the colloidal dispersion further aqueous silver salt solution and further alkali metal or ammonium halide and then finally optionally (d) removing the water-soluble salts formed and chemically sensitizing the emulsion. The new photographic emulsions exhibit a high covering power and contrast on development.
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