Photographic element containing a speed-enhancing compound

A multicolour photographic element comprising a support bearing yellow, magenta and cyan image-dye-forming units comprising at least one blue-, green- or red-sensitive silver halide emulsion layer having associated therewith at least one yellow, magenta or cyan dye-forming coupler respectively, wherein the element may contain a non silver-containing light-insensitive layer, in which at least one of these layers contains a colourless imidazole compound of formula (I) that undergoes less than 10% chemical or redox reaction with oxidized developer and which enables the photographic speed of the element to be increased by at least 0.05 stop without increasing granularity, compared to the same element without the compound, wherein the compound of formula (I) has the structure:—

• wherein
• X is H or a substituent;
• Z represents the atoms necessary to complete an unsubstituted or substituted imidazole ring, which may form part of a fused unsubstituted or substituted ring system containing no further ring heteroatoms;
• wherein there is present at least one —NH group either in the imidazole ring or directly attached to the imidazole ring as part of X;
• provided that
• (a) when Z represents the atoms necessary to complete an unsubstituted or substituted benzimidazole ring, an —NH group is directly attached to the imidazole ring as part of X, and X is located between the imidazole nitrogen atoms;
• (b) when the —NH group is located in the ring adjacent to the carbon atom bearing X, and X is located between the imidazole nitrogen atoms, the imidazole ring is not fused to a phenanthrene ring.

A process of forming an image in a photographic element after the element has been imagewise exposed to light, comprising contacting the element with a colour developing agent, wherein the element comprises at least one light-sensitive silver halide emulsion layer or a non silver-containing light-insensitive layer, in which at least one of these layers contains a colourless imidazole compound of formula (I) that undergoes less than 10% chemical or redox reaction with oxidized developer and which enables the photographic speed of the element to be increased by at least 0.05 stop without increasing granularity, compared to the same element without the compound, wherein the compound of formula (I) has the structure:—

• wherein
• X is H or a substituent;
• Z represents the atoms necessary to complete an unsubstituted or substituted imidazole ring, which may form part of a fused unsubstituted or substituted ring system containing no further ring heteroatoms;
• wherein there is present at least one —NH group either in the imidazole ring or directly attached to the imidazole ring as part of X;
• provided that
• (a) when Z represents the atoms necessary to complete an unsubstituted or substituted benzimidazole ring, an —NH group is directly attached to the imidazole ring as part of X, and X is located between the imidazole nitrogen atoms;
• (b) when the —NH group is located in the ring adjacent to the carbon atom bearing X, and X is located between the imidazole nitrogen atoms, the imidazole ring is not fused to a phenanthrene ring.