1. A process for forming an image in a vesicular imaging material, wherein the vesicular imaging material comprises a plastics vehicle applied to a carrier sheet or film, said plastics vehicle comprising a thermoplastics component and dispersed uniformly therein a sensitising agent which is non-reactive with said thermoplastics component and releases a vesicle-forming gas by decomposition upon exposure to light, said sensitising agent present in an amount of at least 5% by weight based upon the weight of said thermoplastics component, said thermoplastics component being softenable upon heating to permit the gas released by the sensitising agent in the light-struck areas to form light-scattering or reflecting vesicles therein, said plastics vehicle also containing at least one sulphone or sulphonamide the sulphonamide having a maximum molecular weight of 253 wherein the sulphone has the general formula EQU R.sup.1 --SO.sub.2 --R.sup.2 in which R.sup.1 and R.sup.2 are selected from aromatic radicals and the sulphonamide has the general formula: EQU R.sup.3 --SO.sub.2 --NR.sup.4 R.sup.5 in which R.sup.3 is a hydrocarbon radical and R.sup.4 and R.sup.5 are each either hydrogen atoms or hydrocarbon radicals, the sulphone or sulphonamide being present in an amount in the range of 1 to 100% by weight based upon the weight of the thermoplastics component, said image being formed in the plastics vehicle by exposing the vesicular imaging material to a light image thereby causing the sensitising agent to release gas in the light-struck areas and heating the vesicular imaging material to soften the thermoplastics component whereby a recorded image is formed therein by the expansion into imaging vesicles of gas released by the sensitising agent, said sulphone or sulphonamide being present in the plastics vehicle in an effective amount whereby the recorded image has a density ratio of 106:94 exceeding 0.80.