Imaging data processing apparatus and method

An image processing apparatus comprises processing circuitry configured to: obtain an irradiance volume representative of virtual light cast into a volumetric imaging data set, the irradiance volume comprising a respective irradiance value for each of a plurality of points in the irradiance volume; determine, for each of a plurality of reference points in the irradiance volume, a respective value for an accuracy measure, wherein the value for the accuracy measure at each reference point is representative of an accuracy with which irradiance has been determined at or near that reference point; and perform a rendering process using the irradiance volume, wherein the rendering process is performed in dependence on the determined values for the accuracy measure.