Pixelwise illuminant estimation

The disclosure concerns estimating an illumination spectrum of a pixel of hyperspectral or multispectral image data. A processor determines an optimised illumination spectrum for a first point based on an initial illumination spectrum and based on associations between the first point and multiple second points of multiple instances of the image data, wherein each of the multiple instances is associated with a refined amount of image information. It is possible to determine the illumination spectrum per pixel of the original image data without clustering pixels or restrictive assumptions but instead by utilising the associations for the statistical inference on the illumination spectrum.