1. A process for estimating a three-dimensional (3D) radiance field of a combustion process in an enclosure, comprising: performing an on-line intensity-temperature calibration based on an association between an intensity of an image pixel and an actual temperature associated with a selected region in the enclosure; transforming the intensity intensity of the corresponding image to a radiance image based on settings of an image-capturing device and the on-line calibration; performing a registration and alignment estimation of the image based on positional information of the enclosure; aligning the radiance image based on the registration estimation; and estimating, using a computer processor, the 3D radiance field having voxels of the enclosure based on a two-dimensional (2D)-to-3D transforming of the aligned radiance images.