Individual pixels from an image of a uniformly illuminated, constant target will not have the same DN value. A camera’s lens produces a roll-off in intensity, known as vignetting, as you move away from the center of the focal plane. In addition there are detector-to-detector response variations, “bad” detectors and manufacturing induced artifacts that prevent a camera from measuring a uniform value.

Flat-fielding is a calibration process that removes image roll-off along with other local artifacts so that a uniform target, uniformly illuminated produces a uniform image.

I2R performs radiometric camera calibrations that include flat-fielding using uniformly illuminated integrating spheres with exit ports as large as 40 cm.