The calibrated light frame is the same image seen before in the light frame, but the master offset and dark frames have been subtracted, and the result has been divided by the master flat frame. Thus the fixed and thermally-related noise components have been removed, and the vignetting and dust motes caused by the optical train have been eliminated. Most of the noise in the image, however, is random "shot noise" caused by the discrete packet nature of light, and this noise is still present in the calibrated image.