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We present evidence that spots imaged using astronomical CCDs do not exactlyscale with flux: bright spots tend to be broader than faint ones, using thesame illumination pattern. We measure that the linear size of spots or stars,of typical size 3 to 4 pixels FWHM, increase linearly with their flux by up to$2\%$ over the full CCD dynamic range. This brighter-fatter effect affects bothdeep-depleted and thinned CCD sensors. We propose that this effect is a directconsequence of the distortions of the drift electric field sourced by chargesaccumulated within the CCD during the exposure and experienced by forthcominglight-induced charges in the same exposure. The pixel boundaries then becomeslightly dynamical: overfilled pixels become increasingly smaller than theirneighbors, so that bright star sizes, measured in number of pixels, appearlarger than those of faint stars. This interpretation of the brighter-fattereffect implies that pixels in flat-fields should exhibit statisticalcorrelations, sourced by Poisson fluctuations, that we indeed directly detect.We propose to use the measured correlations in flat-fields to derive how pixelboundaries shift under the influence of a given charge pattern, which allows usto quantitatively predict how star shapes evolve with flux. This physical modelof the brighter-fatter effect also explains the commonly observed phenomenonthat the spatial variance of CCD flat-fields increases less rapidly than theiraverage.

The brighter-fatter effect and pixel correlations in CCD sensors