  The photoresponse non-uniformity (PRNU) is a camera-specific pattern, widely
adopted to solve multimedia forensics problems such as device identification or
forgery detection. The theoretical analysis of this fingerprint customarily
relies on a multiplicative model for the denoising residuals. This setup
assumes that the nonlinear mapping from the scene irradiance to the
preprocessed luminance, that is, the composition of the Camera Response
Function (CRF) with the optical and digital preprocessing pipelines, is a gamma
correction. Yet, this assumption seldom holds in practice. In this letter, we
improve the multiplicative model by including the influence of this nonlinear
mapping on the denoising residuals. We also propose a method to estimate this
effect. Results evidence that the response of typical cameras deviates from a
gamma correction. Experimental device identification with our model increases
the TPR by a $4.93\, \%$ on average for a fixed FPR of $0.01$.
