I suggest this publication: S. Pyatykh, J. Hesser and L. Zheng, “Image Noise Level Estimation by Principal Component Analysis”, IEEE Transactions on Image Processing, 2013, Vol. 22, Issue 2, pp. 687-699 http://physics.medma.uni-heidelberg.de/cms/sites/default/files/img/Stanislav/noise_param_estimation/2013_noise_level_estimation_tip.pdf