Improved Fast NLM filter for suppressing digital holographic speckle noise and evaluation index

This study proposes an enhanced fast non-local means filtering algorithm (Fast NLM) incorporating sine-cosine transformation and an optimized similarity metric, along with a total gradient difference (TGD) index derived from gradient analysis of denoised versus noise-free phase maps. The issues of phase transitions and image details damaged by the traditional Fast NLM during phase filtering and lack of comprehensive evaluation of phase noise reduction levels in digital holography can be addressed. Compared with four benchmark filters, our method achieves maximum improvements of 2.68% in structure similarity index measure, 506.27% in signal-to-noise ratio, and 777.08% in equivalent number of looks, while reducing TGD by up to 89.90%. Experimental results confirm superior phase detail preservation and noise suppression performance of the proposed algorithm.