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An effective median filter for salt & pepper impulse noise removal is presented. This computationally efficient filtering technique is implemented by a two pass algorithm; In the first pass, identification of corrupted pixels that are to be filtered are perfectly detected into a flag image using a variable sized detection window approach; In the second pass, using the detected flag image, the pixels to be modified are identified and corrected by a more suitable median. Experimental results show that the proposed algorithm performs far more superior than many of the median filtering techniques reported in terms of retaining the fidelity of the image highly corrupted by impulse noises even to the tune of ninety percent impulse noise. The proposed algorithm is free from patchy effects, does not extend black or white blocks in the image as has been found in many other adaptive median based techniques and is very effective in cases when images are corrupted with large percentage of impulse noises. This algorithm works very well for images with lower percentage of impulse noises also.