One of the things the MNF transform does is put the transformed image data into units of standard deviation. So, then it's easy to say what the PPI threshold should be for an MNF image, because we know that the usual point of the PPI tool is to find the pixels that are outside 2 to 3 standard deviations from the mean. But with PCA transformed data, we don't have that kind of information about the "units". So, there can be no rule of thumb for a threshold for the PPI transform, that applies to all PCA transformed data. That's why people usually use MNF transformed data in the PPI tool. - Peg

Thanks peg, I need to use PCA image for PPI, I did iterations by giving the value like 2, 2.5,3, 4, 5,7, 100, except in the case of 100 the number of pure pixels came out to be in the range of 50-60, Is it justified if take the average of the above numbers (exceot 100) as the threshold. Another thing is there any criteia for the number of pure pixels in an image, I mean the number of pure pixels should be high or low or somewhere in the middle range.