@Dibarkar Pal is throwing error :"Page not found"

This is quite well explained in my paper:

http://www.sciencedirect.com/science/article/pii/S0957417416304456

Image processing is stochastic process. The threshold value used by you for your data set may be not work with others data sets. For mathematical explanation, you are supposed to use  adaptive threshold value selection algorithm.   

In order to understand multilevel thresholding use Matlab builtin function

thres = multitheshold(A,N).

A clear algorithm is given by Madhur Srivastava. Refer his article in the link:

https://arxiv.org/ftp/arxiv/papers/1101/1101.0139.pdf