Otsu method is a well-known thresholding method. In other words, it can find the threshold value to convert your image to two levels (bilevels) 0 and 1.
This work was published lone time ago in 1979.
 Otsu, N., "A Threshold Selection Method from Gray-Level Histograms," IEEE Transactions on Systems, Man, and Cybernetics, Vol. 9, No. 1, 1979, pp. 62-66.