Abstract
We propose the technique of classifying the background area automatically by calculating the color variance in the RGB space of the picture. By using this technique, the background area classification robust to change of the brightness and color of the picture was enabled. Moreover, the validity of this technique was shown by actually performing particle extraction and verifying the result which carried out calculation. We aim at development of the particle detector which performs particle counting automatically about the "distributed staining method" which is one of the processing of asbestos analysis. The pictures were taken by the phase microscope.