The surface structure of an artificial membrane composed of a Millipore filter and a synthetic lipid analogue, dioleylphosphate (DOPH), changes from a highly reflective hydrophilic state to a poorly reflective hydrophobic state in KCl solution with increasing calcium ion concentration in the solution. Using a microscopic digital image processing system the structural changes can be characterized quantitatively, in a histogram of the brightness level and a binary black-and-white picture. The gross area of hydrophilic domains and the mean diameter of hydrophilic particles were measured by processing the binary picture.