Quantitative optical image subtraction by means of pulse-width modulation through one-dimensional contact screens

A quantitative optical image-subtraction method based on the principle of spatial pulse-width modulation through a one-dimensional contact-screen process and superposition of positive and negative halftone photographs is described. The difference in intensity transmittance of two images A and B can be measured by either superposing the halftone positive of A onto the halftone negative of B or vice versa. Not only subtle differences between the two photographs but also the polarity of such differences can be found.