The diffusion of light within paper has an effect on the colors in a halftone image and must be taken into consideration in modeling halftone color. The diffusion can be accounted for by using the point spread function or equivalently the modulation transfer function (MTF) of the paper. A novel method of measurement of the paper's MTF, called the bar-target series expansion method, is proposed here. The experimental results of the method confirm that the shape of the MTF is an exponential function, which corresponds to a Lorentzian line spread function as predicted by the multiple-path model of reflection.