This paper aimed to study the effect of irregular ink dot (non-bilevel halftone image) on the spectral reflections, in order to provide a new angle of thought for prediction model theory. The new extended Clapper-Yule model was established by introducing the two-dimensional Gaussian function to describe the surface of irregular ink dot and discussing the effect of the irregularity of ink dot on the dot size, dot thickness and the ink transmittances. The numerical simulation of our new model showed that the irregularity of ink dot caused the physical dot gain, which increased the absorption of light by the ink and consequently decreased the ultimate spectral reflections. Using the Clapper-Yule model to predict the reflections must take the physical dot gain caused by irregular ink dot into account, and the new extended model can be applied in the color prediction and control of monochrome halftone images.