The analysis of the shadows projected by a particle on a plane may be used as a basis for a study of particle three-dimensional shape and size. In that study, a mathematical function describing the value of the area projected by a particle in every spatial orientation could be a very useful tool. In the present work, a method for calculating the angular projection function (apf) of a flat-sided convex solid has been developed. The standard deviation of the apf signal spectrum has been calculated for different model particles and found to be useful as a distinctive signature of particle shape. The relationship between this signature and the noise associated with the transmittance of a suspension has been discussed.