An Analysis of the Optics of the Crossline Screen.

Three analytical methods are described for the determination of the design of the aperture required in an optical system which uses a crossline screen to make contact screens with specified tone-reproduction characteristics. The zone method is based upon the theory that the aperture may be subdivided into elements or zones whose shapes are determined empirically to give the desired dot shapes, and that the effect of the whole aperture may be calculated from the effects of the individual zones. In the Fourier-analysis method, advantage is taken of the periodic nature of the image. The elements in this case are regarded as two-dimensional harmonic waves whose contributions may be determined individually and the aperture calculated as a weighted sum of these waves. In the matrix method, the elements in the aperture are treated as if they were point sources and the elements in the images are the resulting diffraction pattern of a point source behind the screen.