The geometry and optical properties of the phosphor screen in a cathode‐ray picture tube have been studied in connection with the phoshor particle size distribution. The specific surface area, Sw (cm2/g phosphor), which determines the amount of light scattering in the screen, is equal to . Geometry shows that the total surface area of phosphor particles densely arranged in a single layer on the faceplate is independent of the particle size and is constant. The total surface area of the phosphor particles in the screen, therefore, depends on the number, , of particle layers, rather than on the gross thickness. . The usefulness of as a determinant of the brightness of a phosphor screen has been demonstrated. It was found experimentally that the optimum number of the particle layers producing maximum luminescence with minimum scattering is about 1.4.