An apparatus and method are disclosed for measuring the amounts of a coating material on an applicator roll. A beam of light is projected so that only a first portion thereof is intercepted by the coating material on the applicator roll. At least a fraction of the remaining light portion passes through an aperture defined by a slotted shield member and the coated roll surface, said fraction varying with the thickness of said material on the roll surface, and detector means disposed to measure said variable fraction.