The invention relates to an arrangement for contactless determination of at least one dimension of a moving material web, in particular a material web of opaque material, with a background illumination, with optical detection means for detecting at least one current contrast image and with evaluation means, wherein the background illumination is arranged opposite the optical detection means, wherein the material web moves in a plane between the at least one optical detection means and the background illumination, and wherein the current contrast image has at least one piece of information about at least one light intensity laterally adjacent to the material web. Furthermore, the present invention concerns a corresponding process. For simplifying and improving the non-contact determination of the dimension, the present invention proposes to provide a shadowing element and to compare the current contrast image with a reference contrast image, wherein the current contrast image represents a shadowing of the background illumination by the material web and by the shadowing element.