An assembly is provided for the direct measurement of a vertical intensity profile through a plane of focus along an illuminating beam, a determination of a depth of the focal plane and a maximum intensity of the intensity profile. The assembly includes a plurality of focusing indicia fixed relative to a substrate, the focusing indicia being distributed at different locations along the illuminating beam. The focusing indicia are configured to be illuminated with an intensity corresponding to the position relative to the plane of focus along the axis of the illuminating beam. The location of the respective focusing indicia can be predetermined, such as along a given scale at a given inclination of the scale and the path of the illuminating beam, or not initially known and subsequently determined.