1. A method for evaluating a semiconductor layer at least partially crystallized by exposure to a plurality of laser-radiation pulses having an energy density on the layer, the crystallization producing a first group of periodic surface features on the layer in a first direction, and a second group of periodic surface features in a second direction perpendicular to the first direction, wherein characteristics of the first and second groups of periodic features depend on the energy density of the laser-radiation pulses to which the semiconductor layer has been exposed, the method comprising: illuminating an area of the crystallized semiconductor layer with a beam of light directed normal to the surface of the layer; recording an image of the illuminated area with a detector aligned to capture light centered on an axis normal to the layer and arranged to capture light diffracted from the illuminated area by the first and second groups of periodic features, with the variations in brightness within the recorded image corresponding to the characteristics of the first and second groups of periodic features in the illuminated area of the layer; and determining the energy density from a measure of the contrast in brightness along at least one direction of the image corresponding to one of the first or second directions of the layer.