In one aspect, the present invention is a technique of, and a system and sensor for measuring, inspecting, characterizing and/or evaluating optical lithographic equipment, methods, and/or materials used therewith, for example, photomasks. In one embodiment, the system, sensor and technique measures, collects and/or detects an aerial image (or portion thereof) produced or generated by the interaction between the photomask and lithographic equipment. An image sensor unit may measure, collect, sense and/or detect the aerial image in situ—that is, the aerial image at the wafer plane produced, in part, by a production-type photomask (i.e., a wafer having integrated circuits formed during the integrated circuit fabrication process) and/or by associated lithographic equipment used, or to be used, to manufacture of integrated circuits. A processing unit, coupled to the image sensor unit, may measure the critical dimensions of features of the photomask, using data which is representative of the intensity of light sampled by the image sensor unit, to control at least one operating parameter of the lithographic equipment.