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 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 product-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. In this way, the aerial image used, generated or produced to measure, inspect, characterize and / or evaluate the 
photomask is the same aerial image used, generated or produced during 
wafer exposure in 
integrated circuit manufacturing. In another embodiment, the 
system, sensor and technique characterizes and / or evaluates the performance of the optical lithographic equipment, for example, the optical sub-system of such equipment. In this regard, in one embodiment, an 
image sensor unit measures, collects, senses and / or detects the aerial image produced or generated by the interaction between lithographic equipment and a photomask having a known, predetermined or fixed pattern (i.e., test 
mask). In this way, the system, sensor and technique collects, senses and / or detects the aerial image produced or generated by the test 
mask—lithographic equipment in order to inspect, evaluate and / or characterize the performance of the lithographic equipment.