Apparatus for testing microelectronic components on a substrate, including a scanner operative to scan a light beam over a plurality of thin film transistors disposed on a substrate, one transistor at a time, so as to induce a photoconductive response in the plurality of transistors, one transistor at a time; current sensing circuitry operative, synchronously with said scanner, to measure an output induced by the photoconductive response associated with a transistor and to generate photoconductive response output values, the photoconductive response output values representing a photoconductive response induced by the light beam, for one transistor at a time from among the plurality of transistors; and diagnostic apparatus operative to analyze the electronic response output values and to characterize each of the transistors in accordance therewith.