A wafer containing a plurality of electro-optical devices, each device being enclosed in chamber that has a translucent cover. An X-Y matrix of pairs of interconnections on the wafer are connected to the circuitry of the electro-optical devices for addressing the electro-optical devices. The pairs of interconnections extend outside of the chambers enclosing the devices to testing areas on the periphery of the wafer. Testing is done by signals applied through the interconnections while simultaneously exposing the devices to light through the translucent covers.