Patent ID: 8451437

Claim:
Apparatus for detecting variations in light output of an electroluminescent (EL) device, comprising: a) the EL device, including: i) a transparent substrate having a first edge extending in a first direction, and a face; and ii) a plurality of OLED emitters disposed in direct contact with the face of the transparent substrate in the first direction; b) a power supply for providing electric current through the OLED emitters so that they emit light, wherein some of the light emitted by each EL emitter travels through the substrate and out of the first edge; c) a light sensor for sensing the light travelling out of the first edge, wherein the light sensor is physically separated from the first edge; and d) a controller for storing first sensed light at a first time and second sensed light at a later second time and computing a variation in light output of one or more of the OLED emitters in the EL device using the stored first sensed light and second sensed light.