A method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes (OLEDs) in an OLED display device, calculates and predicts the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. In one exemplary embodiment of the invention, the calculation is based the accumulated current that has been passed through the device. In another exemplary embodiment, the calculation is based on a difference in voltage across the pixel at two instants. The compensation system is best used after the display device has been calibrated to provide uniform light output. The present invention further provides a method for calibrating a display device comprising an array of individually adjustable discrete light emitting devices (pixels) using a camera having an array of radiation sensors or a single photodetector. According to this method, the camera captures respective images of substantially equal-sized first sub-areas and adjusts the driving current for each of said pixels within said first sub-areas to achieve a desired light output. Next, the camera captures images of a plurality of second sub-areas, each of the second sub-areas including multiple ones of the first sub-areas second-level sub-areas are adjusted such that each of the first-level sub areas provide substantially equal light output. In an alternative embodiment, each of the first level sub-areas overlaps and, after adjusting a first sub-area to a desired brightness level, the invention adjusts the pixels in each overlapping sub-area to have the same brightness as the overlapping pixels.