The fill factor indicates the size of the light sensitive photodiode relative to the surface of the pixel. Because of the extra electronics required around each pixel the "fill factor" tends to be quite small, especially for Active Pixel Sensors which have more per pixel circuitry. To overcome this limitation, often an array of microlenses is placed on top of the sensor.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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