Their sensitivity to light is decreased because part of each photosite is covered with circuitry that filters out noise and performs other functions. The percentage of a pixel devoted to collecting light is called the pixel's fill factor . The lower the fill factor, the less sensitive the sensor is and the longer exposure times must be. Too low a fill factor makes indoor photography without a flash virtually impossible. In addition, the circuitry can be reduced so it doesn't cover as large an area.
Fill factor refers to the percentage of a photosite that is sensitive to light.
The higher the fill factor, the more sensitive the sensor. Courtesy of Photobit . Bob Caspe, President of Sound Vision, Inc. knows as much about CMOS image sensors as anyone and tells it like it is. Click the button to get his prespecitive on image sensors.