The electrons are held in each pixel and are converted into electrical charge which can be measured to show the amount of light that has fallen on each pixel. The maximum electrical charge possible is termed "full well capacity". The greater the full well capacity the greater the dynamic range, assuming other influencing factors such as noise and A/D converter quality are equal. As there is a limit to the depth to which pixels can be made, the full well capacity is often proportional to the frontal area of the light gathering element of the pixel.
The fill factor is the measure of the area of the pixel that can gather light. Some sensors with a frame transfer or full frame architec ture boast 100 % fill factor as the entire pixel is photosensitive. Interline transfer CCDs or CMOS sensors may only have a 20-50 % fill factor as much of the pixel is occupied with read-out or charge conversion and digitisation structures. Microlenses are sometimes used with this type of sensor being placed over each pixel to guide the light from a wider area onto the photosensitive area.