Fill factor (image sensor) on:
[Wikipedia]
[Google]
[Amazon]
The fill factor of an 
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
array is the ratio of a pixel's light sensitive area to its total area.
For pixels without microlens
A microlens is a small lens, generally with a diameter less than a millimetre (mm) and often as small as 10 micrometres (µm). The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects ...
es, the fill factor is the ratio of photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
area to total pixel area,
but the use of microlenses increases the ''effective fill factor'', often to nearly 100%, by converging light from the whole pixel area into the photodiode.
Another case that reduces the fill factor of an image is to add additional memory beside each pixel, so as to achieve a global shutter on CMOS sensors.
References
Image sensors {{tech-stub