  The camera's focal plane array (FPA) fill factor is one of the parameters for
digital cameras, though it is not widely known and usually not reported in
specs sheets. The fill factor of an imaging sensor is defined as the ratio of a
pixel's light sensitive area to its total theoretical area. It is generally
believed that the lower fill factor may reduce the accuracy of photogrammetric
measurements. But nevertheless, there are no studies addressing the effect of
the imaging sensor's fill factor on digital image correlation (DIC) measurement
accuracy. We report on research aiming to quantify the effect of fill factor on
DIC measurements accuracy in terms of displacement error and strain error. We
use rigid-body-translation experiments then numerically modify the recorded
images to synthesize three different types of images with 1/4 of the original
resolution. Each type of the synthesized images has different value of the fill
factor; namely 100%, 50% and 25%. By performing DIC analysis with the same
parameters on the three different types of synthesized images, the effect of
fill factor on measurement accuracy may be realized. Our results show that the
FPA's fill factor can have a significant effect on the accuracy of DIC
measurements. This effect is clearly dependent on the type and characteristics
of the speckle pattern. The fill factor has a clear effect on measurement error
for low contrast speckle patterns and for high contrast speckle patterns (black
dots on white background) with small dot size (3 pixels dot diameter). However,
when the dot size is large enough (about 7 pixels dot diameter), the fill
factor has very minor effect on measurement error.
