  To understand the scientific imaging capability, one must characterize the
intra-pixel sensitivity variation (IPSV) of the CMOS image sensor. Extracting
an IPSV map contributes to an improved detector calibration that allows to
eliminate some of the uncertainty in the spatial response of the system. This
paper reports the measurement of the sub-pixel sensitivity variation and the
extraction of the 2D IPSV map of a front-side illuminated CMOS image sensor
with a pixel pitch of 6 {\mu}m. Our optical measurement setup focuses a
collimated beam onto the imaging surface with a microscope objective. The spot
was scanned in a raster over a single pixel and its immediate neighbors in
order to probe its response at selected (sub-pixel) positions. In this work we
introduced a novel technique to extract the IPSV map by fitting (forward
modeling) the measured data to a mathematical model of the image, generated in
a single pixel that allows for a spatially varying sensitivity.
