  Space missions designed for high precision photometric monitoring of stars
often under-sample the point-spread function, with much of the light landing
within a single pixel. Missions like MOST, Kepler, BRITE, and TESS, do this to
avoid uncertainties due to pixel-to-pixel response nonuniformity. This approach
has worked remarkably well. However, individual pixels also exhibit response
nonuniformity. Typically, pixels are most sensitive near their centers and less
sensitive near the edges, with a difference in response of as much as 50%. The
exact shape of this fall-off, and its dependence on the wavelength of light, is
the intra-pixel response function (IPRF). A direct measurement of the IPRF can
be used to improve the photometric uncertainties, leading to improved
photometry and astrometry of under-sampled systems. Using the spot-scan
technique, we measured the IPRF of a flight spare e2v CCD90 imaging sensor,
which is used in the Kepler focal plane. Our spot scanner generates spots with
a full-width at half-maximum of $\lesssim$5 microns across the range of 400 nm
- 900 nm. We find that Kepler's CCD shows similar IPRF behavior to other
back-illuminated devices, with a decrease in responsivity near the edges of a
pixel by $\sim$50%. The IPRF also depends on wavelength, exhibiting a large
amount of diffusion at shorter wavelengths and becoming much more defined by
the gate structure in the near-IR. This method can also be used to measure the
IPRF of the CCDs used for TESS, which borrows much from the Kepler mission.
