The sensitivity across a solid state detector array varies as a result of differences in transmission, diffusion and scattering properties over the sensor. This variation will occur over a range of scale lengths and its knowledge is of importance for improved device design and in a variety of applications, for example, event centroiding in photon counting systems. A measurement of the sensitivity variation on a subpixel scale for a two-phase front-illuminated CCD is reported. The measurement is made using a scanning reflection microscope. A variation in sensitivity between the phases within a pixel is clearly observed, as well as variations on a much smaller spatial scale.