A technique for reducing navigation errors that are caused by uneven
illumination involves using only the portion of the image data that is
properly illuminated in the correlation process. The portion of the image
data that is used for optical navigation is established by summing the
image data on a per-slice basis, comparing the per-slice sums to a
pre-established intensity threshold, and defining boundaries of the
navigation window in response to the comparison. Boundaries are set at
the points where the per-slice sums equal the pre-established intensity
threshold.