In principle, any single-pixel
regional maximum algorithm should be able to locate
particle centroids to within half a pixel.
This is the accuracy estimated by Schaertl and Sillescu [13]
for their particle locating algorithm.
In practice, such an approach suffers from poor noise rejection
and includes false identifications.
It is not difficult, however, to reduce the standard
deviation of the position measurement
to better than 1/10 pixel even with moderate signal noise.
Other information gathered in the process can be used to estimate
the spheres' displacements in the *z*-direction and to reject
spurious identifications.

Having already found a locally
brightest pixel at (*x*,*y*), which presumably is near
a sphere's geometric center at ,
we calculate the offset from (*x*,*y*) to the brightness-weighted
centroid of the pixels in a region around (*x*,*y*):

where is the integrated brightness of the sphere's image. The refined location estimate is then . The background subtraction performed by the convolution kernel in eqn. (4) avoids biasing and toward the center of the fitting region and away from the particle image's centroid. If either nor exceeds 0.5, the candidate centroid location can be moved accordingly and the refinement recalculated.

Mon Mar 11 23:01:27 CST 1996