  Various optical methods for measuring positions of micro-objects in 3D have
been reported in the literature. Nevertheless, majority of them are not
suitable for real-time operation, which is needed, for example, for feedback
position control. In this paper, we present a method for real-time estimation
of the position of micro-objects in 3D; the method is based on twin-beam
illumination and it requires only a very simple hardware setup whose essential
part is a standard image sensor without any lens. Performance of the proposed
method is tested during a micro-manipulation task in which the estimated
position served as a feedback for the controller. The experiments show that the
estimate is accurate to within ~3 um in the lateral position and ~7 um in the
axial distance with the refresh rate of 10 Hz. Although the experiments are
done using spherical objects, the presented method could be modified to handle
non-spherical objects as well.
