Lost Lake -- a solution lake.

http://deepblue.lib.umich.edu/bitstream/2027.42/54047/1/2482.pdfDescription of 2482.pdf : Access restricted to on-site users at the U-M Biological Station

Improved detection of centroids in aberroscope and Hartmann-Shack grid images

Purpose: An aberroscope or a Hartmann-Shack sensor is often used to determine the monochromatic aberrations of the human eye. Today, most of these instruments are equipped with a CCD video camera and a computer which calculates the wavefront error using the displacements in a grid image. These displacements are determined from the centroid locations of the grid points. Methods: The images that are acquired from the camera may be of low resolution and/or low intensity. The main factors that give rise to the insufficient quality of captured images include poor reflectance from the retina, eye aberrations, absorption and scatter within the eye, and limited light source power because of the potential for retinal light damage. Current methods for detecting the centroids in these images are essentially based on a Canny-Deriche oriented edge detecting filter, the performance of which has been found to be insufficient in some clinical applications. The limitations of current methods can be overcome by utilizing an image processing procedure that can not only reduce noise and other side-effects but also significantly improve the accuracy of the detected centroid estimates. Results: We have developed a four step technique comprised of the follower: (i) enhancement of the captured image, (ii) calculation of the initial coarse regions of interest using watershed transformation (iii) refinement of the regions using a constrained watershed, and (iv) centroid detection using shrinking procedure. We have tested this new technique on simulated images with controlled amounts of added noise in addition to real aberroscope and Hartmann-Shack images. The proposed technique performs very well and can accurately detect the centroids in the image grid for signal-to-noise ratios down to SNR-5 dB. Conclusions: Our technique can accurately determine the centroid locations in images of low resolution and intensity where Canny-Deriche filter based techniques fail