Automated scanning and measurements of particle distributions within a holographic reconstructed volume

Scanning and analysis of reconstructed holograms of a one-litre sample volume containing particles with varying sizes and shapes at high resolution is a major challenge. A completely automated system for analysing in-line holograms recorded in the ocean, which resolves particles larger than 10 mum, has been developed. It measures the three-dimensional coordinates of all the particles within the reconstructed volume and records their in-focus images. Scanning and analysing a reconstructed volume of about 500 cm(3) that contains several thousand particles takes about 5 h. The analysis consists of several steps. After compensating for exposure non-uniformities, the reconstructed images are scanned continuously with a digital camera. Then, superposition of thresholded images weighted by depth is introduced as a method compressing the 3D data to a plane to increase the efficiency of segmentation analysis. Subsequently, edge filtering is used for pinpointing the depth coordinate. To detect particles smaller than 50 mum, the reconstructed images are band-pass filtered optically. This approach is based on analysis that identifies interference of the reference beam with off-axis scattered light as the primary contributor to background noise. The scanning, thresholding and edge detection processes are repeated for the small particles. Additional procedures remove duplicate detections, and post-processing classifies the particles based on geometrical parameters, Sample data are presented.