Bacteria detection with thin wetting film lensless imaging

Lensless on-chip imaging is a promising technique to count and monitor cells and micro-objects in liquid sample. In this paper we apply this technique to the observation of mu L sample containing bacteria evaporated onto a microscope slide. Compared with previously reported techniques, a large improvement in signal to noise ratio is obtained due to the presence of a few mu m thick wetting film creating a micro-lens on top of each bacteria. In these conditions, standard CMOS sensor are able to detect micro-objects as small as few mu m, e. g. E. coli and Bacillus subtilis bacteria and 1 mu m polymer beads with a large signal to noise ratio of 45 +/- 10. An overall detection efficiency of 85 +/- 7% and a co-localization error of sigma(1D) = 1.1 mu m compared with reference fluorescence microscopy images are achieved. This novel technique will be used as a pre-positioning tool prior to other optical identification methods, e. g. Raman spectroscopy. (C) 2010 Optical Society of America