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B. L. Petrig, M. S. Muller, A. E. Elsner, D. A. VanNasdale, B. P. Haggerty, R. C. Gustus, E. J. Kollbaum; Low Cost Retinal Imaging and Mapping of Fixation with the Laser Scanning Digital Camera. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5279.
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© ARVO (1962-2015); The Authors (2016-present)
To provide retinal imaging that visualizes low contrast features by utilizing tightly confocal or multiply scattered light. To evaluate various types of scanning elements that bring enough precision but low cost to the Laser Scanning Digital Camera (LSDC),
High precision of our scanning element that sweeps an illumination slit across the retina is required, since there is line-by-line read-out of the CMOS area sensor (Aptina / Micron, Boise, ID) whose electronic rolling shutter was used as a flexible, electronic aperture for de-scanning. We considered two kinds of scanning elements for the illumination slit, a DC-motor driven polygon mirror, and a galvanometer mirror. Each scanning element required a different type of positional control and synchronization with respect to the CMOS aperture. The polygon rotated in open-loop, while the galvanometer oscillated in closed-loop with a saw-tooth pattern. Either approach lends itself for direct and indirect scattered light imaging. We evaluated the performance of the LSDC for various slit widths of 50, 100, 150, 200, 400, 800 µm. We also varied the distance between the illumination slit center and the CMOS aperture from a centered position for direct imaging to an off-center position for indirect imaging.
For aperture widths of 200 µm and above, the 2 scanning elements provided high contrast and full field images. To achieve a narrow confocal aperture, we reduced the aperture width to ≤150 µm, but found that small variations in the polygon speed caused spatio-temporal misalignment that led to uneven illumination across the frame, at speeds of up to 8 fps. The galvanometer produced accurate alignment and a uniform field for all aperture widths, at speeds up to 32 f/sec. The polygon and galvanometer achieved duty cycles of 11% and 90%, respectively.
The galvanometer-based design has better synchronization accuracy for narrow aperture widths, as well as for a uniform field, and has a higher duty cycle and frame rate than the DC-motor-based approach. Low cost, high quality retinal images are readily obtainable.
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