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Automated scanning microscopy of wood microstructure
One of the components of SilviScan is an automated scanning optical microscope that uses light emitting diodes (LEDs) in both reflection and transmission modes to illuminate the polished transverse surface of wood strips cut radially from tree cores (or similar samples). In 1989 I found that light directed to the sides (or bottom) of a polished wood strip cut from an increment core illuminated the cell walls at the polished surface (‘tracheid effect’) to give a high quality image. The sanding dust produced by polishing blocked the lumens which appeared dark, while the fibre walls were relatively bright, giving high contrast images suitable for automated analysis. In reflected light, the texture of the sanding dust in the lumens confuses image analysis algorithms. Initial tests with traditional tungsten filament ‘white’ light showed that only longer wavelengths (red and beyond) were useful for transmitted light imaging, as the shorter wavelengths were much more efficiently absorbed. I therefore designed SilviScan to use long wavelength LEDs for transmission and shorter wavelength LEDs for reflection and autofocussing. In this presentation, I describe the design principles and limitations of the SilviScan automated image analysis system and show example results from a wide range of wood species.