Hand Tools Archive

Moving the light source around

Steve Elliott
When examining blades under a microscope, lighting can be tweaked to make the blade surface look good or to reveal as many scratches and defects as possible.

If the scratches from sharpening are perpendicular to the cutting edge, light from the side of the blade makes the scratches show up. If the same lighting is used on a blade that has been side sharpened (scratches parallel to the cutting edge) the scratches hardly show and the surface of the metal seems more highly polished even though it isn't.

My goal was to reveal as much of the surface texture as possible even if my sharpening results didn't look as good as some of images I found online.

Metallurgical microscopes have a light source that shines down along the optical path of the lens. That's a good way to illuminate shiny surfaces, as I found out when I did some over-the-top honing using fine diamond on a boxwood lap. The scratch pattern disappeared completely and my images showed the blade surface as black. None of the light had been scattered by scratches into the microscope lens. Maybe a physicist could say what level of surface refinement that would indicate based on the wavelength of the light source. David's microscope uses optical path lighting and I imagine his highly polished blades would also show no scratches under Winston's microscope.

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