Scientifically they photograph the top and sides and measure the radius of various sides in microns.
As a rule of thumb, if you think it might be dull then it is.
I am going to second Whit3rd. Not sure how he got his data but he is correct. The only thing I would caution is that a very clean cut is different from a polished cut. Remember you are cutting fibers in wood. You want a very sharp, even edge to sever the fibers cleanly instead of pulling them out and stretching. A dull blade will pull the fibers out, stretch them and leave a fuzzier surface. A duller blade will pull the fibers out and the next tips will run over the pulled out fibers and polish them.
Wood fibers do have certain amount of pullback or snap back but not a great deal. It is best to clearly sever the fibers. This is a lot easier to see when you cut a chunk of hemp rope with a very sharp knife, a dull knife and a really dull knife. Cutting natural wood is a lot like cutting hemp rope as far as the behavior of the fiber is concerned. It is just much more obvious with hemp rope.
As to how I go my data. I manufacture top end, custom saw blades. We work with Forintek
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and have done research for and with hundreds of mills plants and saw shops over the last 28 years. You can see some of our research at
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Having said all that, your question is extremely good. How to determine sharpness is always a question in research. There is no single, definitive way to determine sharpness. I mentioned edge radius above. If you are cutting brittle materials you with something such as an ATAF (alternate top / alternate face) with a high ATB (alternate top bevel) you will want a separate determination for point sharpness and well as edge sharpness.
Finally different materials also help determine how well a blade will cut irrespective of sharpness. A cermet 2 tipped blade has a much lower coefficient of friction then carbide and will cut better than a carbide blade with an identical edge.
Tom Walz