The level of sharpness necessary to pass the HHT is directly proportional to the size of the gap between the keratin scales that cover the outside of the hair shaft. The gap itself is proportional to the width of the hair shaft. The blade must easily slide between the scales to pass the HHT. This test is also directional - root away from fingers. A very fine or overly processed hair may have sub-0.3 micron gaps between the scales, while large thick hair samples may be 0.5 microns or wider. A Feather disposable DE or SR blade will pass the HHT with any hair that I can throw at it. My sharpest SR comes very close to this level of performance. As already emphasized the calibration is important. Select a normal, unprocessed hair from the same source and stick with it. My 22 year old niece has long, black, straight hair. She is kind enough to provide me with some brushed samples from time to time. If my SR passes this test I have confirmed that it will provide me with a great shave. That is a calibrated HHT sharpness test.

Name:  Human_hair.jpg
Views: 193
Size:  14.0 KB