The Kerf of a laser is the width of the blade (laser beam) this varies between machines it changes depending on focus so it could be different for different material thicknesses. Note :- added Feb 2015 The Kerf of the laser is reduced by reducing the power of the laser beam I recently changed my focal lens orientation to be correct thanks to Spooky at the OPL laser forum. For some reason Chinese Laser cutters come with the focal lens upside down.. After researching this I found many drawings from UK and USA laser lens manufactures showing that the convex side of the lens should face down not up . I know it looks logical how it is supplied with the convex side facing up. But the improvement in cutting is noticeable.and i my case the Laser Kerf has gone from 0.2mm down to 0.17 mm. It is with this tool that you can measure the kerf, of your laser beam. The blocks are 8mm x 10mm this ensures you measure them the right way. It might be worth also marking the top with a pen before removing them from the laser bed the you can put them into the measure in a 1 up 1 down sequence this will eliminate any sloping beam problems from the measurement. It could also be used to measure the slope of the beam with a little more maths.the how to use Instructions are on the tool