One of the advantages of laser cutting is that the energy density of the beam is high, which is generally >10W/cm2. Due to the proportional to the energy density and 4 / PI D2, so focal spot diameter as small in order to produce a narrow slit; at the same time of focal spot diameter lens and also is proportional to the focal depth.
The smaller the focal depth, the smaller the focal spot diameter is. But cutting has splash, lens away from the workpiece is too near to will damage the lens, so the general high power CO2 laser cutting technology of industrial application is widely used in the focal length of the 5 "7.5" "127~190mm. The actual focus spot diameter is between 0.1~0.4mm. For high quality cutting, the effective focal depth is also related to the diameter of the lens and the material being cut.
For example, 5 "lens cutting carbon steel, the focal depth of focal length in the range of +2%, which is about 5mm. So it is very important to control the position of the focus with respect to the surface of the material being cut. Concerns about the cutting quality, cutting speed and other factors, in principle, <6mm of metal materials, focus on the surface; >6mm carbon steel, focus on the surface; >6mm stainless steel, focus on the surface. Specific size determined by experiment.