A cutting unit includes a detachable head assembly with a cutting insert and holder. The circuit is connected in series with the nonlinear load and is tuned to the third harmonic of the power source frequency. The first channel formation region, the gate electrode, and the second channel formation region are overlapped with each other. In one variation, a groove in the bottom edge of the edge adapter receives the top edge of the front substrate to further define the position of the edge adapter in relation to the substrate. The base plate defines vents therein and blades thereon corresponding to the air outlet of the fan. At least two optical filters among the plurality of optical filters, which have different optical properties, are different in shape from each other.