| 摘要 |
<p>PROBLEM TO BE SOLVED: To prevent noise to be generated during ultrahigh speed or normal speed rotation from increasing by providing a deflection means having a large number of mirror faces on the side face, means for focusing a deflected laser beam on a focus plane and means for reducing rotary noise arranged on the deflection means thereby reducing the pressure difference between high and low pressure regions. SOLUTION: When a polygon mirror 40 begins to turn at a specified r.p.m. through a motor M50, a high pressure region A is formed by the rotating force on the mirror face on the rotational direction side around a vertical edge 100 of the polygon mirror 40 and a low pressure region B is formed on the mirror face on the opposite side to the rotational direction. Pressure difference between the mirror face on the opposite sides is reduced through an air groove 110 made in the opposite side faces of the vertical edge 100 of polygon mirror 40 and communicating with the low pressure region B and the high pressure region A. A fluid stripping phenomenon takes place on the mirror face where the low pressure region B belongs starting from the starting end of the air groove 110 to reduce vortex flow and a corresponding frequency band is reduced resulting in the reduction of noise by one to several dB.</p> |
