JPS6217203B

PURPOSE:To reduce the effect of dust and flaws on an optical element by making the beam diameter of the minimum circle of confusion much smaller than the required beam diamemter on scanning plane and making the beam diameter on the scanning plane before or after the position of the minimum circle of confusion nearly equal to the necessary beam diameter. CONSTITUTION:In order to obtain a necessary beam diameter on the scanning plane, the beam diameter is once widened, thence it is stopped down by a scanning lens 1. If at this time, the focal length of the lens 1 is made constant, the diameter of the maximum circle of confusion of the exit beam becomes smaller as the incident beam diameter is larger. Thereupon, to obtain the necessary beam diameter on the scanning plane 2, such an optical system as the one in which the necessary beam diameter becomes the diameter of the minimum circle of confusion like a beam 3 and such an optical system as the one in which the diameter of the minimum circle of confusion is much smaller like a beam 4 and the beam diameter becomes equal to the necessary beam diameter on the scanning plane 2 are compared, then the quantity of laser light radiated to dust, flaw 6, etc. on the optical system 5 near the scanning plane 2 becomes much smaller in the beam 4 and therefore the effect of dust, etc. may be considerably reduced by using the optical system of the beam 4.