| 摘要 |
PURPOSE:To provide a lens which has an excellent ftheta characteristic, is satisfactorily corrected in curvature of field and permits printing with high resolution up to the periphery by forming said lens of 3-element constitution consisting, from a light source side, of respectively specific biconcave lens, positive lens and positive lens. CONSTITUTION:The lens system which is to be used for scanning of a beam from a light source at a uniform speed on an image plane by a deflector is made of the 3-element constitution consisting, from the light source side, of the biconcave lens, positive lens and positive lens and satisfies the conditions expressed by the equations (1)-(5). The equation (1) is for maintaining the curvature of field within a suitable range and the equation (2) is the conditions to satisfy the ftheta characteristic. The condition(3) relates to the refractive indices of the 2nd and 3rd lenses each having a positive refracting power. A negative distortion aberration is not substantially generated if the refractive index is large. The equation (4) relates to the radius of curvature on the image side of the 1st lens and is for the correction of the curvature of field. The equation (5) relates to the radius ratio of curvature of the 2nd lens and is mainly for correcting a comatic aberration. |
