| 摘要 |
PURPOSE:To form an ftheta lens which can obtain an image being free from a color shift and a distortion, by correcting a magnification chromatic aberration for laser beams of three or more wavelengths, and also, reducing a distortion. CONSTITUTION:The titled lens is constituted of a negative cemented lens group which has joined the first positive meniscus lens, the second negative lens and the third positive lens, and the fourth lens of a positive biconvex lens in order from an entrance pupil side, and satisfies conditions of inequalities 1-5. In this regard, n2, n3, and nu2, nu3, and f, fII, and d1, and d2 denote refractive indexes by the main wavelength of the second and the third lenses, Abbe numbers of the second and the third lenses, focal distances of the whole lens system and the cemented lens groups, core thickness of the first lens, and an axial air interval of the second lens, respectively. As for this ftheta lens, a magnification chromatic aberration of plural wavelengths, especially, three wavelengths is corrected satisfactorily, and its distortion is also small. |
