| 摘要 |
PROBLEM TO BE SOLVED: To make a separation distance on a three-line sensor surface constant, to improve spectral characteristics of a diffraction grating, and to facilitate the manufacture by making at least one stage of a one-dimensional blazed diffraction grating different in width from others. SOLUTION: An image formation system 3 composed of a nontelecentric optical system images luminous flux based upon a color image on a document surface 21 on the surface of the monolithic 3-line line sensor 5 through a color separating means 4. The color separating means 4 is composed of a transmission type one-dimensional blazed diffraction grating formed by arraying multiple 4-stage gratings periodically in the vertical scanning direction. The grating pitch of the diffraction grating 4 is varied from on the axis to off the axis at right angles to the pitch direction according to the field angle of the incident light beam and the width of the stage of the top stage part among four stages is made different in width from other stages. Further, the width of the stage of the top stage part is made constant from on the axis to off the axis irrelevantly to the field angle of the incident light beam.
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