Image scanner and line period determining method for image scanner

A line period determining method for an image scanner having an operation clock generator configured to generate an operation clock by frequency modulating a reference clock with a particular modulation frequency, a photoelectric converter having a plurality of photodetectors arranged in a main scanning direction, the plurality of photodetectors receiving reflected light, the reflecting light being reflected by an original and is displaced in a sub scanning direction perpendicular to the main scanning direction, at every line period, the plurality of photodetectors serially outputting analog pixel signals corresponding to received light amounts of the plurality of photodetectors, respectively, in accordance with the operation clock, an AD converter configured to convert the analog pixel signals serially output by the photoelectric converter to digital pixel data by sampling the analog pixel signals in accordance with the operation clock, read image data representing an read image of the original being generated based on the pixel data converted by the AD converter, the method comprising:

• determining a line period which is (integer+decimal value) times a modulation period that is inversion of the modulation frequency for each of reading resolutions settable in the image scanner, the integer being greater than one;
• and
• identifying, for each of settable reading resolutions, at least one decimal value making cross-point values less than a threshold value, the cross-point values being amounts of the fluctuation component at cross-points of the fluctuation components of the visual sense limit resolution equivalent first adjacent line and the visual sense limit resolution equivalent second adjacent line superimposed such that coordinate axes thereof in the main scanning direction are coincident to each other,
• wherein the determining includes setting, for each of the settable reading resolutions, the line period corresponding to the reading resolution to (integer+decimal value) times the modulation period determined based on the identified decimal value.