Imaging optical system and image reading apparatus including four, free-form shaped, resin off-axis reflective elements

An imaging optical system for image reading, in which image information on a surface of an original is imaged on a line sensor and the image information is read by the line sensor, comprising:

• a first off-axial optical element, a second off-axial optical element, a third off-axial optical element and a fourth off-axial optical element disposed in order from an original side, wherein each of the first, second, third and fourth off-axial optical elements is a reflection-type off-axial optical element having free-form surface shape and is made of a resin material,
• wherein a main scanning direction is a line direction of the line sensor and a sub-scanning direction is a direction orthogonal to the main scanning direction, a length of each of the first, second, third and fourth off-axial optical elements in the main scanning direction is longer than a length thereof in the sub-scanning direction and a thickness thereof is smaller than the length thereof in the sub-scanning direction,
• wherein in a sub-scanning section in which the main scanning direction corresponds to a normal, the following condition is satisfied,
• −30°<θa1−θb1<30°,
• where θa1 represents an absolute value of a first angle formed between a surface normal to a reflective surface of the first off-axial optical element at a reflection point of the reflective surface of the first off-axial optical element at which a reference axis light beam is reflected and a reference axis light beam incident on the reflective surface of the first off-axial optical element and θb1 represents an absolute value of a second angle formed between a surface normal to a reflective surface of the second off-axial optical element at a reflection point of the reflective surface of the second off-axial optical element at which the reference axis light beam is reflected and the reference axis light beam incident on the reflective surface of the first off-axial optical element, and
• wherein in the sub-scanning section, the following condition is satisfied,
• −30°<θa2−θb2<30°,
• where θa2 represents an absolute value of a third angle formed between a surface normal to a reflective surface of the third off-axial optical element at a reflection point of the reflective surface of the third off-axial optical element at which a reference axis light beam is reflected and the reference axis light beam incident on the reflective surface of the first off-axial optical element and θb2 represents an absolute value of a fourth angle formed between a surface normal to a reflective surface of the fourth off-axial optical element at a reflection point of the reflective surface of the fourth off-axial optical element on which the reference axis light beam is reflected and the reference axis light beam incident on the reflective surface of the first off-axial optical element.