Patent ID: 7808649

Claim:
A fixed-point detector, comprising: a plurality of fixed-point detecting patterns each having a pair of diffraction gratings for diffracting incident light in different directions and being arranged at an appropriate distance from one another in a measurement-axis direction; a fixed-point detecting light source which irradiates the pair of diffraction gratings of each of the fixed-point detecting patterns with light while moving in the measurement-axis direction with respect to the plurality of fixed-point detecting patterns; and a plurality of photoelectric conversion elements which move together with the fixed-point detecting light source, while receiving light beams diffracted by the respective diffraction gratings of the plurality of fixed-point detecting patterns and converting the diffracted light beams into electric signals, wherein, different combinations of two photoelectric conversion elements for receiving two light beams diffracted by the diffraction gratings of the respective fixed-point detecting patterns are employed such that a first fixed point associated with a first fixed-point detecting pattern employs a different combination of two photoelectric conversion elements of the plurality of conversion elements than a second fixed point associated with a second fixed-point detecting pattern the first fixed point detecting pattern includes a different set of diffraction gratings than the second fixed point detecting pattern; and a fixed point is defined as a point at which a level of an electric signal converted by one of the two photoelectric conversion elements is equal to a level of an electric signal converted by the other of the two photoelectric conversion elements.