For measuring the oscillation amplitude of a scanner mirror in a projection system of a motor vehicle headlight, a laser beam generated by a laser source is directed onto the scanner mirror and reflected by the latter so that the laser beam thus reflected is incident on a detector device (20) that has a plurality of photodetector elements (Q1, Q2, Q3, Q4) and there describes a curve (P) based on the oscillation movement of the scanner mirror. The center point of the curve (P) is offset by an offset value (xoffset, yoffset) from the center of the detector device (20). The time period (tON,X, tON,Y) in which the curve passes through the specific detector region (RX, RY) that corresponds to a coordinate to be measured is determined; and the oscillation amplitude (xpp, ypp) in the direction of the specific coordinate is determined using the ratio of the time period (tON,X, tON,Y) determined in this manner to the total duration (T) of an oscillation period and the offsets (xoffset, yoffset).