A method of determining a defect in a grid plate of an encoder-type position measurement system, the method including providing an encoder-type position measurement system to measure a position of a movable object with respect to another object, the encoder-type position measurement system including a grid plate and an encoder head, measuring a quantity of light reflected on each of the two or more detectors, using a combined light intensity of the reflected light on the two or more detectors to determine a reflectivity signal representative for the reflectivity of the grid plate at the measurement location, and determining a presence of a defect at the measurement location on the basis of the reflectivity signal of the grid plate.