A method of determining a parameter of a patterning process applied to an object comprising two features (for example an overlay of the two features) comprises: irradiating the two features of the object with a radiation beam and receiving at least a portion of the radiation beam scattered from the two features of the object. The at least a portion of the radiation beam comprises: a first portion comprising at least one diffraction order and a second portion comprising at least one diffraction order that is different to a diffraction order of the first portion. The method further comprises moderating a phase difference between the first and second portions and combining the first and second portions such that they interfere to produce a time dependent intensity signal. The method further comprises determining the parameter of the patterning process from a contrast of the time dependent intensity signal.