In a first aspect of the invention, there is provided a method of determining a correction for a measurement of a target, the measurement being subject to a target-dependent correction parameter term which has a dependence the target and/or a stack on which the target is comprised, the method comprising: obtaining first measurement data relating to a measurement of a fiducial target, said first measurement data comprising at least a first set of intensity parameter values and a corresponding second set of intensity parameter values; obtaining second measurement data relating to a measurement of the fiducial target, the second measurement data comprising a third set of intensity parameter values; determining a target-invariant correction parameter From said first measurement data and second measurement data, the target-invariant correction parameter being a component of the target-dependent correction parameter which is not dependent on the target and/or a stack; and determining said correction from said target-invariant correction parameter. Also disclosed is processing device and associated program storage, and a computer program, each comprising instructions for a processor which cause the processor to perform the method of the first aspect.
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