Patent ID: 8868387

Claim:
A method of optimizing a model of a spectrometer, the spectrometer being configured to measure a property of a substrate, the model having at least two input parameters, the input parameters comprising a first free input parameter and a fixed input parameter, the method comprising: setting, using a processing device, a first set of values for the input parameters; generating, using a processing device, a first simulated spectrum of the spectrometer from the first set of values using the model; changing, using a processing device, the value of a second input parameter by a first predetermined amount to form a second set of values for the input parameters, the second input parameter being different from the free input parameter; generating, using a processing device, a second simulated spectrum of the spectrometer from the second set of values using the model; using a second model to find a third set of values for the input parameters from the second simulated spectrum, the second model being arranged such that the third set of values generates substantially the second simulated spectrum using the first model and such that the third set of input parameters are the same as the first set of input parameters except for the first free input parameter; determining, using the processing device, a difference between the first free input parameter in the third set of values for the input parameters and the free input parameter in the first set of values; and selecting, using a processing device, the free and fixed input parameters in the model based on the difference.