A method of measuring a damaged structure formed on a semiconductor wafer using optical metrology, the method includes obtaining a measured diffraction signal from a damaged periodic structure. A hypothetical profile of the damaged periodic structure is defined. The hypothetical profile having an undamaged portion, which corresponds to an undamaged area of a first material in the damaged periodic structure, and a damaged portion, which corresponds to a damaged area of the first material in the damaged periodic structure. The undamaged portion and the damaged portion have different properties associated with them. A simulated diffraction signal is calculated for the hypothetical damaged periodic structure using the hypothetical profile. The measured diffraction signal is compared to the simulated diffraction signal. If the measured diffraction signal and the simulated diffraction signal match within a matching criterion, then a damage amount for the damaged periodic structure is established based on the damaged portion of the hypothetical profile used to calculate the simulated diffraction signal.