Aspects of the invention relate to techniques for determining pattern optical similarity in lithography. Optical kernel strength values for a first set of layout features and a second set of layout features are computed first. Based on the optical kernel strength values, optical similarity values between the first set of layout features and the second set of layout features are then determined. Subsequently, calibration weight values for the first set of layout features may be determined based on the optical similarity values, which, along with the first set of layout features, may be employed to calibrate lithography process model parameters.