1. A method of calibrating a manufacturing process model comprising: producing a target structure using a layout for a target structure and a manufacturing process; illuminating the target structure and detecting an electromagnetic scattering property arising from the illumination; providing a manufacturing process model for simulation of the manufacturing process, the manufacturing process model comprising at least one manufacturing process parameter; determining a model electromagnetic scattering property using the manufacturing process model and the layout; comparing the model electromagnetic scattering property to the detected electromagnetic scattering property; and based on the result of the comparison, outputting calibrated manufacturing process parameters for calibrating the manufacturing process model.