A method including illuminating a product test substrate with radiation from a component, wherein the product test substrate does not have a device pattern etched therein and yields a non-zero sensitivity when illuminated, the non-zero sensitivity representing a change in an optical response characteristic of the product test substrate with respect to a change in a characteristic of the radiation; measuring at least a part of the radiation redirected by the product test substrate to determine a parameter value; and taking an action with respect to the component based on the parameter value.