1. A method of defining a profile such that the definition of the profile can be used to reconstruct profiles of objects from diffracted radiation from the objects, the method comprising: defining a profile using information from a known object; describing a surface of the profile using a mathematical function; selecting a variable parameter in the mathematical function and varying the variable parameter iteratively until a calculated diffraction pattern of the profile matches the diffraction pattern of the known object; and storing the profile and the variable parameter.