1. Method for determining the heat transfer characteristics of a heat conducting body having an internal structure for a flow medium to flow through comprising the following steps, passing through the internal structure of the body a first flow medium having a first temperature, registering a first thermal image of an external surface of the body as a function of time using a pixelised image detector, processing first thermal image in order to obtain first measured surface temperatures (T S, meas (t,x,z,y)) for a given set of points in time (t) and a given set of points (x,y,z) on the surface, predicting first surface temperatures (T S, pred (t,x,y,z)) resulting from the transient flow through the body of the same external surface of the body and for the same set of points in time using a numerical model (M num ), applying a best fit method (fit) to the first measured surface temperatures (T S, meas (t,x,z,y)) and the first predicted surface temperatures (T S, pred (t,x,y,z)) and determining a measure of fit between them, optimising the first predicted surface temperatures (T S, pred (t,x,y,z)) by means of an iteration until the measure of fit between the first measured surface temperatures (T S, meas (t,x,z,y)) and the first predicted surface temperatures (T S, pred (t,x,y,z)) is optimised, characterised in that the numerical model (M num ) includes initial values for the heat transfer coefficients (h i ) and wall thicknesses (δ i ) of the internal structure of the body, which are optimised in the iteration until the measure of fit between the first predicted temperature values (T S, pred ) and the first measured temperature values (T S, meas (t,x,z,y)) is optimised, and final heat transfer coefficients (h f (x,y,z)) and wall thicknesses (δ f (x,y,z)) are determined that correspond to the optimised measure of fit.