Fig. 2

Arrow electron beam. The inside and outside of the shell are indicated for a, b, c. In green the “real” diameter, as measured from the inside of the test. a Cross section through pore: what is perceived in the SEM image, in red, is equal to the real diameter. b Differently perceived diameters, none of which is corresponding to the real diameter (in green). c Diameter dimension biased (in red) by an inclined incidence of the electron beam