A perforated carbon film is shadowed with gold and graphitised carbon particles are deposited. These particles viewed over the holes may be used for assessment of factors limiting the microscope performance. The evaporated gold forms small polycrystalline islands and within these islands lattice fringes can be resolved. This specimen can also be used for the measurement of contamination rates in the electron microscope by noting the deposition rate of carbon within the holes found in the gold film.