The nominal value of the effective camera
length of an EM poperating in the selected area
mode is not sufficiently accurate for any
calculations of lattice spacing. The actual value
of camera length must be calibrated as the
same accelerating voltage and objective lens
setting by reference to a known substance
with well defined diffraction spacings. The
normal specimens are evaporated films on a
3.05 mm grid of aluminum or thallous chloride.
The very small crystalline size yields ring
patterns suitable for calibration purposes.