A method of double-beam spectrophotometry for the vacuum ultraviolet region is described. The method is first used to measure the transmission of thin unsupported films of aluminium in the wavelength range 1032 to 240 Å (photon energies from 12 to 50 eV). The results are used to calculate the optical constants
n
and
k
for photon energies from 14 to 36 eV. In a second series of experiments the reflexion of aluminium films deposited on a glass substrate is measured for angles of incidence from 30 to 70° and for photon energies from 12 to 24 eV. It is assumed that the aluminium film is covered with a thin layer of aluminium oxide and the optical constants of this material are obtained from a separate experiment. An electronic computer is then used to find the best values of the optical constants of aluminium to fit the reflexion measurements and to estimate the errors. These results are combined with those obtained from the transmission measurements to obtain values of
n
and
k
for photon energies from 12 to 36 eV. There is reasonable agreement between the results obtained by the two methods in the region where the measurements overlap. The critical wavelength below which aluminium is nearly transparent is found to be 826 Å (15.0 eV). The optical properties are not found to be correlated with the grain size of the films determined by electron microscopy.
