A masked film cassette for utilization of the multiple film technique in precession photography

A film casette equipped with a semi-circular mask rotated at constant angular velocity has been designed for the Buerger precession camera. The mask may be adjusted to eliminate either one of the two reflections which comprise each diffraction spot on a normal precession film, thereby eliminating the `focusing condition' and permitting use of the multiple-film technique for quantitative recording of intensities. The device only approximates the geometry for correct masking of the Laue cone. Expressions giving the incorrectly masked portions of the reciprocal lattice are derived, and show that, even under extreme conditions, only a negligible portion of the record is affected. Use of the device necessitates doubled exposure times. This disadvantage is offset by the ability to obtain films of differing density in a single exposure, an improvement in the shape of the diffraction spots, and economy in subsequent correction of the intensity data for but a single Lorentz and absorption factor.