An evaluation of a rotating drum densitometer and its application to precession photographs of protein crystals

A rotating drum densitometer is used to evaluate x-ray diffraction films. The instrument scans a 12 cm × 12 cm precession photograph in 150 s at a 200 μm raster and records the intensity readings on magnetic tape at 256 grey levels. The densitometer response is judged to be linear to an optical density of 1·9 and is accurate enough to yield integrated intensity values with a reproducibility of 1%. We describe in some detail the rationale of the computer program used to determine these intensities.