Calibrating an area-detector diffractometer: integral response

The quality of diffraction data measured with electronic area-detectors is improved by correcting for non-uniformities in the response of the detector. Many detectors are actually much more uniform than they appear because much of the perceived non-uniformity is an artefact of the distortions in their imaging geometry and of the methods of illumination during calibration. Indeed, every known correction reduces the perceived non-uniformity. Our inability to illuminate the detector uniformly with radiation of the same wavelength as is used during data-collection is a particular worry because of differential absorption. The tails of the point-spread function also perturb the apparent response, particularly near to the edges of the imaging area. These problems are difficult to compensate, so there is no completely satisfactory method of determining the true response of real detectors. However, this does not prevent us from making calibrations of usable accuracy. Although this paper applies to all types of area-detector, the discussion is centred mainly on the ENRAF-NONIUS fast system, which is a commercially available television diffractometer, calibrated using software written by the present author. Calibrating the response of imaging detectors is a general problem, and many of the techniques expounded here are of wide applicability.