International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F, ch. 9.1, p. 190
Section 9.1.11.2. Exposure time^{a}National Cancer Institute, Brookhaven National Laboratory, NSLS, Building 725A-X9, Upton, NY 11973, USA, and ^{b}Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, England |
According to the principles of counting statistics, the longer the exposure, the better the signal in the data. The standard uncertainty of the measurement is equal to the square root of the number of counts, and the signal-to-noise ratio increases with the accumulated counts. In practice there are limitations to this rule.
The dynamic range and saturation limit of the detector is one limiting factor. It may be impossible to measure adequately the strongest as well as the weakest reflection simultaneously, since their intensities differ by several orders of magnitude. If the exposure time is long enough to record the weakest intensities, then in general at low resolution the most intense reflections may saturate some pixels within their profile on the detector. Such reflections are termed `overloads' and this problem will be addressed in Section 9.1.11.3.
Exposure time can be limited by the total time available for the experiment. This is often a particularly acute problem for synchrotron-data collection, with high oversubscription of beamlines. The decisions concerning exposure time depend on the expected application of the data, since different applications have different requirements, as addressed in Section 9.1.13. Within the given time constraints, the first priority should be data completeness, even at the expense of underexposure. In this context it is useful to recall that to increase the statistical signal-to-noise ratio by a factor of two, it is necessary to prolong the exposure time by at least a factor of four.