The use of focussed, monochromatised radiation at the synchrotron has so far yielded the most results in terms of biological molecular structure compared with the other methods being developed. This is readily explained because of the ease with which the monochromatic diffraction data measured at the synchrotron have been processed with existing computer programs for data from monochromatic, emission line, laboratory X-ray sources. In contrast, the Laue method, although it is being very actively developed at the synchrotron (chapter 7), had been abandoned in the home laboratory. Hence, the monochromatic method is covered first in this book. In appendix 1 details are given of the various monochromatic diffraction geometries. These geometries are:

1. (a) monochromatic still exposure;

2. (b) rotation/oscillation geometry;

3. (c) Weissenberg geometry;

4. (d) precession geometry;

5. (e) diffractometry.

Quantitative X-ray crystal structure analysis usually involved methods (b), (c) and (e) although (d) has certainly been used. Photographic film is being replaced by use of electronic area detectors or, even more recently, the IP.

At the various synchrotrons all these geometries have been exploited for macromolecular crystal data collection as they have also on conventional X-ray sources. Once the polychromatic synchrotron X-ray beam has been rendered monochromatic the single crystal data can be measured and processed as for a conventional X-ray source.