van der Meer method
The van der Meer method, named after its inventor Simon van der Meer, is a technique for measuring the transverse distribution of particles in an accelerator beam. It can be used to measure the instantaneous luminosity of an accelerator.
The van der Meer method works for colliding beams, by slightly varying the location of one beam of one of the beams with respect to the other and observing how the rate of reactions changes. These beam spot variations are called van der Meer scans. It is often interesting to observe the rate of some particularly well-known reaction (called a standard candle), and use the scans to measure the instantaneous luminosity of the accelerator. The van der Meer method is particularly useful for establishing the absolute calibration scale of the luminosity measurement.