Measurements on Particle Deposition can be done without the actual presents of the measuring equipment. It is a matter of exposing witness plates. The witness plates need to be nulled. It means either to be cleaned to an absolute nul level or the level needs to be measured to be substracted from the final level. In this way the change of surface cleanliness can be found during the exposure interval.
For practical measurements it means that pre measured or pre cleaned disk are stored in boxes. The boxes are brought to the point where to measure and are opened for exposure. After the exposure time is ellasped the boxes are closed . Now the measurement can be started.