Dust is the unwanted particle coming finally down onto the surface you would like to keep clean.  The particle can origine from various sources. The particle may be organic or non organic, man or machine induced.  It can have the shape of a very small rock or it appears like a curling fiber.  What ever, it is the part you do not want to have.Fibers on the edge of the witness plate

The images to right are made by means of the PDM and show small fibers. The grey area is the measuring area.

The image below shows a hair with a diameter of about 100 micrometer. The calibration dust has an average diameter of 25 micrometer.  The image below is suitable for particle detection down to 8 micrometer.

2018 we are able to detect down to 5 micrometer.

PDM III detail image of a sticky pad with calibration dust and a hair