Surface cleanliness is a snapshot: the particle load at the moment you measure. But contamination risk is driven by accumulation, how much settles onto a critical surface during an exposure period, and how fast. That rate determines how long a cleaned surface stays within specification, how often you need to clean, and whether your environment holds its required classification under operational conditions.

Particle Fall Out (PFO) measures exactly this. Two measurements: one at the start of an exposure period, one at the end and PFO reports the deposited particle load, the deposition rate, and the resulting classification. The same UV-imaging measurement that captures particle count and full size distribution, applied over time to quantify deposition and deposition rate.