Geir Kulia gave a guest lecture in TTK4240 at NTNU walking from light scattering physics (Rayleigh, Mie, and geometric regimes; ISO 7708 dust fractions) through the full optical particle counter signal chain: photodiode, transimpedance amplifier, parallel comparator banks for size binning, and conversion from raw bin counts to mg/m³. The lecture closed with the parts of industrial sensor development that don't show up in coursework — EMC, IP-rated enclosures, ATEX/CE certification, gravimetric calibration, and supply-chain reality — drawn from the development of Arpuro, now deployed at ~20 industrial sites across Norway, Iceland, and Spain.