Mixed optical approaches. There are actually numerous proprietary optical smoke measurement equipment such as the 'nephelometer' or the 'aethalometer' which use numerous distinct optical techniques, such as more than one wavelength of sunshine, within one instrument and use an algorithm to give a very good estimate of smoke. 1600 https://broadfar.com/non-combusted-cigarettes-a-safer-alternative-or-just-a-myth/