Blended optical strategies. You'll find numerous proprietary optical smoke measurement equipment including the 'nephelometer' or maybe the 'aethalometer' which use a number of distinct optical procedures, like more than one wavelength of light, within a single instrument and implement an algorithm to provide a very good estimate of smoke. https://broadfar.com/non-combusted-cigarettes-a-safer-alternative-or-just-a-myth/