Mixed optical approaches. There are actually several proprietary optical smoke measurement units such as the 'nephelometer' or even the 'aethalometer' which use many diverse optical strategies, such as multiple wavelength of light, inside of an individual instrument and utilize an algorithm to provide a very good estimate of smoke. https://nuso-vs-heets77888.wikihearsay.com/3366764/the_basic_principles_of_smoke