Blended optical methods. You can find many proprietary optical smoke measurement devices such as the 'nephelometer' or even the 'aethalometer' which use many distinctive optical procedures, such as more than one wavelength of light, within only one instrument and utilize an algorithm to give a superb estimate of smoke. https://broadfar.com/oraco-pioneering-a-smokeless-future-with-innovative-nicotine-delivery/