Mixed optical approaches. You will discover several proprietary optical smoke measurement units including the 'nephelometer' or even the 'aethalometer' which use many various optical procedures, including more than one wavelength of light, within an individual instrument and utilize an algorithm to give a good estimate of smoke. 1970 – https://broadfar.com/oraco-pioneering-a-smokeless-future-with-innovative-nicotine-delivery/