Before introducing the optical dual-wavelength technology, the aerosol particles, which are detected by fire detectors, have to be explained more in detail. Aerosols, also defined as particulate matter (PM), are microscopic small particles distributed in the atmosphere. For the human eye airborne particles are only visible in a group of many particles forming a cloud in the air. If we know the source of origin, the particles may fall into categories like dust, haze, fog, clouds, sand, hairspray, or smoke. Letting a device detect and categorize these particles is only possible by counting and measuring their particle size. As depicted in figure 2, different types of particles have a range of particles size from small to large. The blue and red area is a graphical representation of a particle size distribution and explains how the amount of particles is distributed according to size (histogram). We can categorize particles like smoke, haze, soot, sea salt, in the range under 1 ?m particle size. Particles like dust, sprays, carbon dust, cement dust, coarse sand or fog can be categorized as big particles over 1 ?m particle size. Some airborne particles like haze or fly ash do have small and big particles which cannot always be discriminated at 1 ?m particle size. We can learn from figure 2 that most smoke particles are under 1 ?m particle size (blue area). Looking at the red area, which represents deceptive phenomena like dust particles, it is easily seen that we can discriminate smoke particles and dust particles at around 1 ?m particle size.