The ADPD188BI is not a particle counter. It is better to think of it as a turbidity measurement but in the air. Even enough particles at PM2.5 or smaller will make the air "cloudy" and will be detected. Turbidity in air similar to a liquid is a function of the particle size, its reflectance, its concentration, and if it is clear, its index of refraction with respect to air. In general experimental data has shown that particle size can be differentiated to some degree by looking at the ratio of blue to infra-red for the same level of overall "turbidity-reflectance" that the datasheet refers to as "PTR" or power transfer ratio. This wavelength vs. particle size determination is using the principles of Mie scattering.
