Optical based PM measurement techniques uses the concept of light scattered or light absorbed by the particles to estimate the size and concentration of the particle as shown in Figure 1-1. Light scattered by a particle depends upon the particle size, particle shape and refractive index of the particle. Apart from these parameters, optical chamber design, light source and detection angle influence the light scattering. Light scattering based PM measurement instruments can be of two types, Photometers and Optical Particle Counters.