Since 1994, the researchers at Air Quality Research Center are measuring the light absorption by particles collected on filters for IMPROVE network. This measurement has been conducted on developed in house instrument called HIPS (Hybrid Integrating Plate and Sphere system) operating at a single wavelength using red light of 633 nm from a He(Ne) laser.
A schematic of the current system is given in Figure 1 below. Light of 633 nm wavelength from a He (Ne) laser is diffused and collimated to provide a uniform beam of light of approximately 0.7cm2 in area at the sample. The filters are mounted in standard 2”x2” slide frames with the particles on the side of the filter away from the incident light. The light transmitted through the sample into a forward cone is collected with an Oriel 7022 photodiode detection system, while the reflected light is collected by a 2” sphere (Labsphere) and measured with a photodiode detector.