Absorption spectra (400 to 700 nm) measured with a new instrument, the integrating cavity absorption meter, were compared to those measured with standard techniques for particles (collected on glass fiber filters) or solutions (in long pathlength cuvettes) . The integrating cavity absorption meter has two advantages over previously used methods: 1) low absorption signals can easily be measured due to the long effective pathlength created by the highly reflective walls of the cavity, and 2) scattering averages to zero because of the isotropic light source. For particulate absorption, the locations of absorption peaks were offset up to 5 nm in the integrating cavity spectra due to the use of interference filters for determining wavelength. The filter wheel has bands centered every 10 nm, while the spectrophotometer has a monochromator with 1 nm spectral resolution. Modifications of the integrating cavity, presently in progress, will include replacement of the filter wheel with a monochromator. Particulate absorption measured with the integrating cavity was similar to that measured with the glass fiber filters through a portion (500 to 700 nm) of the visible spectrum. However, from 400 to 500 nm, absorption measured in the integrating cavity was less than that measured on glass fiber filters. For example, at 400 nm, cavity values were only 45 to 83% of the glass fiber filter values. Possible causes of this difference are discussed.
Absorption spectra for dissolved materials measured in the integrating cavity were similar to those measured in the