The integrating cavity absorption meter (ICAM) has been used successfully to measurethe low absorption coefficient of pure water. The ICAM produces an effective total pathlength of several meters or even longer, although the physical size of the instrument isonly several centimeters. The long effective total path length ensures a high sensitivitythat enables the ICAM to measure liquid mediums with low absorption. Compared to theconventional transmission type of instruments that were used to measure the samemedium with the same path length, the ICAM eliminates the effect of scattering byintroducing isotropic illumination in the medium, and consequently measures the trueabsorption coefficient of the medium in stead of the attenuation coefficient. The originalICAM was constructed with Spectralon and used in the wavelength range from 380 nmto 700 nm. Later studies showed that Spectralon is not suitable for measurements in theUV region because of its relatively lower reflectivity in this region and, even worse, thecontinuously decaying reflectivity under the exposure to UV radiation. Thus, we havedeveloped a new way to construct the ICAM utilizing the material fumed silica. The resulting ICAM has a high sensitivity even in the UV region and doesn't have thedeterioration problem. The measurement results from the new ICAM are in goodagreement with the existing results. The absorption coefficients of pure water atwavelengths between 250 nm and 400 nm are presented here.
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