Measuring optical absorption coefficient of pure water in UV using the integrating cavity absorption meter.

The integrating cavity absorption meter (ICAM) has been used successfully to measure the low absorption coefficient of pure water. The ICAM produces an effective total path length of several meters or even longer, although the physical size of the instrument is only several centimeters. The long effective total path length ensures a high sensitivity that enables the ICAM to measure liquid mediums with low absorption. Compared to the conventional transmission type of instruments that were used to measure the same medium with the same path length, the ICAM eliminates the effect of scattering by introducing isotropic illumination in the medium, and consequently measures the true absorption coefficient of the medium in stead of the attenuation coefficient. The original ICAM was constructed with Spectralon and used in the wavelength range from 380 nm to 700 nm. Later studies showed that Spectralon is not suitable for measurements in the UV region because of its relatively lower reflectivity in this region and, even worse, the continuously decaying reflectivity under the exposure to UV radiation. Thus, we have developed 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 the deterioration problem. The measurement results from the new ICAM are in good agreement with the existing results. The absorption coefficients of pure water at wavelengths between 250 nm and 400 nm are presented here.