A recent breakthrough solution for measuring absorption in the presence of scattering is the Integrating Cavity Absorption Meter (ICAM). This device provides isotropic illumination of the sample (i.e. at any point in the sample, light is incident from 4 pi steradians) and absorption measurements are therefore insensitive to scattering effects. Specifically, if light in the sample is already propagating in all directions, time averaged scattering in the sample cannot change anything. But absorption does change something; it reduces the amount of light in the sample. Thus, measurement of the latter provides an accurate measurement of absorption by both the medium and the scattering particulates suspended in it, and that absorption measurement is essentially unaffected by scattering in the medium.
In principle, the QFT has unlimited sensitivity; however, although it has a long and successful history in concentrating the particulates from dilute natural aquatic samples, it suffers from significant errors associated with measurement of absorption by the filter. This is where Röttgers and Gehnke stepped in and cleverly combined the QFT with their Point Source ICAM (or PSICAM). Specifically, they measured the absorption of the filter by placing it in their PSICAM; their paper describes extensive tests and comparisons to show the efficacy of this approach. They show that overall precision is high and does not suffer from the scattering errors of usual T (transmission) or T-R (transmission/reflection) measurements. Errors due to varying filter properties are avoided, and the PSICAM inherently eliminates the need for a zero absorption point determination. Due to the very strong scattering in the filter, there is a path-length amplification effect in the filter; this also occurs with T and T-R measurements, but the amplification effect was shown to be about twice as big in the PSICAM. Of course, the greater effective path length in the PSICAM makes it more sensitive. The path length amplification is a variable depending on filter characteristics and particle loading but the relative variability was found to be significantly less for measurements in the PSICAM; nevertheless, this does appear to be the major source of error.
The final conclusion is that a combination of QFT with a PSICAM could lead to measurements of particulate absorption in natural waters with overall errors less than 5%.
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