Traditionally, one measures the optical absorption in a medium by measuring the transmission of a beam of light through it. Unfortunately, this does not work well if there are particulates suspended in the medium, e.g. phytoplankton particles in natural waters. Specifically, the light lost from the transmitted beam due to absorption in the medium cannot be distinguished from the light lost due to scattering by the particles. As a consequence, accurate measurements of particulate absorption have generally been very difficult. One solution that has often been used for the measurement of particulate absorption is to collect (and thereby concentrate) the particles on/in a glass fiber filter and to then measure the transmittance of the filter; this is the so-called Quantitative Filter Technique (QFT). A further refinement is to also measure back reflections from the filter/entrained particles, and thereby provide a correction for the loss of light from the transmitted beam due to reflections. But there are still errors associated with optical properties of the sample and filter composite, e.g. scattering losses to the side, filter to filter variations, multiple scattering effects within the filter, particle sizes and size distributions within the filter, and determining the zero of absorption (null point correction).

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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