Tech Forum-Absorption

## In Situ Spectral Absorption Meter

### by Ronald Zaneveld

An in situ spectral absorption and attenuation meter has been developed that measures the absorption coefficient at multiple wavelengths at a rate of several Hz. This instrument is based on the reflective tube absorption meter principle. A collimated beam of light is transmitted through a reflecting tube. Both the directly transmitted light and most of the scattered light are collected using a diffuser and photodiode at the end of the tube. Since most of the scattering in the ocean is near-forward only a small amount of light is lost. The instrument uses precisely the same principle as a spectrophotometer and is perhaps best thought of as an in situ spectrophotometer. Wavelengths are selected by means of a wheel containing interference filters in the light source housing. The device contains a beam splitter and a reference detector to correct for drift in lamp output. The device uses nine wavelengths. Figures 1-3 show profiles and absorption spectra obtained in East Sound, Washington. The difference of the particulate absorption coefficients at 676 and 712 nm (ap(676)- ap(712)) is an indicator of chlorophyll concentration, whereas ap(712) is an indicator of scattering error and therefore estimates particulate concentration. Note from Figure 1 and Figure 2 the fine-scale of some of the vertical structure, with some of the layers only ca. 10 cm thick. Since the device combines the measurement of spectral absorption with spectral attenuation, we can calculate the scattering coefficient from the difference of the attenuation and absorption coefficients. We can then obtain the scattering error for the absorption meter at any wavelength.

### Current Status

WET Labs of Philomath, Oregon [(503) 929-5650] now produces the 9-wavelength, absorption and attenuation meter (ac-9) commercially. They also produce a three-wavelength version (ac-3) and a device that measures the chlorophyll absorption at the red absorption peak. These devices sample at 12 Hz and consume 3 watts at 12 volts. They measure 46 cm x 10 cm (length by diameter) and weigh 2 kg in air. Signal output is either analog or RS-232 (serial), so that they can be readily interfaced to CTD's.

### Future Development

WET Labs, together with Satlantic, has also received a NASA-SBIR grant to develop a combination spectral absorption, backscattering, attenuation and irradiance sensor. If the second phase is funded the combination device should be commercially available in about one year. Oregon State University, with a subcontract to WET Labs, has requested funding from the NSF to develop an in situ spectral fluorescence sensor. Development should take two years and make the instrument available in mid-1995. (Ron Zaneveld is with the College of Oceanic and Atmospheric Sciences at Oregon State University)

### References

Zaneveld, J. R. V., J. C. Kitchen, A. Bricaud and C. Moore. 1992. Analysis of data. Ocean Optics XI. G. D. Gilbert, Ed., Proc. SPIE, 1750, 187-200.

Moore, C., J. R. V. Zaneveld and J. C. Kitchen. 1992. Preliminary results from an in situ spectral absorption meter. Ocean Optics XI, G. D. Gilbert, Ed., Proc. SPIE, 1750, 330-337.