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.
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.
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)
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,
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.