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HydroBeta: Measuring Volume Scattering Function from 10° to 170°

HydroBeta is a new instrument for measuring the volume scattering function (VSF) of water. It utilizes a collimated and depolarized 532 nm illumination and a ring of fixed, narrow field-of-view radiometers to measure the angular distribution of scattered light. This instrument is designed for rapid profiling and detailed time series measurements, with calibration traceable to basic radiometric standards.

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HydroBeta: Measuring Volume Scattering Function from 10° to 170°

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  1. HydroBeta:A New Instrument For Measuring the Volume Scattering Function from 10° to 170° In Situ David R. Dana & Robert A. Maffione Hydro-Optics, Biology, & Instrumentation Laboratories 55 Penny Lane Tel: (831) 768-0680 Watsonville, CA 95076 Fax: (831) 768-0681 Email: dana@hobilabs.com Web: www.hobilabs.com

  2. Volume Scattering Function (VSF) The VSF describes the angular distribution of scattered light exiting an illuminated volume of water. Incident beam Transmitted beam F0 Scattered light Scattering angle  DFs()

  3. Previous VSF Measurements Petzold, Ref. 1. See also Kullenberg, Ref 2.

  4. HydroBeta Approach • HydroBeta approximates the geometry used to define the VSF • Collimated, depolarized 532 nm illumination • Ring of fixed, narrow field of view radiometers simultaneously view the beam at different angles • Typical angles: 0 (beam transmission), 10, 15, 20, 30, 50, 70, 90, 120, 140, 160, and 170 degrees • Absolute VSF calibration integrated into design

  5. Sample Volumes Optical Layout

  6. Cross-section View

  7. Ready for Deployment

  8. Features • No moving parts • Synchronous detection rejects solar background • Continuously measures 12 channels simultaneously for rapid profiling and detailed time series • Calibration traceable to basic radiometric standards • Low power

  9. Calibration Background • Extension of approach used for fixed-angle HydroScat backscattering sensors (ref. 3) • W(c,z) is the receiver’s weighting function—its response to scattering at a distance z along the path of the source beam. • W(c,z) cannot be accurately modeled, but can be accurately measured.

  10. Calibration Methodology • To measure W(c,z) • Move a diffusing target throughout the sample volume. • Change target angle to accommodate various scattering angles • Use transmitting diffuser for forward angles • Assumes only that target is Lambertian from 0º to 45º, and has known reflectivity or transmittance • All geometric imperfections accounted for by measurement

  11. In the Calibration Tank

  12. Measured Weighting Functions

  13. Microspheres and Mie Theory? • Alternate approach calls for use of calibrated suspension of microspheres as calibration standard, with VSF calculated from Mie theory. • Mie predictions highly sensitive to particle size distribution • Particle size distribution difficult to measure and maintain • Still requires knowledge of sensor weighting function • From a scientific perspective, one should verify Mie calculations for a suspension of microspheres using an independently calibrated instrument

  14. Formazin Time Series

  15. Formazin VSF

  16. Maalox VSF

  17. HydroBeta in Action

  18. Monterey Bay Profile

  19. Montery Bay Profile

  20. Monterey Bay Profile

  21. Monterey Bay Profile

  22. Summary And Conclusions • Measuring the VSF is hard! • The HydroBeta approach is successful and will soon make VSF measurements routine in our deployments. • We have successfully extended the calibration methodology developed for fixed-angle sensors to the complete VSF • Preliminary measurements in lab and field demonstrate variations in scattering phase function

  23. Future Plans • Modify for multi-wavelength operation • Compare calibrated particles to compare with Mie theory • Measure VSFs of phytoplankton cultures and inorganic particles • Investigate accuracy of bbestimates made with fixed-angle sensors • Measure VSF simultaneously with other IOPs, in variety of ocean waters • Complementary instrument for measuring VSF at angles from 0.1° to 6° in design phase

  24. Acknowledgement & References • HydroBeta development supported by • Office of Naval Research • Naval Air Warfare Center • References • Petzold, T. J., 1972. “Volume scattering functions for selected ocean waters,” Scripps Institution of Oceanography Ref. No. 72-78 (Scripps Institution of Oceanography, La Jolla, Calif., 1972). • Kullenberg, G., 1974. Observed and computed scattering functions; chapter 2 in Optical Aspects of Oceanography, edited by N.G. Jerlov and E.S. Nielsen, Academic Press, NY, 25-49. • Maffione, R.A., and D.R. Dana, 1997. Instruments and methods for measuring the backward-scattering coefficient of ocean waters, Appl. Opt., 36, 6057-6067.

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