Transcript HydroBeta: a New Instrument For Measuring the Optical

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: [email protected]
Web: www.hobilabs.com
Volume Scattering Function
(VSF)
The VSF describes the angular distribution of scattered
light exiting an illuminated volume of water.
0
Incident
beam
Scattered light
Ds()
Transmitted
beam
Scattering
angle 
   
   
EV
2
Previous VSF Measurements
1000
1
100
0.1
10
1
0.01
0.1
0.01
0.001
0.001
0.0001
0.0001
0.1
1
10
100
Petzold, Ref. 1. See also Kullenberg, Ref 2.
40
80
120
160
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
Optical Layout
Sample Volumes
10 Degrees
90°
70°
90 Degrees
140 Degrees
170 Degrees
15°
LASER
TRANSMISSION
10°
170°
20°
160°
30°
140°
SUPPORT
RING
120°
50°
Cross-section View
LASER
POSITIONER
LASER DRIVER
LASER
MAIN ELECTRONICS
HOUSING
SUPPORT
STRUT
BEAM
EXPANDER
CALIBRATION FIXTURE (NOT
DEPLOYED IN SITU)
REFERENCE
DETECTOR
SUPPORT RING
LEAD SCREW
DRIVES TARGET
PREAMPLIFIER
CIRCUIT
MIRROR
APERTURE CALIBRATION
TARGET
DEPOLARIZER
SPECTRAL FILTER
LENS
BAFFLE
DETECTOR
Ready for
Deployment
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
Calibration Background
• Extension of approach used for fixed-angle HydroScat
backscattering sensors (ref. 3)

   W (c, z )dz
   0      W (c , z )dz   0  *
0
 
 

0

*

 0  W (c , z )dz
0
• 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.
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
In the Calibration Tank
Measured Weighting Functions
1.0
NormalizedW(z)
0.8
0.6
0.4
90° 160°
0.2
170°
30° 10°
0.0
5
10
15
Distance from Laser (cm)
20
25
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
Formazin Time Series
1
10
15
0.1
VSF (/m/sr)
20
30
0.01
50
140
70
155
165
0.001
120
90
0.0001
0
2
4
6
Formazin Concentration (arbitrary)
8
Formazin VSF
1
10
6
4
2
Normalized VSF
VSF (/m/sr)
0.1
0.01
1
6
4
2
0.1
6
0.001
4
2
0.0001
40
80
120
Angle (degrees)
160
0.01
40
80
120
Angle (degrees)
160
Maalox VSF
1
10
Normalized VSF
VSF (/m/sr)
0.1
0.01
1
0.1
0.001
0.01
0.0001
40
80
120
Angle (degrees)
160
40
80
120
Angle (degrees)
160
HydroBeta in Action
Monterey Bay Profile
0
120
90
70
50
30
15
20
10
140
10
155
Depth (m)
165
20
30
40
0.0001
0.001
0.01
VSF (/m/sr)
0.1
1
Montery Bay Profile
1
Normalized VSF
VSF (/m/sr)
0.1
0.01
Sur face
2m
4m
6m
8m
10m
12m
14m
16m
1
0.1
0.001
0.01
0.0001
40
80
120
Angle (degrees)
160
40
80
120
Angle (degrees)
160
Monterey Bay Profile
0
165
2
120
155
30
50
Depth (m)
4
140
70
15
20
10
6
8
10
90
12
14
16
0.0001
0.001
0.01
VSF (/m/sr)
0.1
1
Monterey Bay Profile
1
Normalized VSF
VSF (/m/sr)
0.1
0.01
Sur face
6m
12m
18m
24m
30m
36m
42m
1
0.1
0.001
0.01
0.0001
40
80
120
Angle (degrees)
160
40
80
120
Angle (degrees)
160
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
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 bb estimates made with fixedangle 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
Acknowledgement &
References
•
•
HydroBeta development supported by
– Office of Naval Research
– Naval Air Warfare Center
References
1. 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).
2. 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.
3. 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.