U.S. patent number RE41,682 [Application Number 12/134,855] was granted by the patent office on 2010-09-14 for apparatus and method for direct measurement of absorption and scattering coefficients in situ.
This patent grant is currently assigned to The Texas A&M University System. Invention is credited to Edward S. Fry, Deric J. Gray, George S. Kattawar, Zheng Lu, Xianzhen Zhao.
United States Patent |
RE41,682 |
Fry , et al. |
September 14, 2010 |
Apparatus and method for direct measurement of absorption and
scattering coefficients in situ
Abstract
An apparatus for measuring an absorption coefficient includes a
first diffusive material, a second diffusive material inside the
first diffusive material separated from the first diffusive
material by a cavity, and a transparent material proximate to an
inner surface of the second diffusive material that holds an
absorptive material. First and second light detectors measure light
intensities in the .[.first and second diffusive materials.].
.Iadd.cavity and the transparent material .Iaddend.respectively. An
absorption coefficient for the absorptive material may be
determined based on the first and second light intensities measured
when the cavity is illuminated by a light source.
Inventors: |
Fry; Edward S. (College
Station, TX), Kattawar; George S. (College Station, TX),
Gray; Deric J. (Nacogdoches, TX), Zhao; Xianzhen
(College Station, TX), Lu; Zheng (College Station, TX) |
Assignee: |
The Texas A&M University
System (College Station, TX)
|
Family
ID: |
32326413 |
Appl.
No.: |
12/134,855 |
Filed: |
June 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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60426733 |
Nov 15, 2002 |
|
|
|
Reissue of: |
10715336 |
Nov 17, 2003 |
07057730 |
Jun 6, 2006 |
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Current U.S.
Class: |
356/432;
356/440 |
Current CPC
Class: |
G01N
21/532 (20130101); G01N 21/03 (20130101); G01N
21/51 (20130101); G01N 2201/0634 (20130101); G01N
2201/0614 (20130101) |
Current International
Class: |
G01N
21/00 (20060101) |
Field of
Search: |
;356/336-342,432-446
;250/573-576 ;73/170.33,170.34 ;436/805,356,250,73,436 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
E Friedman, et al.; Absorption Coefficient Instrument for Turbid
Natural Waters; Applied Optics; vol. 19, No. 10; XP-002277936; pp.
1688-1693, May 15, 1980. cited by other .
Robert A. Leathers, et al.; Analysis of a Point-Source
Integrating-Cavity Absorption Meter; Applied Optics; vol. 39, No.
33; XP-002277937; pp. 6118-6127, Nov. 20, 2000. cited by other
.
PCT Notification of Transmittal of the International Search Report
or the Declaration for International Application No.
PCT/US03/36668; 8 pages, May 13, 2004. cited by other.
|
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Ton; Tri T
Attorney, Agent or Firm: Baker Botts L.L.P.
Parent Case Text
RELATED APPLICATIONS
This application claims the benefit under 35 U.S.C. section 119(e)
of the priority of U.S. Provisional Application No. 60/426,733,
filed Nov. 15, 2002, entitled "An In Situ Device, System and Method
to Directly Measure Both the Absorption and Scattering Coefficients
of Natural Waters."
Claims
What is claimed is:
1. An apparatus, comprising: a first diffusive material having a
generally cylindrical shape with a length greater than twice its
diameter; a second diffusive material inside the first diffusive
material separated from the first diffusive material by a cavity; a
transparent material proximate an inner surface of the second
diffusive material operable to hold an absorptive material, the
transparent material having an inner surface that is curved along
its length in a substantially sinusoidal pattern; first light
detectors, comprising: first optical fibers operable to carry light
from the .[.first diffusive material.]. .Iadd.cavity.Iaddend.; and
first photodetectors operable to measure a first light intensity
for the light carried from the .[.first diffusive material.].
.Iadd.cavity.Iaddend.; second light detectors, comprising: second
optical fibers operable to carry light from the .[.second
diffusive.]. .Iadd.transparent .Iaddend.material; and second
photodetectors operable to measure a second light intensity for the
light carried from the .[.second diffusive.]. .Iadd.transparent
.Iaddend.material; first light sources operable to illuminate the
cavity; a second light source operable to illuminate the absorptive
material with a collimated beam having an incident light intensity;
and a processor operable to: determine an absorption coefficient
for the absorptive material based on a first diffused light
intensity measured by the first light detectors in the .[.first
diffusive material.]. .Iadd.cavity .Iaddend.and a second diffused
light intensity measured by the second light detectors in the
.[.second diffusive.]. .Iadd.transparent .Iaddend.material, wherein
the first and second diffused light intensities are measured while
the cavity is illuminated by the first light sources; and determine
a scattering coefficient for the absorptive material based on a
third diffused light intensity measured by the second light
detectors in the .[.second diffusive medium.]. .Iadd.transparent
material.Iaddend., wherein the third light intensity is measured
while the absorptive material is illuminated by the collimated beam
from the second light source.
2. A method for measuring an absorption coefficient, comprising:
introducing light into a cavity between a first diffusive material
and a second diffusive material, wherein at least some of the light
in the cavity passes into the first and second diffusive material
and at least some of the light passing into the second diffusive
material passes through a transparent material proximate to the
second diffusive material and into an absorptive material;
measuring a first intensity comprising the intensity of the light
in the .[.first diffusive material.]. .Iadd.cavity.Iaddend.;
measuring a second intensity comprising the intensity of the light
in the .[.second diffusive.]. .Iadd.transparent .Iaddend.material;
and determining an absorption coefficient for the absorptive
material based on the first and second intensity measurements.
3. The method of claim 2, wherein the first and second diffusive
materials are Spectralon cavities.
4. The method of claim 2, wherein the first and second diffusive
materials comprise concentric cylindrical shells; and the cavity
comprises a cylindrical space between the first and second
diffusive materials.
5. The method of claim 2, wherein the transparent material
comprises a quartz tube.
6. The method of claim 2, wherein the transparent material
comprises an inner surface that is curved along the length of the
transparent material.
7. The method of claim 6, wherein the curvation of the inner
surface is substantially sinusoidal.
8. The method of claim 2, further comprising: stopping the
introduction of light into the cavity; after stopping the
introduction of light into the cavity, illuminating the absorptive
material using a collimated beam, wherein scattered light from the
absorptive material passes through the transparent material to the
second diffusive material and light is not introduced into the
cavity while the collimated beam is illuminating the absorptive
material; after illuminating the absorptive material with the
collimated beam, measuring a new value of the second light
intensity in the .[.second diffusive.]. .Iadd.transparent
.Iaddend.material; determining an incident light intensity for the
collimated beam; and determining a scattering coefficient of the
absorptive material based on the incident light intensity and the
new value of the second light intensity.
9. An apparatus, comprising: a first diffusive material; a second
diffusive material inside the first diffusive material separated
from the first diffusive material by a cavity; a transparent
material proximate to an inner surface of the second diffusive
material operable to hold an absorptive material; first light
detectors operable to measure a first light intensity in the first
diffusive material; and second light detectors operable to measure
a second light intensity in the second diffusive material, wherein
an absorption coefficient for the absorptive material in the
transparent material .[.may be.]. .Iadd.is .Iaddend.determined
based on the first and second light intensities measured when the
cavity is illuminated by a light source.
10. The apparatus of claim 9, further comprising a processor
operable to determine the absorption coefficient of the absorptive
materials based on the first and second light intensities.
11. The apparatus of claim 9, wherein the first and second
diffusive materials are Spectralon cavities.
12. The apparatus of claim 9, .Iadd.wherein: the .Iaddend.first and
second diffusive materials comprise concentric cylindrical shells;
and the cavity comprises a cylindrical space between the first and
second diffusive materials.
13. The apparatus of claim 9, wherein the transparent material
comprises a quartz tube.
14. The apparatus of claim 9, wherein the transparent material
comprises an inner surface that is curved.
15. The apparatus of claim 14, wherein the curvation of the inner
surface is substantially sinusoidal.
16. The apparatus of claim 9, further comprising an additional
light source operable to illuminate the absorptive material with a
collimated beam having an incident light intensity, wherein a
scattering coefficient .[.may be.]. .Iadd.is .Iaddend.determined
based on the second light intensity then the absorptive material is
illuminated by the collimated beam and the incident light
intensity.
17. A method for measuring a scattering coefficient, comprising:
illuminating an absorptive material with a collimated beam having
an incident light intensity, wherein the absorptive material is
within a transparent material substantially surrounded by a
diffusive material; measuring a diffused light intensity in the
.[.diffusive.]. .Iadd.transparent .Iaddend.material; and
determining a scattering coefficient for the absorptive material
based on diffused light intensity and the incident light
intensity.
18. The method of claim 17, wherein the diffusive material is a
Spectralon cavity.
19. The method of claim 17, wherein the diffusive material
comprises a cylindrical shell.
20. The method of claim 17, wherein the transparent material
comprises a quartz tube.
21. The method of claim 17, wherein the diffusive material is
enclosed by a reflective material having a reflective inner
surface.
22. The method of claim 17, wherein the diffusive material is a
first diffusive material, and the first diffusive material is
substantially surrounded by a second diffusive material separated
from the first diffusive material by a cavity.
23. The method of claim 22, further comprising: illuminating the
cavity; measuring a first diffused light intensity in the .[.first
diffusive.]. .Iadd.transparent .Iaddend.material; measuring a
second diffused light intensity in the .[.second diffusive
material.]. .Iadd.cavity.Iaddend.; and determining an absorption
coefficient for the absorptive material based on the first and
second diffused light intensities.
24. An apparatus, comprising: a transparent material operable to
hold an absorptive material; a diffusive material substantially
surrounding the transparent material; light detectors operable to
detect a diffused light intensity in the diffusive material; a
light source operable to illuminate an absorptive material with a
collimated beam having an incident light intensity, wherein a
scattering coefficient for the absorptive material .[.may be.].
.Iadd.is .Iaddend.determined based on the incident light intensity
and the diffused light intensity.
25. The apparatus of claim 24, further comprising a processor
operable to determine the scattering coefficient for the absorptive
material based on the incident light intensity and the diffused
light intensity.
26. The apparatus of claim 24, wherein the diffusive material is a
Spectralon cavity.
27. The apparatus of claim 24, wherein the diffusive material
comprises a cylindrical shell.
28. The apparatus of claim 24, wherein the transparent material
comprises a quartz tube.
29. The apparatus of claim 24, wherein the diffusive material is
enclosed by a reflective material having a reflective inner
surface.
30. The apparatus of claim 24, wherein the diffusive material is a
first diffusive material, and the first diffusive material is
substantially surrounded by a second diffusive material separated
from the first diffusive material by a cavity.
31. The apparatus of claim 30, wherein: the light source is a first
light source, the light detectors are first light detectors, and
the diffused light intensity is a first diffused light intensity;
the apparatus further comprises: a second light source operable to
illuminate the cavity; second light detectors operable to measure a
second diffused light intensity in the second diffusive material;
and an absorption coefficient for the absorptive material .[.may
be.]. .Iadd.is .Iaddend.determined based on the first and second
diffused light intensities when the cavity is illuminated.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to measuring absorption and
scattering coefficients of materials, and more particularly to an
apparatus and method for direct measurement of absorption and
scattering coefficients in situ.
BACKGROUND OF THE INVENTION
It is often useful in analyzing materials to characterize the
response of those materials to incoming light (electromagnetic
radiation). Two known coefficients for measuring these properties
are absorption and scattering coefficients. An absorption
coefficient can be expressed as the property of a medium that
describes the amount of absorption of radiation per unit path
length within the medium. It can be interpreted as the inverse of
the mean free path that a photon will travel before being absorbed
(if the absorption coefficient does not vary along the path). The
unit quantity for an absorption coefficient is inverse length. A
scattering coefficient can be expressed as the property of a medium
that describes the amount of scattering of radiation per unit path
length for propagation in the medium. It can be interpreted as the
inverse of the mean free path that a photon will travel before
undergoing scattering (if the scattering coefficient does not vary
along the path). The unit quantity for a scattering coefficient is
inverse length. Along with the scattering coefficient, an
absorption coefficient describes the change in radiation intensity
per unit length along the path through the medium.
Existing approaches to obtaining a scattering coefficient, b,
involve measuring a volume scattering function using an array of
detectors placed at various angles around the scattering material.
The scattering coefficient is then calculated by integrating the
volume scattering function over all angles. Traditionally, the
scattering coefficient is obtained by measuring the extinction
coefficient, c, using the relation b=c-a, where a is the absorption
coefficient. The extinction coefficient, c, is a constant that
predicts the attenuation or dissipation of light at a certain
wavelength. In pure water, light is highly absorbed in the infrared
region of the light spectrum and poorly absorbed in the blue
region. Extinction coefficients are influenced by water absorption,
suspended organic and inorganic particles, and dissolved compounds.
Thus, the visible color in a water sample is the light that is
refracted, reflected or re-emitted by substances in water because
it has not been absorbed to produce heat or chemical reactions. The
absorption coefficient, a, is also measured and, using the relation
b=c-a, the scattering coefficient, b, can be calculated. However,
sometimes this method can yield unphysical values of the scattering
coefficient, b.
As for the measurement of the absorption coefficients of light in
water, U.S. Pat. No. 5,424,840 to C. C. Moore and J. R. V. Zaneveld
describes a method that can measure the absorption of chlorophyll
for a single wavelength, and which was adapted by C. C. Moore and
J. R. V. Zaneveld to be usable for nine discrete wavelengths. The
major disadvantages of this device are: (1) it cannot measure the
scattering coefficient directly; and (2) the accuracy of the
measured absorption coefficient depends on how the scattered light
is collected and accounted for. Therefore, there remains a need for
a device, system and method for measuring more directly the above
coefficients with relative accuracy.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, an
apparatus for measuring an absorption coefficient includes a first
diffusive material, a second diffusive material inside the first
diffusive material separated from the first diffusive material by a
cavity, and a transparent material proximate to an inner surface of
the second diffusive material that holds an absorptive material.
First and second light detectors measure light intensities in the
first and second diffusive materials respectively. An absorption
coefficient for the absorptive material may be determined based on
the first and second light intensities measured when the cavity is
illuminated by a light source.
In accordance with another embodiment of the present invention, an
apparatus for measuring a scattering coefficient includes a
transparent material that holds an absorptive material, a diffusive
material that substantially surrounds the transparent material, and
light detectors that detect a diffused light intensity in the
.[.diffusive material.]. .Iadd.cavity and the transparent
material.Iaddend.. A light source illuminates the absorptive
material with a collimated beam. A scattering coefficient for the
absorptive material may be determined based on the incident light
intensity of the collimated beam and the diffused light intensity
measured by the light detectors when the absorptive material is
illuminated.
Important technical advantages of certain embodiments of the
present invention include direct measurements of absorption and
scattering coefficients. As noted above, previous techniques often
require indirect measurements of absorption and scattering
coefficients, such as by measurement of the extinction coefficient,
that can introduce errors in real-world situations. Other such
techniques involve volume integration of scattered light, which
requires mathematical assumptions that may not apply perfectly to
real-world situations. In contrast with such methods, certain
embodiments of the present invention provide direct measurement of
the scattered light intensity.
Other important technical advantages of certain embodiments of the
present invention include the use of open-ended detectors. Previous
detectors required enclosed containers in order to accurately
account for all of the light incident on the sample, because of the
need for enclosure, introducing samples into the detector proved
difficult, often requiring liquid samples to be pumped in and out
of the detector. In contrast with such methods, certain embodiments
of the present invention may accurately measure absorption and
scattering coefficients even when the detector is open-ended, thus
allowing the sample to be introduced into the container with
relative ease compared to previous methods.
Still other technical advantages of certain embodiments of the
present invention include durability under real-world use
conditions. Previous enclosed detectors often included highly
reflective surfaces that were prone to damage, which resulted in
inaccurate measurements. Certain embodiments of the present
invention protect optically sensitive components likely to be
damaged by exposure to samples, thus providing a more durable
detector for in situ use.
Other technical advantages of the present invention will be readily
apparent to one skilled in the art from the following figures,
descriptions, and claims. Moreover, while specific advantages have
been enumerated above, various embodiments may include all, some,
or none of the enumerated advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and its
advantages, reference is now made to the following description,
taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a rectangular cross-section of a cylindrical detector
according to a particular embodiment of the present invention;
FIG. 2 is a perspective view from one end of the detector of FIG.
1;
FIG. 3 is a flow chart illustrating one example of a method for
measuring an absorption coefficient; and
FIG. 4 is a flow chart illustrating one example of a method for
measuring a scattering coefficient.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION
FIG. 1 shows a rectangular cross section of a cylindrical detector
100 used for detecting absorption and/or scattering coefficients
for an absorptive material 110 within detector 100, and FIG. 2
illustrates a perspective view from the end of detector 100 taken
along lines 2--2 of FIG. 1. In the depicted embodiment, detector
100 includes an outer diffusive material 102, an inner diffusive
material 104, a transparent material 106 between absorptive
material 110 and inner diffusive material 104, light sources 114
and 116 for introducing light into detector 100, and detectors 118
and 120 used to measure light intensity levels at different
locations in detector 100. In general, detector 100 permits the
direct measurement of absorption and scattering coefficients for
adsorptive material 110.
Outer diffusive material 102 and inner diffusive material 104
comprise materials that diffuse incoming light such that the light
in the medium of diffusive materials 102 and 104 is approximately
isotropic. Thus, scattered photons or other light energy in
diffusive materials 102 and 104 contributes to the overall
intensity of light in diffusive materials 102 and 104, but does not
produce local variations in light intensity. This allows a
measurement of the overall intensity of light in .[.inner diffusive
material 102.]. .Iadd.cavity 108 .Iaddend.and .[.outer diffusive
material 104.]. .Iadd.transparent material 106 .Iaddend.without
regard to positioning sensors in order to capture particular light
rays. In particular embodiments in which detector 100 is
substantially cylindrical, it is desirable for detector 100 to be
significantly longer than its diameter in order to minimize leakage
from the ends, thus increasing the accuracy of detector 100. In a
particular embodiment, outer diffusive material 102 and inner
diffusive material 104 may be formed from highly diffusive
materials having high reflectance, such as those manufactured by
SPECTRALON, which are known in the art as "Spectralon
cavities."
Transparent material 106 forms a tube within detector 100 that
separates absorptive material 110 from inner diffusive material
104. Transparent tube 106 should be substantially transparent to
the wavelength of interest being measured for absorptive and
scattering properties. Transparent tube 106 is in contact with
diffusive material 104 allowing light to leak from inner diffusive
material 104 to absorptive material 110 and vice versa. This
protects the surface of inner diffusive material 104 from contact
with absorptive material 110, which may increase its durability and
protect inner diffusive surface 104 from damage resulting from
contact with absorptive medium 110. In the depicted embodiment,
transparent material 106 has a wavy inner surface that follows a
generally sinusoidal curvature. The purpose for this feature will
be described in greater detail below.
Cavity 108 between outer diffusive material 102 and inner diffusive
material 104 provides a space for introduction of light into
detector 100. Because cavity 108 is separated from absorptive
material 110 by inner diffusive material 104, light introduced into
cavity 108 will reach absorptive material 110 in a nearly isotropic
manner. Accordingly, the directional effects for absorption in
absorptive material 110 are diminished by the isotropy of the
illumination of absorptive material 110. Furthermore, the
separation between outside diffusive material 102 and inside
diffusive material 104 created by cavity 108 permits measurement of
light intensity both as introduced into detector 100 measured in
.[.outer diffusive material 102.]. .Iadd.cavity 108.Iaddend., and
after absorption by absorptive material 110, measured in .[.inner
diffusive material 104.]. .Iadd.transparent material 106.Iaddend..
Cavity 108 may in principle be filled with any transparent material
including air.
Absorptive material 110 represents any suitable substance to be
studied that absorbs and/or scatters light. Substances of interest
may include water with suspended particles, plastics, tissues,
fluids, or any other substance to be characterized by absorptive or
scattering properties. In a particular embodiment, detector 100 may
be used to study ocean water 110 in real world settings, such as
oceanographic vessels.
Light sources 114 represent any suitable source of illumination for
cavity 108 in order to allow measure of absorption coefficients. In
a particular embodiment, light sources 114 may be optical fibers
coupled to light sources that deliver light from the sources to
cavity 108. However, light sources 114 may include wave guides,
phosphorescent materials, filaments, or other suitable sources of
illumination. Depending on the nature of light sources 114, light
sources 114 may be isolated from outer diffusive region 102 by a
suitable coating or other barrier between light sources 114 and
diffusive material 102, so that light from light source 114 does
not increase the light intensity in outer diffusive material 102
other than by leakage from cavity 108. This prevents photons from
light source 114 that may have directional properties from
interfering with the isotropy of light in outer diffusive material
102.
Light source 116 introduces light into absorptive material 110 to
measure scattering from absorptive material 110. It is desirable
for light emitted from light source 116 to be well collimated so
that the energy from light source 116 is effectively delivered into
absorptive material 110. This allows the determination of the
amount of scattered light to be assessed accurately based on the
intensity of incoming light from light source 116. The curved inner
surface of transparent material 106 facilitates scattering
measurements by increasing the probability that light scattered at
small angles will not be returned into absorptive material 110 by
specular reflection and travel outside of the end of detector 100.
For similar reasons, it is desirable that the light from light
source 116 not impinge directly on the surface of transparent
material 106.
Light detectors 118 and 120 may include any suitable device for
measuring intensity of light at a desired wavelength. In a
particular embodiment, light detectors 118 and 120 are optical
fibers that carry light from .[.outer diffusive material 102.].
.Iadd.cavity 108 .Iaddend.and .[.inner diffusive material 104.].
.Iadd.transparent material 106 .Iaddend.to photodetectors that
measure light intensity. Photodetectors may include photodiodes,
photomultipliers, photoelectric detectors or any other suitable
form of light detection. In order to increase the accuracy of light
intensity determination, it may be desirable to isolate light
detectors 118 and 120 from particular regions of detector 100. For
example, light detector 118 may be encased in an opaque covering
such as aluminum foil in the region of outer diffusive material 102
and cavity 108 so that the measurement of light intensity is solely
in .[.inner diffusive material 104.]. .Iadd.transparent material
106.Iaddend..
Processor 122 comprises any suitable hardware of software for
processing information. In particular, processor 122 includes
electronic or other types of components for receiving information
from light detectors 118 and 120 and calculating absorption and
scattering coefficients based on that information. Processor 122
may include components for information storage (such as magnetic
memory), input devices for receiving information from detectors
and/or users, output devices for displaying or otherwise generating
an output of results, and any other appropriate component useful
for performing tasks related to the measurement of absorption and
scattering coefficients.
End caps 112 hold the components of detector 100 in a fixed
arrangement. End caps 112 may be ring shaped and may be composed of
any suitable material that can be affixed to outer diffusive
material 102 and inner diffusive material 104. End caps 112 may
also secure transparent material 106 in place as well. End caps 112
may also form part of a housing (not shown), which may be an
integral piece or formed from several pieces. Such a housing may
encase the components of detector 100 to protect them from exposure
to the elements and other hazards and potentially damaging
influences in the environment.
In general, detector 100 should be sized so as to allow adequate
optical separation between inner diffusive material 102 and outer
diffusive material 104, typically several millimeters. Furthermore,
the length of detector 100 along its longitudinal axis (the
direction of lines 2--2 in FIG. 1) should be significantly longer
than the transverse dimension of the sample space holding
absorptive material 110 in order to avoid light leakage. As an
example, for a cylindrical, meter-long detector 100, the inner
diameter of transparent material 106 could be around ten
millimeters.
In operation, detector 100 may function in one of two modes. In the
first mode, detector 100 measures the absorption coefficient of
absorptive material 110. In the second mode of operation, detector
100 measures the scattering coefficient of absorptive material
110.
For the first mode of operation, detector 100 is filled with
absorptive material 110. Light is introduced into cavity 108 by
light sources 114. Once equilibrium light state is achieved in
detector 100, the light intensity levels are measured in .[.outer
diffusive material 102.]. .Iadd.cavity 108 .Iaddend.and .[.inner
diffusive material 104.]. .Iadd.transparent material 106
.Iaddend.by light detectors 120 and 118, respectively. Based on
these measurements the absorption coefficient of absorptive
material 110 may be determined.
In the second mode of operation, light is introduced into detector
100 by light source 116. Light is scattered by absorptive material
110, passing through transparent material 106 to diffusive material
104. Diffusive material 104 diffuses scattered light so that the
intensity of light in .[.diffusive material 104.].
.Iadd.transparent material 106 .Iaddend.represents the intensity of
light scattered by absorptive material 110. This intensity is
measured by light detectors 118, and the scattering coefficient may
then appropriately determined based on the amount of light
introduced into detector by light source 116. Note that the
measurement of light intensity in .[.outer diffusive region 102.].
.Iadd.cavity 108 .Iaddend.is not necessary to determine the
scattering intensity. Accordingly, for a detector 100 that is used
to measure only scattering coefficients, detector 100 may omit
.[.outer diffusive region 102.]. .Iadd.cavity 108.Iaddend., light
sources 114, and detectors 120. In such a case, inner diffusive
material 104 may be encased in a reflective material to prevent
light from escaping or being absorbed.
The absorption and scattering coefficients may be determined by
modeling the properties of detector 100 based on the particular
materials and construction used. One example is described for an
embodiment in which diffusive materials 102 and 104 are Spectralon
cavities, transparent material 106 is a quartz tube, and absorptive
material 110 is ocean water. The Spectralon cavities can be modeled
as an ideal Lambertian emitter and reflector, which emits equal
radiance into all directions. The surface albedo of the Spectralon
is taken as 0.994 as an assumption, although other values can be
used. It is also reasonably assumed that the quartz tube neither
absorbs nor scatters the light but does produce some specular
reflection. The index of refraction of the quartz is assumed to be
1.46. The water itself has an index of refraction of 1.338. A
Henyey-Greenstein phase function, given by
.function..theta..PHI..times. .times..pi..times..times.
.times..times. .times..times. .times..theta..times. .times..theta.
##EQU00001## is used to model the directional dependence of the
scattering, as this allows easy exploration of the effect of the
shape of the volume scattering function on the recorded signal. The
cylindrical symmetry of detector 100 means that the phase function
is independent of azimuthal angle .phi.. The g-parameter is equal
to the average cosine of the scattering angle .theta.. The g=0 case
represents isotropic scattering. The g=0.97 case is chosen to
represent a "characteristic upper limit" for real ocean water, in
this case most of the scattered light travels forward. For an
absolute upper limit the value g=0.99 is chosen, in that this case
can be considered as one in which almost all the scattered light
travels forward. For most applications with ocean water, the
calculation is not particularly sensitive to variation in g-values
within an ordinary range, and accordingly, this variation can be
ignored, although the calibration curve may include variation for
g-values as well if desired. The described relationships can be
exploited to determine calibration curves for detectors 118 and
120, which in turn allow subsequent measurements of scattering
coefficients using detectors 118 and 120.
FIG. 2 illustrates a perspective view from the end of detector 100
taken along lines 2--2 in FIG. 1. From this perspective, the ring
shape of end cap 112 is clearly visible. The inside structure of
detector 100 is shown by dashed lines, which illustrate cavity 108
separating outer diffusive material 102 and inner diffusive
material 104. Transparent material 106 extends out from the inner
surface of inner diffusive material 104 so that transparent
material 106 is visible looking through the hole at the end of
detector 100. Within the space enclosed by transparent material 106
is absorptive material 110. As depicted on the sides, light sources
114 and detectors 118 and 120 (hidden from this perspective by
detector 118) extend into the sides of detector 100.
Although particular embodiments of detector 100 have been described
in detail, it should be understood that these embodiments are only
examples, and that numerous modifications and variations of the
basic principles are possible. For example, as discussed above,
detector 100 may be enclosed in a variety of housings, and may be
assembled in any suitable manner. If detector 100 does not need to
be used in measuring absorption coefficient, outer diffusive medium
102 and associated components may be left out of detector 100. In
the opposite case, in which scattering is not measured, specular
reflection from the interface between transparent material 106 and
absorptive material 110 is a much less significant concern, and
accordingly, the inner surface of transparent material 106 may be
made smooth. Other physical configurations of detector 100 may also
be used in different geometrical symmetries may be designed,
although other geometries may require more complicated placements
of detectors 118 and 120 and the calculations performed based on
resulting measurements. These and other variations should be
understood to be encompassed within the embodiments described
above.
FIG. 3 is a flow chart 200 illustrating a method for measuring an
absorption coefficient of an absorptive material 110. The first
three steps of the illustrated method are calibration steps to
determine the proper settings for detectors 118 and 120, in which
detector 100 may be filled with a reference material having known
optical properties. Cavity 108 is illuminated at step 202. At step
204, light intensity is measured in .[.inner diffusive material
104.]. .Iadd.transparent material 106 .Iaddend.and .[.outer
diffusive material 102.]. .Iadd.cavity 108.Iaddend.. Detectors 118
and 120 are calibrated according to the known absorptive value at
step 206. This step may also involve adjusting the light intensity
to determine whether the scaling of the results is appropriate to
enable determination of the accuracy of detectors 118 and 120.
To measure the absorptive qualities of absorptive material 110,
absorptive material 110 is inserted into detector 100 at step 208.
Cavity 108 is illuminated using light sources 114 at step 210.
Light intensities are measured in .[.diffusive regions 102 and
104.]. .Iadd.cavity 108 and transparent material 106 .Iaddend.using
detectors 118 and 120 at step 212. Based on the previous
calibration, and the results of the measurement, absorptive
coefficient is determined at step 214.
FIG. 4 is a flowchart 220 that illustrates an example of a method
for measuring scattering coefficient. The first three steps of the
illustrated method are calibration steps, in which detector 100 may
be filled with a reference material with known optical properties.
The inside of the tube formed by transparent material 106 is
illuminated at step 222 while detector 100 is filled with a
reference material. The light intensity in the .[.inner and outer
diffusive materials 102 and 104.]. .Iadd.cavity 108 and transparent
material 106 .Iaddend.is measured at step 224. During this step the
light intensity produced by light source 116 may be adjusted to
ensure that the proper relationship between intensity and
scattering coefficient is being observed by detectors 118 and 120.
Based on those measurements, detectors are calibrated at step
226.
To measure scattering coefficient, a light-scattering material is
introduced into detector at step 228. The light-scattering material
110 is illuminated using light source 116 at step 230. Light
intensity in .[.inner diffusive material 104.]. .Iadd.transparent
material 106 .Iaddend.is measured using detectors 118 at step 232.
The incoming light intensity from light source 116 is determined at
step 234. This light intensity may be known from the properties of
light source 116, or may be measured using photo detectors or other
suitable techniques. Based on the measured light intensity in
.[.diffusive material 104.]. .Iadd.transparent material 106
.Iaddend.and the light intensity incident on light scattering
material 110, the scattering coefficient may be determined.
Although the present invention has been described with several
embodiments, a myriad of changes, variations, alterations,
transformations, and modifications may be suggested to one skilled
in the art, and it is intended that the present invention encompass
such changes, variations, alterations, transformations, and
modifications as fall within the scope of the appended claims.
* * * * *