U.S. patent application number 12/133949 was filed with the patent office on 2009-12-10 for optical reference, and a method of using same.
This patent application is currently assigned to RAYTHEON COMPANY. Invention is credited to Douglas J. Brown, Geoffrey G. Harris, Daniel B. Mitchell.
Application Number | 20090302206 12/133949 |
Document ID | / |
Family ID | 41350882 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090302206 |
Kind Code |
A1 |
Harris; Geoffrey G. ; et
al. |
December 10, 2009 |
OPTICAL REFERENCE, AND A METHOD OF USING SAME
Abstract
A detector calibration reference is disposed along a path of
travel for radiation that extends from a radiation source to a
radiation detector. The detector calibration reference has mutually
exclusive first and second portions that are offset in a direction
transverse to the path of travel, the first portion being
substantially opaque to radiation from the source, and the second
portion being substantially transmissive to radiation from the
source. The detector calibration reference is moved relative to the
path of travel in a manner so that the first and second portions
become successively aligned with the path of travel.
Inventors: |
Harris; Geoffrey G.;
(Midland, CA) ; Mitchell; Daniel B.; (Port
McNicoll, CA) ; Brown; Douglas J.; (Midland,
CA) |
Correspondence
Address: |
HAYNES AND BOONE, LLP;IP Section
2323 Victory Avenue, Suite 700
Dallas
TX
75219
US
|
Assignee: |
RAYTHEON COMPANY
Waltham
MA
|
Family ID: |
41350882 |
Appl. No.: |
12/133949 |
Filed: |
June 5, 2008 |
Current U.S.
Class: |
250/252.1 ;
250/393 |
Current CPC
Class: |
G01J 1/04 20130101; G01J
1/08 20130101; G01J 1/0418 20130101 |
Class at
Publication: |
250/252.1 ;
250/393 |
International
Class: |
G12B 13/00 20060101
G12B013/00; G01J 1/42 20060101 G01J001/42 |
Claims
1. An apparatus comprising: a source that emits radiation, the
radiation traveling away from said source along a path of travel; a
radiation detector disposed along said path of travel at a location
spaced optically from said source; a detector calibration reference
disposed along said path of travel at a location optically between
said source and said detector, said detector calibration reference
having mutually exclusive first and second portions that are offset
in a direction transverse to said path of travel, said first
portion being substantially opaque to radiation from said source,
and said second portion being substantially transmissive to
radiation from said source; and support structure supporting said
detector calibration reference for movement relative to said path
of travel in a manner so that said first and second portions become
successively aligned with said path of travel; wherein said
detector calibration reference includes a member made of a material
that is opaque to radiation from said source, said second portion
defining an opening through said member, and said first portion
being a part of said member that is free of an opening through said
member, said opening defined by said second portion having opposite
side edges that each extend transversely to a direction of movement
of said second portion; and wherein said member includes
anti-reflection structure adjacent each said side edge, said
anti-reflection structure including said member tapering in
thickness toward each said side edge.
2. An apparatus according to claim 1, wherein said support
structure supports said detector calibration reference for rotation
about an axis, said first and second portions being successively
aligned with said path of travel in a cyclic manner, and said side
edges of said opening defined by said second portion each extending
substantially radially with respect to said axis.
3. An apparatus according to claim 2, wherein said support
structure rotates said detector calibration reference at a speed
that causes a beam of radiation from said source to be interrupted
at a frequency having a period significantly shorter than a
response time associated with said detector.
4. (canceled)
5. (canceled)
6. (canceled)
7. An apparatus according to claim 1, wherein said detector
calibration reference has third and fourth portions that are
mutually exclusive with respect to each other and with respect to
said first and second portions, said third portion being
substantially opaque to radiation from said source, and said fourth
portion being substantially transmissive to radiation from said
source, movement of said detector calibration reference by said
support structure causing said first, second, third and fourth
portions to become successively aligned with said path of travel in
a cyclic manner, said fourth portion defining an opening through
said member, and said third portion being a part of said member
that is free of an opening through said member; said opening
defined by said fourth portion having opposite side edges that each
extend transversely to a direction of movement of said fourth
portion, and said anti-reflection structure including said member
tapering in thickness toward each said side edge of said opening
defined by said fourth portion.
8. (canceled)
9. An apparatus according to claim 7, wherein said support
structure supports said detector calibration reference for rotation
about an axis, and wherein said side edges of said openings defined
by said second and fourth portions each extend substantially
radially with respect to said axis.
10. (canceled)
11. An apparatus according to claim 1, wherein said detector
calibration reference is configured to permit variation of an
effective size, in said direction of movement of said second
portion, of said opening defined by said second portion.
12. An apparatus comprising: a source that emits radiation, the
radiation traveling away from said source along a path of travel; a
radiation detector disposed along said path of travel at a location
spaced optically from said source; a detector calibration reference
disposed along said path of travel at a location optically between
said source and said detector, said detector calibration reference
having mutually exclusive first and second portions that are offset
in a direction transverse to said path of travel, said first
portion being substantially opaque to radiation from said source,
and said second portion being substantially transmissive to
radiation from said source; and support structure supporting said
detector calibration reference for movement relative to said path
of travel in a manner so that said first and second portions become
successively aligned with said path of travel; wherein said
detector calibration reference is configured to permit variation of
an effective size in said direction of said second portion, and
includes: a first member having mutually exclusive first and second
sections, said first section being substantially opaque to
radiation from said source, said second section being substantially
transmissive to radiation from said source, and said second section
being larger than said second portion of said detector calibration
reference; a second member made of a material substantially opaque
to radiation from said source, and supported for movement relative
to said first member between two positions in which said second
member provides different degrees of obstruction to radiation
travel through said second section, wherein at any given point in
time, said second portion is the portion of said second section
currently unobstructed by said second member; and securing
structure for releasably physically securing said second member to
said first member so that said second member is held in a selected
position with respect to said first member.
13. A method comprising: emitting radiation from a source, the
radiation traveling away from said source along a path of travel;
detecting radiation with a radiation detector disposed along said
path of travel at a location spaced optically from said source;
positioning a detector calibration reference along said path of
travel at a location optically between said source and said
detector, said detector calibration reference having mutually
exclusive first and second portions that are offset in a direction
transverse to said path of travel, said first portion being
substantially opaque to radiation from said source, and said second
portion being substantially transmissive to radiation from said
source; moving said detector calibration reference relative to said
path of travel in a manner so that said first and second portions
become successively aligned with said path of travel; and
configuring said detector calibration reference to include a member
made of a material that is opaque to radiation from said source,
said second portion defining an opening through said member, and
said first portion being a part of said member that is free of an
opening through said member, said opening defined by said second
portion having opposite side edges that each extend transversely to
a direction of movement of said second portion, and said member
having adjacent each said side edge anti-reflection structure that
includes said member tapering in thickness toward each said side
edge.
14. A method according to claim 13, wherein said moving includes
rotating said detector calibration reference about an axis, said
first and second portions being successively aligned with said path
of travel in a cyclic manner.
15. A method according to claim 14, wherein said rotating is
carried out at a speed that causes a beam of radiation from said
source to be interrupted at a frequency having a period
significantly shorter than a response time associated with said
detector.
16. (canceled)
17. A method according to claim 14, including configuring said
detector calibration reference to have third and fourth portions
that are mutually exclusive with respect to each other and with
respect to said first and second portions, said third portion being
substantially opaque to radiation from said source, and said fourth
portion being substantially transmissive to radiation from said
source, said fourth portion defining an opening through said
member, and said third portion being a part of said member that is
free of an opening through said member, said opening defined by
said fourth portion having opposite side edges that each extend
transversely to a direction of movement of said fourth portion, and
said anti-reflection structure including said member tapering in
thickness toward each said side edge of said opening defined by
said fourth portion; and wherein said rotating is carried out so
that said first, second, third and fourth portions become
successively aligned with said path of travel in a cyclic
manner.
18. (canceled)
19. A method according to claim 13, including varying an effective
size, in said direction of movement of said second portion, of said
opening defined by said second portion.
20. A method according to claim 13, including measuring a level of
radiation detected by said detector while carrying out said moving
of said detector calibration reference.
Description
FIELD OF THE INVENTION
[0001] This invention relates in general to calibration of optical
measurement systems and, more particularly, to optical references
for calibration, and calibration techniques that use optical
references.
BACKGROUND
[0002] Optical systems have been developed that are used to make
optical measurements. For example, a spectrophotometer is an
optical system than can be used to measure the level of
transmission or absorption of a sample material with respect to a
number of different wavelengths of radiation. A spectrophotometer
has a radiation source that transmits radiation along a path of
travel to a radiation detector. During operational use, the sample
under test is positioned optically between the source and the
detector, along the path of travel. Radiation from the source that
is traveling along the path of travel must pass through the sample,
and the detector measures the intensity of received radiation,
which represents the amount of radiation that is able to pass
through the sample. The accuracy of optical measurements provided
by such a system depends on the accuracy of the calibration of the
system.
[0003] It is relatively simple to calibrate a spectrophometer for a
transmissivity of 0% and/or a transmissivity of 100%. In
particular, it is easy to completely block the radiation beam, or
to leave it completely unblocked. However, radiation detectors are
typically nonlinear, and in fact there may be differences in the
nonlinearity of equivalent detectors that in theory should be
identical. Consequently, calibrating for only 0% and/or 100% is not
sufficient. It is desirable to perform calibration for one or more
different levels of transmissivity that are between 0% and 100%.
This can improve the accuracy of the calibration, for example by an
average of a factor of ten.
[0004] A related consideration is that radiation detectors are not
always spatially uniform. For example radiation impinging on one
portion of the detector may produce a different measurement than if
that same radiation were to impinge on a different portion of the
same detector.
[0005] To calibrate for a level of transmissivity between 0% and
100%, a traditional approach is to insert a stationary optical
reference (or several successive stationary references) between the
source and detector. Each such optical reference has a known
transmissivity. One known type of optical reference is a filter
with a known transmissivity, typically a neutral density filter.
However, filters of this type work only for particular wavelength
ranges. Further, materials in the filter may gradually deteriorate
and change performance, due to handling, exposure and/or aging.
Care must be taken to avoid abrading, scratching or otherwise
altering the filter. Moreover, contaminates from the air can
accumulate on the filter, altering performance. Cleaning the
surface of the filter to remove contaminates may alter the
performance of the filter.
[0006] A different type of known optical reference is made from a
material that is well characterized. For example, the optical
reference may be a piece of calcium fluoride (CaF.sub.2). This type
of reference can be more stable than a neutral density filter, but
is still subject to some of the same problems. Further, only a
limited selection of transmissivity levels may be available. For
example, in the visible spectrum, there are very few materials
having a transmissivity in the 0% to 70% range.
[0007] Thus, although existing optical references and calibration
techniques have been generally adequate for their intended
purposes, they have not been satisfactory in all respects. For
example, existing optical references used for calibration are not
always durable, stable and highly accurate, and cannot always be
obtained for every desired level of transmissivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A better understanding of the present invention will be
realized from the detailed description that follows, taken in
conjunction with the accompanying drawing, in which:
[0009] FIG. 1 is diagrammatic view of an apparatus that is a
spectrophotometer embodying aspects of the invention, and that
includes a detector calibration reference.
[0010] FIG. 2 is a diagrammatic sectional view, taken along the
section line 2-2 in FIG. 1.
[0011] FIG. 3 is a diagrammatic fragmentary sectional view, taken
along the section line 3-3 in FIG. 2.
[0012] FIG. 4 is a diagrammatic fragmentary sectional view similar
to FIG. 3, but showing part of a detector calibration reference
that is an alternative embodiment of the detector calibration
reference in the embodiment of FIGS. 1-3.
[0013] FIG. 5 is a diagrammatic exploded perspective view of a
detector calibration reference that is an alternative embodiment
of, and can be substituted for, the detector calibration reference
in the embodiment of FIGS. 1-3.
DETAILED DESCRIPTION
[0014] FIG. 1 is a diagrammatic view of an apparatus that is a
spectrophotometer 10, and that embodies aspects of the invention.
The spectrophotometer 10 includes a base 12. A radiation source 16
of a known type is fixedly supported on the base 12, and emits a
beam of radiation that propagates along a path of travel 18. The
beam includes radiation having a range of different
wavelengths.
[0015] A radiation detector 26 of a known type is fixedly supported
on the base 12, at a location that is spaced optically from the
source 16, and that is at an end of the path of travel 18 remote
from the source 16. A control unit 28 controls the source 16, and
receives signals from the detector 26.
[0016] A support 31 is fixedly provided on the base 12. During
normal operation, a sample 33 can be removably and stationarily
supported on the support 31. The sample 33 is shown in broken lines
in FIG. 1, because the focus of the present discussion is
calibration of the spectrophotometer 10, and the sample 33 is not
present during calibration. During normal operation, radiation from
the source 16 propagates along the path of travel 18 to the sample
33. A portion of that radiation will be absorbed and/or reflected
by the sample. The rest of the radiation will pass through the
sample 33, and continue along the path of travel 18 to the detector
26. For each of a number of different wavelengths, the detector 26
measures the amount of radiation at that wavelength arriving at the
detector, which represents the level of transmissivity of the
sample 33 for that particular wavelength.
[0017] In order to ensure that measurements taken with the
spectrophotometer 10 are accurate, the spectrophotometer must be
periodically calibrated in relation to a known reference. It is
relatively straightforward to calibrate for transmissivity levels
of 0% and 100%. For 100%, radiation is allowed to travel from the
source 16 along the path of travel 18 to the detector 26, without
encountering or passing through any physical structure. For 0%, the
source 16 can be turned off, or a not-illustrated part that is
completely non-transmissive can be provided along the path of
travel, for example in place of the sample 33. But it is desirable
to calibrate for more than just a transmissivity of 0% and/or a
transmissivity of 100%. This is because the detector 26 is
nonlinear, and in fact the nonlinearity may differ from one
detector 26 to another detector that in theory should be identical
to the detector 26. As explained earlier, the traditional
calibration approaches for transmissivities between 0% and 100%
have been adequate for their intended purposes, but have not been
completely satisfactory. The spectrophotometer 10 therefore
includes some additional structure that is provided for the purpose
of calibration.
[0018] In more detail, a motor 51 of a known type is fixedly
supported on the base 12. In the disclosed embodiment, the motor 51
is a stepper motor, but it could alternatively be any other
suitable type of motor. The motor is controlled by the control unit
28. The motor 51 has a shaft 52 that rotates about an axis 53. The
axis 53 extends approximately parallel to the path of travel 18. A
detector calibration reference 61 is fixedly mounted on the shaft
52, for rotation therewith. FIG. 2 is a diagrammatic sectional view
of the shaft 52 and the calibration reference 61, taken along the
section line 2-2 in FIG. 1.
[0019] As discussed above, the axis 53 in the disclosed embodiment
extends approximately parallel to the path of travel 18. however,
it would alternatively be possible for the axis 53 to extend at an
angle to the path of travel 18. For example, the detector 26 may
emit a small amount of heat, and where the detector 26 is used to
measure infrared radiation, it is desirable that the calibration
reference 61 not take heat emitted by the detector 26 and reflect
that heat directly back to the detector 26. If the axis 53 is
oriented at an angle to the path of travel 18, so that side
surfaces of the calibration reference 61 are not perpendicular to
the path of travel 18, then the calibration reference 61 will
reflect heat from the detector 26 in a direction other then
directly back to the detector 26.
[0020] In the disclosed embodiment, the calibration reference 61 is
made of a material that fully blocks radiation from the source 16.
In the disclosed embodiment, the calibration reference 61 is made
from a material that is non-transmissive to radiation (0%
transmissive), and in particular is made from a metal such as
steel. However, it could alternatively be made from any other
suitable material. As evident from FIGS. 1 and 2, the calibration
reference 61 is a platelike circular disk. The calibration
reference 61 has two openings 71 and 72 extending axially
therethrough, on diametrically opposite sides of the shaft 52. In
the disclosed embodiment, the calibration reference 61 has two
openings 71 and 72. However, it would alternatively be possible to
have only one opening, or to have more than two openings. In FIG.
2, the opening 72 has edges 76 and 77 on opposite sides thereof,
and the edges 76 and 77 each extend radially with respect to the
shaft 52. In addition, the opening 72 has inner and outer edges 78
and 79, each of which is an arc concentric to the shaft 52. The
distance between the edges 78 and 79 is greater than the width of
the beam of radiation produced by the source 16. The opening 71 has
a configuration that is identical to that of opening 72, and the
opening 71 is therefore not separately described here in
detail.
[0021] FIG. 3 is a diagrammatic fragmentary sectional view taken
along the section line 3-3 in FIG. 2. As shown in FIG. 3, an
optional anti-reflection coating of a known type is provided on the
edges of the opening 72, and on adjacent portions of the
calibration reference 61. For simplicity and clarity, the coating
86 has been omitted in FIGS. 1 and 2. The coating 86 is made of a
known material, and a similar coating would be provided in the
region of the opening 71. In fact, the entire calibration reference
61 could be coated. During calibration of the system 10 of FIG. 1,
the coating 86 prevents the edges 76 and 77 of the openings from
reflecting light into the detector 26.
[0022] FIG. 4 is a diagrammatic fragmentary sectional view similar
to FIG. 3, but showing part of a detector calibration reference 161
that is an alternative embodiment of the detector calibration
reference 61 of FIGS. 1-3. The calibration reference 161 is
generally identical to the calibration reference 61, except for
differences that are discussed below. The calibration reference 161
has an opening 172 that is generally equivalent to the opening 72
except that, adjacent each of the radially extending edges 176 and
177, the calibration reference 161 tapers in thickness in a
direction toward the opening 172. The edges 176 and 177 each have a
shape that is referred to figuratively as a knife edge, although of
course neither edge is actually as sharp as a knife. The tapering
thickness adjacent these knife edges is an alternative technique
for minimizing undesired reflections from the regions adjacent the
edges 176 and 177.
[0023] With reference to FIGS. 1 and 2, during calibration the
motor 51 effects rotation of the calibration reference 61. When the
path of travel 18 is aligned with either one of the openings 71 or
72, radiation from the source 16 will travel through that opening
and reach the detector 26. When neither of the openings 71 and 72
is aligned with the path of travel 18, the opaque material of the
calibration reference 61 will completely block the radiation from
the source 16, so that none of the radiation reaches the detector
26.
[0024] With reference to FIG. 2, it can be seen that radiation from
the beam will be blocked during about 90% of the angular movement
of the calibration reference 61, and will be passing through one or
the other of openings 71 and 72 during the other 10% of angular
movement. With reference to FIGS. 1 and 2, the motor 51 rotates the
calibration reference 61 at a sufficiently high speed so that the
radiation beam is chopped or interrupted at a frequency
significantly higher than the sampling frequency of the detector
26, for example an order of magnitude higher. Stated differently,
the radiation beam is chopped or interrupted with a frequency
having a period that is much shorter than the sampling interval or
response time of the detector 26. To avoid a beating effect, the
calibration reference 61 should not be rotated at a speed that
interrupts the beam at a direct multiple of the measurement
frequency of the detector 26. But if the speed of rotation of the
calibration reference 61 is sufficiently high, the likelihood of a
beating effect becomes negligible.
[0025] Since the calibration reference 61 is rotated at relatively
high speed, the detector 26 effectively sees an average of all the
radiation passing through the rotating calibration reference 61,
rather than alternating bursts of 0% and 100% radiation. Stated
differently, the level of the average depends on the relative
circumferential lengths of the openings 71 and 72 and the solid
regions between these openings. In the case of the calibration
reference 61, approximately 90% of the radiation emitted by the
source 16 will be blocked by the calibration reference 61, while
the other 10% will pass through the openings 71 and 72, and
ultimately reach the detector 26. By altering the size of the
openings and/or the number of openings in the calibration reference
61, the calibration reference 61 can be set to provide any desired
transmissivity between 0% and 100%. At the completion of the
calibration process, the motor 51 is stopped in a position where
the shaft 52 is stationary, and holds the calibration reference 61
in a position where radiation from the source 16 passes through one
of the two openings 71 and 72, without contacting any portion of
the calibration reference 61. Alternatively, the calibration
reference 61 could be removed from the shaft 52.
[0026] The calibration reference 61 shown in FIGS. 1-3 provides an
optical reference for a selected but fixed level of transmissivity,
such as 10%. In order to provide a different level of
transmissivity, the calibration reference 61 would be detached from
the shaft 52 of the motor 51, and replaced with a different
calibration reference that is effectively identical to the
calibration reference 61, except that it would have openings with a
configuration and/or size different from the openings 71 and
72.
[0027] FIG. 5 is a diagrammatic exploded perspective view of a
detector calibration reference 261 that is an alternative
embodiment of, and can be substituted for, the detector calibration
reference 61 of FIGS. 1-3. The calibration reference 261 includes
two circular plates 263 and 264. The plate 263 is fixedly secured
to the motor shaft 52, and the plate 264 is rotatably supported on
the shaft 52, so that it can be pivoted in relation to the plate
263. The plate 263 has two openings 271 and 272 that are generally
similar to the openings 71 and 72 in FIG. 2, except that the
openings 271 and 272 each have a circumferential length that is
significantly longer than the circumferential length of the
openings 71 and 72. The plate 264 has similar openings 273 and
274.
[0028] The plate 264 has an arcuate slot 282 that is concentric to
the axis 53 of the motor shaft 52, and that has an angular length
of approximately 90.degree.. A screw 281 has a threaded shank that
is slidably received within the slot 282, and that engages a
threaded opening 283 provided in the calibration reference 263. If
the screw 281 is tightened, the plate 264 is forced against the
plate 263, so that friction prevents relative rotation of the
plates 263 and 264. If the screw is 281 is loosened slightly, the
plate 264 can be rotated with respect to the plate 263, while the
shank of the screw slides within the slot 282. This permits
variation of the amount of overlap between the openings 271 and
273, and the amount of overlap between the openings 272 and 274.
This has the effect of varying the effective size of the openings
through the overall calibration reference 261.
[0029] Not-illustrated indicia can be provided along the
circumferential edges of the two plates 263 and 264. The indicia on
one plate can be selectively aligned with indicia on the other
plate to identify relative rotational positions of the plates 263
and 264 that would, for example, provide 5% transmissivity, 10%
transmissivity, 15% percent transmissivity, and so forth. After the
plates have been positioned so as to provide a desired level of
transmissivity, the screw 281 can be tightened in order to
releasably hold the two plates in that position.
[0030] The disclosed calibration references each limit the beam of
radiation mechanically, such that calibration is not based on a
sample that is referenced to a measurement previously made by a
different optical device. The disclosed calibration references can
be manufactured to great accuracy, thereby providing much more
accurate reference values. Further, The disclosed calibration
references can be readily manufactured to provide any desired level
of transmissivity from 1% to 99%. In addition, the disclosed
calibration references are not limited to particular wavelength
ranges, but can be used for virtually any wavelength ranges of
interest. Also, the disclosed calibration references are each made
of metal, and are thus more durable than existing references.
Scratches and/or contamination do not affect the performance of the
disclosed calibration references, and the disclosed calibration
references are not affected by temperature variations. Although the
disclosed calibration references are discussed in association with
a spectrophotometer, they can alternatively be used for calibrating
other types of optical instruments.
[0031] Although selected embodiments have been illustrated and
described in detail, it should be understood that a variety of
substitutions and alterations are possible without departing from
the spirit and scope of the present invention, as defined by the
claims that follow.
* * * * *