U.S. patent application number 09/740316 was filed with the patent office on 2002-11-21 for instrument for rapidly characterizing material reflectance properties.
Invention is credited to Davis, Keith J..
Application Number | 20020171840 09/740316 |
Document ID | / |
Family ID | 24975982 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020171840 |
Kind Code |
A1 |
Davis, Keith J. |
November 21, 2002 |
INSTRUMENT FOR RAPIDLY CHARACTERIZING MATERIAL REFLECTANCE
PROPERTIES
Abstract
A reflectometer characterizes the reflectance properties of a
test material. The reflectometer includes a radiation subsystem
that generates and directs radiation onto a test material at a
plurality of incident angles. An elliptical reflector assembly has
one or more reflectors with a first and second foci. A holder
positions the test material at the first focus of the reflectors.
One or more lenses are located within a first focal length of the
second focus of the reflectors. The lenses receive a first angular
image that is reflected by the reflector. The holder is rotatable
relative to the radiation subsystem. Stepper motors and encoders
vary the angular position of the incident angle and an azimuth
angle of the test material. A computer records an angular image for
each azimuth and incident angle to completely characterize the
reflectance properties of the test material.
Inventors: |
Davis, Keith J.; (Seattle,
WA) |
Correspondence
Address: |
Harness, Dickey & Pierce, P.L.C.
P.O. Box 828
Bloomfield Hills
MI
48303
US
|
Family ID: |
24975982 |
Appl. No.: |
09/740316 |
Filed: |
December 18, 2000 |
Current U.S.
Class: |
356/445 |
Current CPC
Class: |
G01N 21/474
20130101 |
Class at
Publication: |
356/445 |
International
Class: |
G01N 021/55 |
Claims
What is claimed is:
1. A reflectometer for characterizing reflectance properties of a
test material, comprising: a radiation subsystem that generates and
directs radiation onto a test material at a plurality of incident
angles; an elliptical reflector assembly having a first reflector
with a first and second foci; a holder that positions said test
material at said first focus of said first reflector; and a first
lens that is located within a first focal length of said second
focus of said first reflector and that receives a first angular
image that is reflected by said first reflector.
2. The reflectometer of claim 1 wherein said elliptical reflector
assembly further includes a second reflector having a third and
fourth foci, and wherein said holder positions said test material
at said third focus.
3. The reflectometer of claim 2 further comprising: a second lens
that is located within a second focal length of said fourth focus
of said second reflector and that receives a second angular image
that is reflected by said second reflector.
4. The reflectometer of claim 1 wherein said holder is rotatable
relative to said radiation subsystem.
5. The reflectometer of claim 3 wherein said first and third foci
are approximately co-located on said test material.
6. The reflectometer of claim 3 wherein said radiation subsystem
further comprises: a housing that is moveable relative to said
elliptical reflector assembly to alter said incident angle; and a
focusing mirror that is connected to said housing.
7. The reflectometer of claim 6 wherein said radiation subsystem
further comprises: a slit that controls a shape of said radiation
that illuminates said test material and that is moveable relative
to said housing to keep said shape relatively constant as said
housing moves.
8. The reflectometer of claim 7 further comprising: a cam connected
to said elliptical reflector assembly; and an arm that is biased by
said cam to move said slit.
9. The reflectometer of claim 1 further comprising: a shutter that
blocks said radiation when in a closed position and that passes
said radiation when said shutter is in an open position.
10. The reflectometer of claim 6 further comprising: a first
stepper motor that adjusts an angular position of said housing
relative to said elliptical reflector assembly to adjust an
incident angle of said radiation on said test material.
11. The reflectometer of claim 10 further comprising: a first
position encoder for generating a position signal that is related
to the angular position of said housing.
12. The reflectometer of claim 1 wherein said elliptical reflector
assembly includes a slot through which said radiation passes.
13. The reflectometer of claim 11 further comprising: a second
stepper motor that adjusts an angular position of said holder.
14. The reflectometer of claim 13 further comprising: a second
position encoder for generating a position signal related to said
angular position of said holder.
15. The reflectometer of claim 14 further comprising: a computer
that is connected to said first and second stepper motors and said
first and second position encoders; a first imaging assembly that
receives said first angular image and generates a first angular
image signal; a second imaging assembly that receives said second
angular image and generates a second angular image signal, wherein
said computer generates a first difference signal by subtracting an
ambient first image signal from said first image signal and a
second difference signal by subtracting an ambient second image
signal from said second image signal.
16. The reflectometer of claim 15 wherein said computer generates a
calibrated first product signal by multiplying said first
difference signal by a first set of calibration factors and
generates a calibrated second product signal by multiplying said
second difference signal by a second set of calibration
factors.
17. The reflectometer of claim 16 wherein said computer combines
said calibrated first difference signal with said calibrated second
difference signal to create a hemispherical angular image
signal.
18. A method for characterizing reflectance properties of a test
material, comprising the steps of: generating and directing a
radiation beam onto said test material at an incident angle;
reflecting radiation that is reflected by said test material using
a first reflector with first and second foci; positioning said test
material at said first focus of said first reflector; and receiving
a first angular image that is reflected by said first reflector
using a first lens.
19. The method of claim 18 further comprising the steps of:
reflecting radiation that is reflected by said test material using
a second reflector with third and fourth foci; positioning said
test material at said third focus of said second reflector; and
receiving a second angular image that is reflected by said second
reflector using a second lens.
20. The method of claim 19 further comprising the steps of:
incrementally changing said incident angle; and adjusting said beam
of radiation to keep a shape of said radiation on said test
material relatively constant as said incident angle is changed.
21. The method of claim 20 further comprising the steps of:
incrementally rotating said test material.
22. The method of claim 21 further comprising the steps of:
generating a first image signal from said first angular image;
generating a second image signal from said second angular image;
generating a first difference signal by subtracting an ambient
first image signal from said first image signal; and generating a
second difference signal by subtracting an ambient second image
signal from said second image signal.
23. The method of claim 22 further comprising the steps of:
generating a calibrated first product signal by multiplying said
first difference signal by a first set of calibration factors; and
generating a calibrated second product signal by multiplying said
second difference signal by a second set of calibration
factors.
24. The method of claim 23 further comprising the steps of:
combining said calibrated first difference signal with said
calibrated second difference signal to create a hemispherical image
signal.
Description
TECHNICAL FIELD
[0001] The present invention relates to reflectometers for
measuring the reflectance of a test material. More particularly,
the present invention relates to reflectometers that measure the
angular distribution pattern of light reflecting off the test
material.
BACKGROUND OF THE INVENTION
[0002] In a number of disciplines such as remote sensing, computer
graphics, and aircraft signature prediction, the reflection
properties of materials must be precisely determined. In
particular, the bi-directional reflectance distribution function
(BRDF) defines the distribution of the reflected light rays that
are associated with each possible incident direction of light. For
a particular wavelength of light, the BRDF is a function of four
variables. Two of the variables define the direction of incident
light. The remaining two variables define the direction of
reflected light. For isotropic materials, the BRDF is independent
of the azimuth orientation of the sample. Therefore, for isotropic
materials, only three angles are needed to describe the BRDF.
Anisotropic materials, however, require the four variables to
describe the BRDF and are much more difficult to characterize.
[0003] In practice, the BRDF of anisotropic materials is extremely
difficult to measure with any degree of completeness due to the
large number of angle combinations for the incident and reflected
light. For example, if the BRDF measurements were made by moving a
light source and a detector in two degree increments, over 65
million separate measurements are required. If each individual
measurement could be accomplished in one second, the complete BRDF
measurements would take over 2 years.
[0004] Surface Optics markets a portable measurement device that
operates in the infrared (IR) region. The portable measurement
device uses a movable source and detector. Furthermore, in U.S.
Pat. No. 5,637,873, which is incorporated by reference, a hand-held
instrument uses angular imaging to measure the directional
reflectance of materials after they have been applied to a vehicle.
This instrument is suitable for verifying compliance of in situ
coatings with their reflectance specifications. Both devices,
however, do not provide a complete and automated characterization
of the BRDF of a material.
SUMMARY OF THE INVENTION
[0005] A reflectometer according to the invention characterizes the
reflectance properties of a test material. The reflectometer
includes a radiation subsystem that generates and directs radiation
onto a test material at a plurality of incident angles. An
elliptical reflector assembly has one or more reflectors with first
and second foci. A holder positions the test material at the first
foci of the reflectors. One or more lenses are located within a
first focal length of the second focus of the reflectors. The
lenses receive angular images that are reflected by the
reflectors.
[0006] According to other features of the invention, the elliptical
reflector assembly includes a first reflector having first and
second foci and a second reflector having a third and fourth foci.
A first lens is located at said second focus of said first
reflector. A second lens is located at said fourth focus of said
second reflector. The holder positions the test material at the
first and third foci of the first and second reflectors.
[0007] According to other features of the invention, the holder is
rotatable relative to the radiation subsystem. The radiation
subsystem includes a housing that is movable relative to the
elliptical reflector assembly to alter the incident angle. A
focusing mirror is connected to the housing. A slit controls the
shape of the radiation that is illuminated by the test material.
The slit is movable relative to the housing to keep the shape and
size of the illumination spot relatively constant as the housing
moves.
[0008] According to still other features of the invention, a
shutter blocks the radiation when in a closed position and passes
the radiation when the shutter is in an open position. Ambient
reflection and sample emissions measurements are made when the
shutter is in the closed position.
[0009] According to still other features of the invention, a first
stepper motor adjusts an angular position of the housing relative
to the elliptical reflector assembly to adjust an incident angle of
the radiation on the test material. A position encoder generates a
position signal that is related to the angular position of the
housing.
[0010] According to still other features of the invention, a second
stepper motor adjusts an angular position of the holder. A second
position encoder generates a position signal that is related to the
angular position of the holder.
[0011] In still other features of the invention, a computer is
connected to the first and second stepper motors. The computer is
also connected to the first and second position encoders. A first
imaging assembly receives the first angular image and generates a
first angular image signal. A second imaging assembly receives the
second angular image and generates a second angular image signal.
The computer generates a first difference signal by subtracting an
ambient first image signal from the first image signal. The
computer generates a second difference signal by subtracting an
ambient second image signal from the second image signal. The
computer generates a calibrated first product signal by multiplying
the first difference signal by a first set of calibration factors.
The computer generates a calibrated second product signal by
multiplying the second difference signal by a second set of
calibration factors. The computer combines the calibrated first
difference signal with the calibrated second difference signal to
create a hemispherical angular image signal.
[0012] Other objects, features and advantages will be apparent from
the specification, the drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The various advantages of the present invention will become
apparent to one skilled in the art by reading the following
specification and subjoined claims and by referencing the following
drawings in which:
[0014] FIG. 1 is a cross-sectional view of an imaging reflectometer
according to the present invention;
[0015] FIG. 2 is a perspective view of an exterior of the imaging
reflectometer of FIG.1;
[0016] FIG. 3 is a side view of the imaging reflectometer of FIG. 1
that shows a measurement position and an adjustment position;
[0017] FIG. 4 is a perspective view of a test material positioning
assembly, lenses, imaging arrays, and image processing
electronics;
[0018] FIG. 5 is a perspective view of an imaging array and lens
assembly;
[0019] FIG. 6A is a top-side, perspective view of an elliptical
reflector assembly;
[0020] FIG. 6B is a bottom-side, perspective view of the elliptical
reflector assembly;
[0021] FIG. 7 is a schematic diagram of a computer for controlling
the automated BRDF characterization of a test material;
[0022] FIG. 8 is a data flow diagram illustrating the processing of
the BRDF characterization;
[0023] FIG. 9 is a flowchart illustrating steps for characterizing
the BRDF of a test material;
[0024] FIG. 10 is a side cross-sectional view of a mirror arm
according to an alternate embodiment of the present invention;
[0025] FIG. 11 is a plan view illustrating the mirror arm of FIG.
10 used with a double ellipsoid mirror;
[0026] FIG. 12 is a side view of the mirror arm and the double
ellipsoid mirror of FIG. 11;
[0027] FIG. 13 is a plan view of the mirror arm of FIG. 10 used
with a single ellipsoid mirror;
[0028] FIG. 14 is a side view of the mirror arm and the single
ellipsoid mirror of FIG.13;
[0029] FIGS. 15A -15C illustrate the reciprocity principle by
showing incident and reflected light on an anisotropic
material;
[0030] FIG. 16 illustrates a plan view of a ellipsoid mirror with
an offset slot; and
[0031] FIG. 17 illustrates a side view of the ellipsoid mirror with
the offset slot of FIG. 16.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The ensuing detailed description provides preferred
exemplary embodiments only and is not intended to limit the scope,
applicability, or configuration of the invention. Rather, the
ensuing detailed description of the preferred exemplary embodiments
will provide those skilled in the art with an enabling description
for implementing a preferred exemplary embodiment of the invention.
It being understood that various changes may be made in the
function and arrangement of elements without departing from the
spirit and scope of the invention as set forth in the appended
claims.
[0033] Referring now to FIG. 1, an imaging reflectometer 10 is
illustrated and includes a radiation source assembly 12, an
elliptical reflector assembly 14, a test material positioning
assembly 16, a lens positioning assembly 18, a translation stage
19, and an enclosure 20.
[0034] The radiation source assembly 12 includes a radiation source
housing 30 that is movable in directions indicated by arrow 31. The
radiation source assembly 12 further includes a radiation source 32
that provides radiation that illuminates a slit 36 to produce a
beam. A shutter 38 selectively blocks and passes the beam 34.
Baseline or ambient radiation measurements are made with the
shutter 38 blocking the beam 34. The baseline measurements are
subtracted from subsequent measurements to remove system background
radiation. An elliptical reflector 44 reflects and focuses the beam
34 through a slot 46 in the elliptical reflector assembly 14 onto a
test material 50. In a preferred mode, the radiation source
assembly 12 rotatably moves to provide an angle of incidence of the
beam 34 relative to the test material 50 that is between 90.degree.
and 0.degree..
[0035] The elliptical reflector assembly 14 includes a forward or
first elliptical reflector 54 having a first focus 56 that is
located at a target area on the test material 50. A second focus 60
is located above a lens and filter assembly 64. Preferably the
second focus 60 is located approximately within one focal length of
the lens and filter assembly 64. The lens collimates radiation that
is received from the first elliptical reflector 54. An imaging
array 66 receives the first angular image from the lens assembly
64. Image processing electronics 70 are connected to the imaging
array 66. The lens and filter assembly 64 is adjustably connected
to a base bracket 72 that is connected to a bottom surface 74 of
the imaging reflectometer 10.
[0036] The backward or second elliptical reflector 58 has a third
focus 80 that is located at the target area on the test material 50
and a fourth focus 82 that is located above a lens and filter
assembly 84. Preferably, the fourth focus 82 is located
approximately within one focal length of the lens and filter
assembly 84. The lens collimates radiation that is received from
the second elliptical reflector 58. An imaging array 86 receives
the second angular image from the lens and filter assembly 64.
Image processing electronics 88 are connected to the imaging array
86. A lens and filter assembly 84 is adjustably connected to a base
bracket 90 that is connected to the bottom surface 74 of the
imaging reflectometer 10.
[0037] The translation stage 19 includes a translation base 100
that is connected to the bottom surface 74. A male translation
guide 102 is slideably connected to a female translation guide 104.
The female translation guide 104 is connected to the test material
positioning assembly 16. The translation base 100, the male
translation guide 102, and the female translation guide 104 allow
the test material positioning assembly 16 to be moved between a
measurement position and an adjustment position that are shown and
described in FIG. 3.
[0038] Referring now to FIG. 2, the imaging reflectometer 10 is
illustrated in further detail. An arm 120 is biased by an
adjustment cam 124 to vary the angular position of the slit 36
relative to the beam 34 to control the size of the beam spot on the
test material 50. In a preferred mode, the beam spot is
approximately 2 millimeters (mm) by 2 mm. As the angle of incidence
varies between 90.degree. to 0.degree., the angular position of the
slit 36 adjusts to maintain a constant-sized beam spot on the
target area of the test material 50.
[0039] A stepper motor 128 controllably rotates arms 130 and 132
relative to an axis that is defined by bearings 134. The stepper
motor 128 preferably includes a position encoder for generating a
position signal that is related to the relative angular position of
the arms 130 and 132. A slot cover 140 covers the slot 46 and is
movable with the radiation source housing 30.
[0040] The enclosure 20 includes sides 142-1 and 142-2 and ends
144-1 and 144-2. A top 146 includes an access opening 148 and cover
149 for accessing the interior of the enclosure 20, for example
when the test material positioning assembly 16 is in the adjustment
position. The stepper motor 128 is attached to the enclosure 20
adjacent to the end 144-1 and the top 146.
[0041] Referring now to FIG. 3, the test material positioning
assembly 16 of the imaging reflectometer 10 can be positioned in
the measurement position 150 and the adjustment position 154. An
upper portion 156 of the test material positioning assembly 16 is
rotatable about an axis 158. The upper portion 156 is rotatable
90.degree. in first and second directions to provide additional
clearance when moving between the measurement and adjustment
positions 150 and 154, respectively. A height adjustment device 160
of the test material positioning assembly 16 allows an upper
surface 162 of the test material 50 to be positioned at the height
of a target plane 164. The translation stage 19 also allows the
test material to be positioned relative to a center line 166. The
upper portion 156 is an arm with two equivalent ends, each of which
can hold a test sample. One of these ends can be used to hold a
reference material of known reflectance to verify the stability of
the calibration.
[0042] Referring now to FIG. 4, a positioner housing 180 sits on
top of a rotation stage 184 to rotate a test material holder 187 on
which the test material 50 sits. A position encoder connected to
the rotation motor generates a rotation signal that is related to
an azimuth angle of the test material 50. Translation assemblies
190 and 192 permit the manual interchanging of two lens/filter
combinations corresponding to two different wavelength bands.
[0043] Referring now to FIG. 5, a vertical translation stage 200
allows the adjustment of the lens and filter assembly 64 in a
vertical direction. The translation assembly 190 allows the
adjustment of the lens and filter assembly 64 in a horizontal
direction as well as interchange of the two lens/filter
combinations. Referring now to FIG. 6, the elliptical reflector
assembly 14 is illustrated in further detail. A cusp 192 separates
the first elliptical reflector 54 from the second elliptical
reflector 58. The slot 46 allows the beam 34 to pass through the
elliptical reflector assembly 14 onto the test material 50.
[0044] Referring now to FIG. 7, a control system for automating the
BRDF characterization for the test material 50 is illustrated at
230. The control system 230 includes a controller with an
input/output (I/O) interface 234, a microprocessor 236 and memory
238. The memory 238 includes random access memory (RAM), read only
memory (ROM), and/or external storage such as a hard drive, a
floppy drive, optical storage or other suitable electronic memory
storage. An additional I/O card 240 may be provided for connecting
peripheral devices 244. Alternatively, the peripheral devices 244
can be directly connected to the I/O interface 234.
[0045] The peripheral devices 244 include a position encoder 246
and the stepper motor 128 that are associated with the radiation
source assembly 12. The position encoder 246 is associated with the
stepper motor 128. As the stepper motor 128 incrementally changes
the angle of incidence of the beam 34 on the target area of the
test material 50, the position encoder 246 generates an angular
position signal.
[0046] The peripheral devices 244 further include an encoder 250
that is associated with the rotation stage 184 and a stepper motor
252. As the stepper motor 252 rotates the test material holder 187
and the test material 50, the position encoder 250 generates an
azimuth angle signal. The control system 230 controllably adjusts
the stepper motors 128 and 252 when measuring the BRDF as will be
described further.
[0047] The peripheral devices 244 further include the imaging
arrays 66 and 86 and/or the image processing electronics 70 and 80
that are likewise connected to the I/O card 240 and/or the I/O
interface 234. A display 256, a keyboard 258 and a mouse 260 are
also connected to the I/O interface 234. Other I/O devices 262 such
as printers, scanners, and other suitable devices are connected to
the I/O interface 234. The memory 238 loads an operating system
(OS) module 266 when booted up. An image processing module 268 is
also loaded into the memory 238 during use. In a preferred
embodiment, the control system 230 is a computer.
[0048] Referring now to FIG. 8, the first imaging array 66 and
image processing electronics 70 generate a first illuminated image
signal 300 when the shutter 38 is open and the radiation source 32
is on. The first illuminated image signal 300 is input to a
difference calculator 302. A first unilluminated or ambient image
signal that is output by the first image processing electronics 70
when the shutter 38 is closed is subtracted using the difference
calculator 302. The first difference signal 306 is input to a first
input of a product calculator 308. Calibration factors 310 are
input to a second input of the product calculator 308.
[0049] The calibration factors are set by using a sample diffuse
gold reflector with a known reflectance. The intensity value
corresponding to each pixel of the image are measured and a
calibration factor is computed to provide the known or expected
BRDF. These same calibration factors are then used for computing
the BRDF for the test material 50.
[0050] A second illuminated image signal 320 is output by the
second image processing electronics 80 when the shutter 38 is open.
The second illuminated image signal 320 is input to a second
difference calculator 322. A second unilluminated or ambient image
signal 324 that is output when the shutter 38 is closed is input to
the difference calculator 322. A second difference signal 326 is
input to a first input of a second product calculator 328.
Calibration factors 330 are likewise input to the second product
calculator 328.
[0051] A first calibrated product signal 334 is output by the
product calculator 308. A second calibrated product signal 338 is
also output by the second product calculator 328. A merge
calculator 342 merges the first and second calibrated product
signals 334 and 338. A hemispherical image signal 344 is output by
the merge calculator 342. The hemispherical image signal 344 is an
angular image of all of the radiation that is reflected into an
upper hemisphere above the test material except for light that hits
the slot 46 in the elliptical reflector assembly 14. With the
exception retro-reflection in the slot area, the BRDF that is
generated completely characterizes the reflectance properties of
the sample.
[0052] Referring now to FIG. 9, the steps for automatically
controlling the first and second stepper motors 128 and 252 when
characterizing the BRDF of the test material 50 is shown and is
generally designated 358. Control starts at step 360. In step 364,
the incident angle is set equal to 90 degrees. In step 366, the
azimuth angle is set equal to zero degrees. In step 368, control
determines whether the incident angle is less than or equal to zero
degrees. If it is, control ends at step 370. Otherwise, control
continues with step 372 where control determines if the azimuth
angle is greater than or equal to 360 degrees. If it is, control
decrements the incident angle in step 374. Otherwise, the shutter
is closed in step 376. In step 378, an unilluminated signal is
recorded. In step 380, the shutter is open. In step 384, the
illuminated signal is recorded. In step 386, the azimuth angle is
incremented. Control continues from step 386 to step 372.
[0053] Referring now to FIG. 10, an alternate mirror arm 400 is
shown and includes a hub 404 that is rotatably mounted on a bearing
408. The alternate mirror arm 400 does not require a slot in the
elliptical reflector assembly. The hub 404 and the bearing 408
define an open central cavity 412 through which an incident beam of
light 416 travels. A first mirror 420 and a second mirror 424
redirect the light 416 onto a sample 426. The sample 426 can be
isotropic or anisotropic. The first and second mirrors 420 and 424
are connected to and supported by an arm portion 430 that extends
from the hub 404.
[0054] Referring now to FIGS. 11 and 12, the mirror arm 400 is
shown rotatably mounted inside of a double ellipsoid mirror 434
(which is similar to the elliptical reflector assembly shown above
without a slot). The double ellipsoid mirror 434 includes a front
half hemisphere 438 having a first focus 440 and a rear half
hemisphere 442 having a second focus 444. The mirror arm 400 sweeps
through an arc 450 as it rotates on the bearings 408. A driving
mechanism such as a belt and pulley, a geared mechanism or any
other suitable driving mechanism can be used to position the mirror
arm 400. A position encoder can also be employed to generate a
rotational position signal that is used as an input signal for a
controller.
[0055] Referring now to FIGS. 13 and 14, the mirror arm 400 of FIG.
10 is rotatably mounted and extends inside or below a single
ellipsoid mirror 460 having a focus 464. As can be appreciated, the
single ellipsoid mirror 460 reduces the complexity of the
reflectometer by eliminating the need for multiple imaging arrays
and their associated electronics. While the mirror 460 misses many
of the outgoing rays, reflection at those angles can be determined
by reciprocity as discussed below.
[0056] Referring now to FIG. 15A, an incident beam of light 470
travels in incidence plane 474 onto an anisotropic sample 426. The
anisotropic sample 426 (having an orientation indicated by lines
478) reflects a reflected beam of light 482 at a first angle 488 in
reflectance plane 490. If an obstruction 492 is located in the
reflected plane 490, reflectance properties of the sample cannot be
measured for the first angle unless another technique is
employed.
[0057] Referring now to FIG. 15B, the reciprocity principle states
that the anisotropic sample 426 will have the same reflectance
characteristics when the source and the detector locations are
interchanged. In other words, an incident beam of light 500
replaces the reflected beam of light 482 in FIG. 15A. A reflected
beam of light 502 replaces the incident beam of light 470 in FIG.
15A. This particular configuration can not be realized because the
mirror arm only allows the beam to be incident in plane 474.
However, as discussed below an equivalent configuration can be
realized simply by turning the sample.
[0058] Referring now to FIG. 15C, the reciprocity principle can be
employed in a modified fashion. The anisotropic material 426 is
rotated such that the incident beam 510 forms the same angle with
the sample 426. The reflected beam 512 is measured at an angle
equal to the first angle 488 (identified at 514 in FIG. 15C) on an
opposite side of the incidence plane. Thus, the configuration in
15C permits successful measurement of the reflectance value that
could not be measured in 15A due to obstruction.
[0059] Referring now to FIGS. 16 and 17, an offset slot 520 is a
shown in an elliptical mirror 522. An offset slot mirror arm 523
includes a first mirror 524 and a second mirror 528. The first and
second mirrors 524 and 528 redirect an incident beam of light 530
through an elliptical reflector assembly onto the sample 426. The
first and second mirrors 524 and 528 are attached to and supported
by tubes 534 and 538. The tubes 534 and 538 are attached to an arm
540 that is positioned using stepper motors and encoders in a
manner similar to the arms 130 and 132. By the method described
above, reciprocity can be used to fill in reflectance values at the
outgoing angles obscured by the offset slot 520 and arm 523.
[0060] Using conventional measurement methods and apparatus, a
complete characterization of the BRDF for a test material requires
over 65 million separate measurements when using a two degree
increment for the source and the detector. If each individual
measurement could be accomplished by the conventional devices in
one second, the complete measurement of the BRDF function would
take over 2 years. By contrast, the imaging reflectometer according
to the present invention can accomplish the task in 8 hours or less
assuming a five second measurement at each combination of source
incident angle and sample azimuth. Isotropic materials can be fully
characterized in under four minutes since the BRDF function is
independent of sample azimuth.
[0061] For anistropic test materials, the present invention
generates the complete hemispherical image signal that is the
angular image for a given incident angle and a given azimuth angle.
In other words, for the given incident angle and the given azimuth
angle, the present invention records all of the variables
associated with the reflected light at the same time. As a result,
the BRDF measurements can be completed more quickly. For isotropic
materials, the BRDF is independent of the azimuth angle. Therefore,
only the incident angle is varied when measuring the BRDF.
[0062] Those skilled in the art can now appreciate from the
foregoing description that the broad teachings of the present
invention can be implemented in a variety of forms. Therefore,
while this invention has been described in connection with
particular examples thereof, the true scope of the invention should
not be so limited since other modifications will become apparent to
the skilled practitioner upon a study of the drawings,
specifications and following claims.
* * * * *