Infrared Laser
Gonioreflectometer Instrument (ILGRI) |
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The bidirectional reflectance distribution function (BRDF) is a
measure of the amount of light reflected by a surface from one
direction into another. Integrating it over specified incident and
reflected solid angles defines the reflectance, which also can be easily
related to the absorptance (or emissivity) of a sample.
The BRDF can thus be taken as a fundamental quantity for the optical
characterization of an object and it correspondingly is important in
a large variety of applications. BRDF is critical to computer image
analysis and reconstruction, for applications in targeting,
reconnaissance and surveillance, virtual reality systems and
simulators, radiative heat transfer, computer graphics, optical
testing, telemedicine, robotics, etc.
The Infrared Laser Gonioreflectometer Instrument (ILGRI)
is intended to measure the BRDF (as well as the BTDF - bidirectional
transmittance distribution function) of flat and structured samples
(such as V-grooved surfaces or cavities) in the infrared spectral
range. The ILGRI enables the measurement in absolute units (1/sr) of
the BRDF and BTDF as functions of 4 variables
(2 angular coordinates for incident direction and 2 - for the
detector angular position). |
Bi-directional Reflectance
Distribution Function (BRDF)
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The complete instrument consists of three main parts: the infrared
source section, the sample manipulation section, and the detection
section. ILGRI layout showing the main components is depicted below.
System features:
- Out-of-plane capability with sample tilt and normal rotation
- Retro-reflection setup with beamsplitter
- Set of interchangeable sample mounts
- Mueller Matrix with linear polarizers and retarders
- Fixed and tunable wavelength infrared laser to be added
The infrared source section is a line-tunable, stable
CO2 laser, together with a suite of optics including
mirrors and lenses for beam direction and adjustment, polarizers
and retarders for beam polarization control, and polarizers and
filters for beam power level adjustment from 1 nW to
1 W. The addition of other laser wavelengths in the
infrared is currently underway. The sample manipulation section
consists of computer controlled motorized rotation, goniometer
and translation stages for alignment and setting the polar and
azimuthal angles of incidence with respect to the sample
surface. The detector section consists of pyroelectric and MCT
detectors, along with an aperture, a lens and polarizers. Two
setups are used: one with a beamsplitter and mirror that enable
measurements at and close to the retroreflection directions,
where the reflected light is folded to the detector and its
associated optics (aperture, lens and polarizer), and a second
with a direct path to the detector and its optics.The ILGRI also
contains several quadrant detectors, which enable alignment of
the stages, axes of rotation, sample surface, and input laser
beam, all to within 50 µm of each other.
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Photo of the input source optics section,
including CO2 laser, beam conditioning optics and
polarizers. The beam is directed to the sample and detector
sections located behind the aluminum panels at the top
center-left of the picture. |
Photo of the sample and detector sections of
the ILGRI. The CO2 beam is incident from the left
onto a mirror sample mounted at the intersection of the
rotation and cradle goniometer stages. Pyroelectric detectors
and viewing optics are located at either end of the double arm
held by the detector rotation stage on the optical
table. |
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BRDF example of two similar
graphite samples with different surface finishes. The BRDF is
measured at 10.6 µm, in the plane of incidence, each curve
represents a different angle of incidence and is plotted versus
the viewing angle relative to the sample normal. |
Reference
IR optical scattering instrument with out-of-plane
and retro-reflection capabilities,
Jinan Zeng and L. Hanssen,
Conference on Lasers and Electro-Optics (CLEO), 22-27 May 2005;
3, 1882-1884 (2005).
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