An apparatus denoted a laser image contrast enhancement system (LICES) increases the contrast with which one can view a target glowing with blackbody radiation (a white-hot object) against a background of blackbody radiation in a furnace at a temperature as high as approximately 1,500 C. The apparatus utilizes a combination of narrowband illumination, along with band-pass filtering and polarization filtering to pass illumination reflected by the target while suppressing blackbody light from both the object and its background

Holmes, Richard R.

Kurtz, Robert L.

Witherow, William K.

NASA Technical Reports Server

NASA Tech Briefs, January 2006 25
Physical Sciences
Enhanced-Contrast Viewing of White-Hot Objects in Furnaces
Band-pass- and polarization-filtered laser light exceeds polarization-suppressed blackbody light.
Marshall Space Flight Center, Alabama
An apparatus denoted a laser image
contrast enhancement system (LICES)
increases the contrast with which one
can view a target glowing with black-
body radiation (a white-hot object)
against a background of blackbody ra-
diation in a furnace at a temperature
as high as ≈1,500 °C. The apparatus
utilizes a combination of narrowband
illumination, along with band-pass fil-
tering and polarization filtering to
pass illumination reflected by the tar-
get while suppressing blackbody light
from both the object and its back-
ground.
In a typical application, the target is
about 1 cm in size and located as far as
30 in. (≈76 cm) into the furnace. In the
absence of this or another contrast-en-
hancing apparatus, a white-hot target
in a furnace is nearly or totally indistin-
guishable from the white-hot back-
ground. Unlike a prior contrast-en-
hancing apparatus that utilizes two
intersecting optical axes for viewing
and illumination of the target and re-
quires a furnace opening as wide as 3
in. (≈8 cm) the LICES provides for
both illumination and viewing of the
target along the same path. Hence, the
LICES makes it possible to utilize a nar-
rower opening into the furnace: the
LICES can function with an illumina-
tion/viewing tube only about half an
inch (≈1.3 cm) wide.
The LICES (see figure) includes a
laser aimed perpendicularly to the opti-
cal path to the target. (Optionally, an-
other source of narrowband illumina-
tion could be used.) The laser light
impinges on a polarizing beam splitter
that turns the light onto the optical
path to the target. The laser light passes
through a quarter-wave retardation
plate, which causes the light to become
Dual-Laser-Pulse Ignition
This scheme provides a more reliable ignition source and more efficient energy delivery than a
single-pulse format.
Marshall Space Flight Center, Alabama
A dual-pulse laser (DPL) technique
has been demonstrated for generating
laser-induced sparks (LIS) to ignite
fuels. The technique was originally in-
tended to be applied to the ignition of
rocket propellants, but may also be ap-
plicable to ignition in terrestrial settings
in which electric igniters may not be suit-
able. Laser igniters have been sought as
alternatives to such conventional devices
as electrical spark plugs and torch ignit-
ers for the following main reasons:
1. A typical electric spark igniter gener-
ates sparks at its electrode near a
wall, which potentially quenches
combustion. Hence, more spark en-
ergy is needed to ensure ignition. A
large combustion chamber would re-
quire a torch igniter, which com-
prises an electric-spark source, a pre-
mixing chamber, and propellant
valves.  In contrast, the laser igniter is
capable of creating sparks directly in
a main chamber at specific optimal
locations, which can be out away
from the chamber walls, and without
the need of other subsystems.
2. Laser igniters can generate LIS with
very precise timing, on the order of
nanoseconds.  This accurate timing
precision may be helpful in certain ig-
nition applications.  Furthermore,
this ignition generates significantly
less electromagnetic emission noise
than electrical igniters.  Such noise
can interfere with other electronic sig-
nals of engine sensors and control
components.
Years of research on laser ignition
have produced viable single-pulse laser
ignition concepts; however, the trans-
mission of high laser energy through
fiber optics required by these single-
pulse schemes has been problematic
due to potential fiber damage and re-
duction in transmission efficiency.  In
comparison, optical energy for the DPL
method can be stretched out and trans-
mitted through multiple fiber lines, ef-
fectively reducing the energy intensity.
In addition, the lifetime of the plasmas
generated by use of the DPL exceeds
those of plasmas generated by single-
laser pulses, which increases their effi-
cacy as an ignition source.
In the present DPL technique, the
first pulse is used to generate a small
plasma kernel. The second pulse is sub-
sequently used to irradiate the plasma
kernel. The transfer of laser energy into
the kernel is much more efficient be-
cause the radiation-absorption charac-
teristics of the plasma kernel are greatly
enhanced, relative to the single-pulse
approach. Consequently, the kernel can
develop into a more effective ignition
source.
Comparative experiments on single-
and dual-pulse laser ignition were per-
formed in a small test-bed rocket
thrust chamber, at Marshall Space
Flight Center, using gaseous oxygen
and kerosene. Sapphire windows were
used for optical access to the chamber.
In the tests, the DPL technique was
found to provide a repeatable ignition
source for combustion with an optimal
energy level. 
This work was done by Huu Trinh of
Marshall Space Flight Center, James W.
Early of Los Alamos National Laboratory,
Matthew E. Thomas of CFD Research
Corp., and John A. Bossard, formerly of
CFD Research Corp. Further information
is contained in a TSP (see page 1).
MFS-31922
https://ntrs.nasa.gov/search.jsp?R=20100014072 2019-08-30T09:19:41+00:00Z
26 NASA Tech Briefs, January 2006
circularly polarized. The circularly po-
larized laser light passes undisturbed
through a vertical polarizer, then trav-
els along the optical axis to the target.
Reflection from the target reverses the
circular polarization. The reflected
laser light passes again through the ver-
tical polarizer and then through the
quarter-wave retardation plate, which
converts the reverse circular polariza-
tion to horizontal polarization.
The polarizing beam splitter passes
the horizontally polarized reflected laser
light, which then passes through a lens,
a field aperture (which helps to increase
contrast by blocking background light),
a horizontal polarizer, and a filter having
a 20-nm-wide wavelength pass band. The
reflected laser light ultimately comes to
focus in a charge-coupled-device (CCD)
camera. At the same time, the crossed
polarizers and the band-pass filter dis-
criminate the wideband, randomly po-
larized blackbody light from both the
target and the background.
Because the intensity of blackbody
radiation is proportional to the fourth
power of absolute temperature, it
could be necessary to increase the
laser power to maintain adequate con-
trast at higher temperatures. A proto-
type LICES has been found to yield
high-contrast images at temperatures
1,500 °C.
This work was done by William K. With-
erow and Richard R. Holmes of Marshall
Space Flight Center and Robert L. Kurtz
of Pace & Waite, Inc. 
This invention has been patented by NASA
(U.S. Patent No. 6,366,403). Inquiries con-
cerning nonexclusive or exclusive license for its
commercial development should be addressed to
Sammy Nabors, MSFC Commercialization As-
sistance Lead, at sammy.a.nabors@nasa.gov.
Refer to MFS-31561-1.
CCD
Camera
Band-Pass
Filter
Horizontal
Polarizer
Lens
Laser or Other Narrowband
Light Source
Polarizing Beam Splitter
Quarter-Wave Retardation Plate
Vertical Polarizer
Target
Field
Aperture
MFS31561
4-19-02    bs
Only One Optical Path is used for both illuminating and viewing the target with laser light. Blackbody
radiation from the target is suppressed by the crossed (horizontal and vertical) polarizers.
Improved quantum-cascade lasers
(QCLs) are being developed as electri-
cally tunable sources of radiation in the
far infrared spectral region, especially
in the frequency range of 2 to 5 THz.
(Heretofore, the wavelengths of QCLs
have been adjusted by changing tem-
peratures, but not by changing applied
voltages or currents.) In comparison
with gas lasers now used as far-infrared
sources, these QCLs would have larger
wavelength tuning ranges, would be
less expensive, and would be an order
of magnitude less massive and power-
hungry. It is planned to use the im-
proved QCLs initially as the active com-
ponents of local oscillators in
spaceborne heterodyne instruments
for studying infrared spectral lines of
molecules of scientific interest. On
Earth, the QCLs could be used as far-
infrared sources for medical glucose-
monitoring and heart-monitoring in-
struments, chemical-analysis and
spectral-imaging systems, and imaging
instruments that exploit the ability of
terahertz radiation to penetrate cloth
and walls for detection of contraband
weapons.
The structures of QCLs and the
processes used to fabricate them have
much in common with those of multi-
ple-quantum-well infrared photodetec-
tors described in numerous previous
NASA Tech Briefs articles. In one of four
approaches being followed in the pres-
ent development effort, the focus is
upon designing and fabricating the
structures to obtain heterogeneous cas-
cades for different electric fields and
different wavelengths in order to enable
electrical tuning of laser emission wave-
lengths. Both the variation of the emis-
sion wavelength and the targeted range
of the electric-field strength for each
cascade would be kept small so the spec-
tral gains of adjacent cascades at any
given electric-field strength in the target
range would overlap. This approach is
expected to afford the desired variation
of the gain maximum with electric-field
strength, so that a change in applied
bias voltage would result in a wave-
length change.
In the second approach, layers of a QCL
structure are to be graded to modify the
shapes and depths of quantum wells, such
that the electronic wave functions in the
quantum wells and the transition energies
between them would change with an in-
tentional variation of the applied bias elec-
trical field. A change of the bias applied to
such a structure would result in a change
in the energy and, hence, of the wave-
length of the lasing transition.
In the third approach, the focus is on ex-
ploiting distributed-feedback and distrib-
uted-Bragg-reflector QCL architectures to
achieve wavelength tuning through varia-
tion of applied electric current.
In the fourth approach, which is com-
plementary to the other three, the focus
Electrically Tunable Terahertz Quantum-Cascade Lasers
These devices would supplant gas lasers as far-infrared sources.
NASA’s Jet Propulsion Laboratory, Pasadena, California


Enhanced-Contrast Viewing of White-Hot Objects in Furnaces

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