the counter-rotating beams. Of course, the
effect wasn’t caused by ether, but simply
the fact that one of the light beams had
slightly farther to travel than the other.
This “Sagnac effect,” which wasn’t
exploited until the era of lasers and fiber
optics, is the basis for modern interferometers and ring laser gyroscopes. The latter
are able to detect rotation because they
have two counter-propagating beams over
the same path in order to detect rotation;
interference between the beams indicates
shifts in the standing wave pattern and
hence rotation. Ring laser gyroscopes are now key components
of the inertial guidance systems of the Boeing 777 and Airbus
A320, as well as many military aircraft.
move in tandem down a track placed next
to the target of inspection.
Understanding the exact mechanisms
of composite degradation as a function
of time, environment and load is still
an active area of materials science,
Cramer said. His team focuses on the
measurement and characterization of
Another commercially available
aircraft inspection technique is called
laser shearography. These systems start by
recording the laser speckle reflected from
a surface of interest. Then the inspectors apply a small stress
load to the aircraft component and take a second image of the
specular reflections. By subtracting one image from the other
on a computer, someone can see the characteristic fringes that
represent surface strain. “Originally a lot of the systems used
a birefringent crystal that would provide a sheared image,”
Cramer said. “Now they can do that electronically.”
According to a recent issue of National Geographic, NASA
recently unveiled a “personal air vehicle” prototype called the
Puffin that may someday enable one-person flight. While the
Puffin may or may not allow humans to fly like Icarus did, one
thing is for sure: Light-based technology will surely play a major
role in 21st century aviation. t
developing micro-and nano-textured
surfaces that would
onto the external
surfaces of aircraft.
Nondestructive testing of materials
How can inspectors tell when part of the structure of an airplane has been weakened and should be replaced? Not just by
looking—the flaws may be too small to detect with the naked
eye. And pilots can’t exactly pull their vehicles off to the side of
the road to check a wavering wing.
Thermal imaging is one method used for aircraft inspection as well as for quantitative characterization of metallic and
composite materials. Researchers can measure changes in the
density or thickness of corroding metals, or the fiber volume
fraction of composites. They can also assess changes in density
that may be due to porosity, entrapped voids in the material,
or delaminations or microcrack density in the material. (In
delamination, stress causes the layers of a carbon-fiber-based
composite material to separate. As in metal fatigue, the early
stages of the process occur below the surface of the material.)
All these flaws could be precursors to potentially catastrophic
Beyond merely looking for defects, researchers want to
relate their material property measurement to the remaining
useful life or the residual strength in the material, so that they
can understand how it will perform in service, said Cramer at
Today, composites are used in aircraft control surfaces
such as rudders, ailerons and elevators—although they may
compose much larger percentages of tomorrow’s airplanes.
Infrared thermography is one of the methods that can be
used to look for water entrapped in those materials or to look
To examine an aircraft part, whether aluminum or composite, inspectors inject a small heat flux into the part—its
temperature needs to be raised only one or two degrees Celsius above ambient levels. A commercially available infrared
camera records the way the heat spreads out within the material. For checking aluminum fuselages, inspectors might use
a cryogenically cooled HgCd Te detector working at wavelengths of 8 to 12 µm. The heat source and infrared camera
Patricia Daukantas ( email@example.com) is a freelance writer specializing in
optics and photonics.
[ References and Resources ]
>> American Airlines. “Fuel Smart,” online at www.aa.com/i18n/
>> A.D. Kersey et al. “Fiber optic gyroscope technology,” Opt. News
15( 11), 12 (Nov. 1989).
>> K.E. Cramer and W.P. Winfree. “Thermal characterization of defects
in aircraft structures via spatially controlled heat application,” Proc.
SPIE 2766, 202 (1996).
>> K.E. Cramer. “NASA thermographic inspection of advanced composite materials,” keynote address, 7th International Conference
on Quantitative Infrared Thermography (QIRT; Rhode-St-Genèse,
>> A. Helfrick. Electronics and the Evolution of Flight (College Station:
Texas A&M University Press, 2004).
>> M. Naeem. “Advancement in Laser Drilling for Aerospace Gas
Turbines,” in Proceedings of the 3rd Pacific International Conference on Applications of Lasers and Optics (PICALO; Beijing, China,
2008), paper 401.
>> R.E. Bailey and J. Arthur. “Head-Worn Displays for Commercial and
Business Aircraft,” SPIE Newsroom, May 14, 2009; online at spie.
>> R. Vandel and E.F. Weener. “Head-Up Guidance System Technology—A Clear Path to Increasing Flight Safety” (Flight Safety Foundation report, November 2009).
>> R.E. Bailey et al. “Head-Worn Displays for NextGen,” Proc. SPIE
8041, 80410G (2011); doi: 10.1117/12.885847.
>> G. Overton. “First laser-sintered UAV takes flight,” posted on
OptoIQ.com August 3, 2011.
>> D. Pugh-Thomas et al. “CdSe(ZnS) nanocomposite luminescent
high temperature sensor,” Nanotechnology 22, 185503 (2011).