Digital focusing schlieren relies on accurate mapping between a camera and a digital modulator or display system. Our early development of the technology was challenged by higher-order distortions in the optical systems. A simple perspective transform was adequate for lower-power lenses and smaller fields of view, but for wide fields and for highly sensitive configurations such as dark-field schlieren, correction terms were necessary. The Brown-Conrady model fitting implemented by OpenCV offered little improvement, so we developed a new distortion correction method that empirically optimizes the schlieren filter performance and constructs a map with phase unwrapping techniques borrowed from interferometry. This approach allows operation even with wider-angle lenses and finally makes dark-field schlieren practical. It also appears to improve the resilience of the grid generation with lower resolution camera systems, which is important for implementation in high speed schlieren imaging.
This paper presents results from simultaneous measurements of fluid density and the resulting wavefront distortions in a
sonic underexpanded jet. The density measurements were carried out using Rayleigh scattering, and the optical
distortions were measured using a wavefront sensor based on phase shifting interferometry. The measurements represent
a preliminary step toward relating wavefront distortions to a specific flow structure. The measured density field is used
to compute the phase distortions using a wave propagation model based on a geometric-optics approximation, and the
computed phase map shows moderate agreement with that obtained using the wavefront sensor.
Schlieren imaging has been an essential method for studying aerodynamic effects, particularly thermal convection, shock waves, and turbulent flows. This paper describes a compact portable digital focusing schlieren system that can be used to visualize relatively large fields for applications in ventilation design and aerodynamics research. Visualizing large fields is difficult using classical schlieren systems that employ collimated light because their field of view is limited by the size of the mirrors or lenses. Background-oriented schlieren systems are well-suited for visualizing large fields, but their sensitivity is limited by the need to simultaneously maintain focus on the background pattern and the test area. Lens and grid-based focusing schlieren systems are essentially hybrids between classical and background-oriented systems. They can visualize fields that are much larger than possible with classical schlieren systems, while providing more sensitivity than background-oriented schlieren systems. Using commercially available camera lenses and optics, fields up to several square meters can be visualized. A key innovation in the system presented here is that digital display devices are used to display the background pattern, which simplifies the optical system and reduces its size. To calibrate the system, proprietary software is used to analyze images acquired by the system’s digital camera, and then a background pattern is computed that is complementary to the cutoff grid. The calibration software also provides real-time background subtraction and contrast enhancement. The schlieren system is portable enough that it can be set up quickly in industrial facilities.
Modern digital recording and processing techniques combined with new lighting methods and relatively old schlieren visualization methods move flow visualization to a new level, enabling a wide range of new applications and a possible revolution in the visualization of very large flow fields. This paper traces the evolution of schlieren imaging from Robert Hooke, who, in 1665, employed candles and lenses, to modern digital background oriented schlieren (BOS) systems, wherein image processing by computer replaces pure optical image processing. New possibilities and potential applications that could benefit from such a capability are examined. Example applications include viewing the flow field around full sized aircraft, large equipment and vehicles, monitoring explosions on bomb ranges, cooling systems, large structures and even buildings. Objectives of studies include aerodynamics, aero optics, heat transfer, and aero thermal measurements. Relevant digital cameras, light sources, and implementation methods are discussed.
Since its invention in the 19th century, schlieren imaging has been an essential method for studying many aerodynamic effects, particularly convection and shock waves, but the classical method using parabolic mirrors is extremely difficult to set up and very expensive for large fields of view. Focusing schlieren methods have made large- area schlieren more feasible but have tended to be difficult to align and set up, limiting their utility in many applications We recently developed an alternative approach which utilizes recent advances in digital display technology to produce simpler schlieren system that yields similar sensitivity with greater flexibility.
Low coherence interferometry (LCI) methods have been investigated for the detection of damage on coated and
uncoated airfoils in advanced gas turbines, and in particular methods for implementing LCI for in situ inspection using
borescope-type instrumentation. The work reported in this paper includes design of prototype instrumentation and some
test results, as well as results using commercial instruments obtained on TBCs. LCI techniques can provide significant
advance over currently employed visual inspection of gas turbine airfoils. The instrumentation provides a significant
advance over currently employed visual inspection of gas turbine airfoils. For instance, with thermal barrier coatings
(TBCs), these techniques allow the detection and quantification of incipient spalls, delamination, and changes in TBC
porosity which typically go unnoticed with visual inspection methods. The methods are well suited for use with
borescopes and thus provide a large potential to be developed into commercial optical diagnostics instruments for use
during maintenance and inspection of on-wing airfoils in advanced gas turbines.
A high-speed digital streak camera designed for simultaneous high-resolution color photography and focusing schlieren imaging is described. The camera uses a computer-controlled galvanometer scanner to achieve synchroballistic imaging through a narrow slit. Full color 20 megapixel images of a rocket sled moving at 480 m/s and of projectiles fired at around 400 m/s were captured, with high-resolution schlieren imaging in the latter cases, using conventional photographic flash illumination. The streak camera can achieve a line rate for streak imaging of up to 2.4 million lines/s .
Systems that attempt to image or project optical energy or information through a turbulent atmosphere are limited by
aberrating, refractive index variations. The processes can be improved in a variety of ways if the complex wave function
of the aberrated wave can be recorded, reconstructed and analyzed at a sufficient speed. This paper describes application
of digital holography for recording, reconstructing, and processing complex wave functions to complement methods such
as adaptive optics and lucky imaging. Having the complex waveform provides all of the information required by
adaptive optical procedures and also enables improved image processing that is not applicable to real images. Unlike
intensity averaging, when complex wave functions are averaged, the random fluctuations in the phase cancel since phase
terms include both positive and negative values. In this paper we describe the application of digital holography for
recording, reconstructing, and processing complex wave functions of atmospherically aberrated wave functions and
report demonstrations in correcting for atmospheric turbulence.
A highly compact laser-scanning device that can be used as an in situ detection and monitoring device for metal fatigue is described. A prototype is built on a Mica2 wireless sensor mote platform with TinyOS-based firmware. This device is shown to be capable of detecting fatigue-related changes in the surface bidirectional reflectance distribution (BRDF) of an aluminum test coupon, and detectable changes in BRDF are measured at a very early stage of fatigue development, when no cracks larger than 50 µm in length have yet developed at the monitored location. The system power requirements are compatible with standard sensor mote architectures such as the Mica and Telos series, enabling the potential for multiyear lifetimes without battery replacement. Such an in situ optical fatigue sensor could be used in a variety of structural health monitoring applications, including aerospace, rail transport, and civil structures.
Thermally assisted electro-luminescence may provide a means to convert heat into electricity. In this process, radiation
from a hot light-emitting diode (LED) is converted to electricity by a photovoltaic (PV) cell, which is termed
thermophotonics. Novel analytical solutions to the equations governing such a system show that this system combines
physical characteristics of thermophotovoltaics (TPV) and the inverse process of laser cooling. The flexibility of having
both adjustable bias and load parameters may allow an optimized power generation system based on this concept to
exceed the power throughput and efficiency of TPV systems. Such devices could function as efficient solar thermal,
waste heat, and fuel-based generators.
It has been proposed recently that thermally assisted electroluminescence may in principle provide a means to convert solar or waste heat into electricity. The basic concept is to use an intermediate active emitter between a heat source and a photovoltaic (PV) cell. The active emitter would be a forward biased light emitting diode (LED) with a bias voltage, Vb, below bandgap, Eg (i.e., qVb < Eg), such that the average emitted photon energy is larger than the average energy that is required to create charge carriers. The basic requirement for this conversion mechanism is that the emitter can act as an optical refrigerator. For this process to work and be efficient, however, several materials challenges will need to be addressed and overcome. Here, we outline a preliminary analysis of the efficiency and conversion power density as a function of temperature, bandgap energy and bias voltage, by considering realistic high temperature radiative and non-radiative rates as well as radiative heat loss in the absorber/emitter. From this analysis, it appears that both the overall efficiency and net generated power increase with increasing bandgap energy and increasing temperature, at least for temperatures up to 1000 K, despite the fact that the internal quantum yield for radiative recombination decreases with increasing temperature. On the other hand, the escape efficiency is a crucial design parameter which needs to be optimized.
Metal components subjected to cyclic stress develop surface-evident defects (microcracks, slip bands, etc). Monitoring the formation and evolution of these fatigue damage precursors (FDPs) with increasing numbers of cycles can be an effective tool for determining the fatigue state of the component, which can be used in remaining fatigue life prognostics. In this paper a laser scanning technique for FDP detection is described and experimental results from examination of specimens of several metal types are presented. This technique is based on scanning a focused laser beam over the specimen surface and detecting variations in the characteristics of the scattered light signal. These variations can indicate the presence of surface abnormalities and therefore can be associated with fatigue damage formation. Particular patterns of spatial, angular, and optical characteristics can be used to identify and discriminate many types of FDP, which can provide a means to enhance the accuracy of surface defect frequency estimates and to eliminate the false counts that typically occur on surfaces in uncontrolled environments. Experiments during fatigue testing in the laboratory have shown that the technique can produce a defect frequency estimate that relates well to remaining fatigue life, but previous experiments showed large "plateau" regions, in which the slow defect frequency change made life estimation difficult. New data collection and analysis techniques have therefore been developed, and new experiments have been performed to test the ability of this modified approach to improve the utility of defect frequency measurements over the whole of fatigue life.
Metal components subjected to cyclic stress develop surface-evident defects (microcracks, slip bands, etc). Monitoring the formation and evolution of these fatigue damage precursors (FDPs) with increasing numbers of cycles can be an effective tool for determining the fatigue state of the component, which can be used in remaining fatigue life prognostics. In this paper a laser scanning technique (LST) for FDP detection is described and experimental results from examination of specimens made of nickel-based superalloy and aluminum are presented. The proposed detection technique is based
on scanning a focused laser beam over the specimen surface and detecting variations in spatial characteristics of the scattered light signal. These variations indicate the presence of surface abnormalities and therefore can be associated with incremental fatigue damage formation. The studies performed show that the proposed LST can serve as a basis for design of a portable non-contact instrument for in situ structural health monitoring.
In the past 30 years, the pattern and intensity of scattered light from small illuminated bio-features suspended in fluids have been analyzed to obtain information on feature size and composition. However, recent advances by the investigators related to rapid, non-invasive detection of sub-micron particles on silicon wafers in semiconductor processing suggest an attractive new approach to the problem of surface bio-feature detection and characterization. Our objective is to develop a rapid light scattering sensory method for the detection and identification of surface bacteria microcolonies to meet the needs for rapid identification techniques by the food and health industries. Scatterometer measurements of light scattering from Listeria monocytogenes ATCC 191 13 and Listeria innocua ATCC 33090 microcolonies on enriched agar plates has been performed. The prediction of light scattering from bacteria cells on surfaces has also been conducted using numerical modeling based on the discrete-dipole approximation. These studies show the variation of light scattering for various shaped and sized bacteria.
The detection of surface particles is an important part of contamination control in semiconductor manufacturing. However, the minimum particle size required to be detected has been becoming smaller as integrated-circuit geometries shrink. Current visible-light detection systems can detect particles down to around 50 nm in polystyrene-latex-equivalent size and so are adequate for current geometries, but in the near future even particles as small as around 20 nm in diameter will become significant contaminants. This is beyond the capability of current visible-light scanners, but previous work has shown that deep UV scattering by such particles should be sufficient to enable their detection. Consequently, we have constructed a deep/vacuum UV scatterometer capable of measuring scattering from semiconductor samples.
The detection of surface particles has become important in contamination control over the years. However, the minimum particle size required to be detected has been becoming smaller as IC geometries shrink. Current visible-light detection systems can detect particles down to around 60 nm in polystyrene-latex-equivalent size and so should be adequate for geometries down to around 0.18 micrometers , but a quick glance at the National Technology Roadmap for Semiconductors shows that geometries are expected to become as small as 0.07 micrometers in a little over ten years, requiring the ability to detect particles around 20 nm in diameter. This is beyond the capability of current visible-light scanners, so the Semiconductor Research Corporation has recently commissioned our group to conduct research into the limits of optical defect detection and potential of alternative detection techniques. This research centers on short-wavelength optical systems and scanned electron-beam systems as the most likely candidate technologies for high- speed nanoparticle detection. In this paper we develop a model for the analysis of the performance of hypothetical short-wavelength surface inspection systems and examine the manifold difficulties involved with using those wavelengths. The properties of scattering in the transitional region between the UV and X-ray regimes are also examined.