In the illusory-flash effect (Shams, L., Kamitani, Y., & Shimojo, S. (2000). Illusions. What you see is what you hear. Nature, 408, 788), one flash presented with two tones has a tendency to be seen as two flashes. Previous studies of this effect have been ill-equipped to establish whether this illusory-flash is the result of a genuine percept, or that of a shift in criterion. We addressed this issue by using a stimulus comprising two locations. This enabled contrast-threshold measurement by means of a location detection task. High-contrast white or black flashes were presented simultaneously to both locations, followed by threshold contrast flashes of the same contrast polarity at the two locations in half of the trials; observers reported whether or not the low-contrast flashes had been present. Irrelevant to the task, half of the trials contained one tone, the other half contained two tones. In this way, we were able to compute the change in sensitivity and shift in criterion between illusory and non-illusory trials. We observe both a decrease in visual sensitivity and a criterion shift in the illusory-flash conditions. In a second experiment, we were interested in determining whether this change in visual sensitivity gave rise to measurable visual attributes of the illusory-flash. If it has a contrast, it should interact with a spatio-temporally concurrent real flash. Using a similar two-location stimulus presentation, we found that under certain conditions, we were able to infer the polarity of the perceived illusory-flash. We conclude that the illusory-flash is indeed a perceptual effect with psychophysically assessable characteristics.
Abstract-We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO 2 and soot immediately behind the engine. The systems are designed to accommodate the largest civil aviation engines currently in service, without obstruction of the exhaust or bypass flows and with negligible effect upon the entrained flow behavior. We describe the physical construction and installation status of each system. In the case of the CO 2 system, we examine the challenges of achieving the structural rigidity necessary for adequate suppression of pointing error within 126 laser-based transmittance measurements, each utilizing a 7 m overall path length. We describe methods developed for efficient implementation of co-planarity and 4-degree-of-freedom alignment of individual paths within this beam array. We also present laboratory performance data for three alternative optical designs that differ in their approach to the management of pointing error and turbulence-induced beam wander and spread. The FLITES soot monitoring capability is based on laser induced incandescence (LII) and uses a short-pulse fiber laser and two CCD cameras, in an autoprojection arrangement. We describe the measurement geometry currently being implemented in the test cell and discuss optical design issues, including once again the effect of the plume itself.
Abstract-We report on the development of three systems intended to provide fast, non-intrusive measurement of cross sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.We describe a CO2 imaging system that performs wavelength modulation spectroscopy (WMS) simultaneously on 126 beam paths. Its novel architecture uses a Tm-doped fiber amplifier to generate sufficient optical power for the entire beam array (> 3 W) from a single 1997.2 nm diode-laser seed, reducing cost and enabling fully parallel detection and signal recovery. Various optical propagation issues are considered, including those arising from the varying degrees of interaction with the exhaust flow that exist within the beam array, as well as pointing errors arising from the limited rigidity of the measurement system's structure.We also report first steps towards a similar UHC measurement system, operating in the mid-infrared (MIR) region and targeting partially decomposed or oxidized fuel constituents, including formaldehyde and propene. Progress towards the chalcogenide glasses and fibers, needed for light delivery and/or amplification at these wavelengths is described. Finally, we report on the development status of a tomographic soot imaging system, based on laser induced incandescence (LII). We have demonstrated both long (192 ns) and short (17 ns) pulse variants of LII using fiber laser sources. Single path tests on a laboratory soot generator and, in the long pulse case, on a jet engine have confirmed that the energy and beam quality available from the fiber lasers is sufficient to enable an autoprojection approach, using just two intensified CCD cameras having 'near-orthogonal' views, with respect to the excitation laser.
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