Quantitative 3D imaging of scattering media using structured illumination and computed tomography

An imaging technique capable of measuring the extinction coefficient in 3D is presented and demonstrated on various scattering media. The approach is able to suppress unwanted effects due to both multiple scattering and light extinction, which, in turbid situations, seriously hampers the performance of conventional imaging techniques. The main concept consists in illuminating the sample of interest with a light source that is spatially modulated in both the vertical and horizontal direction and to measure, using Structured Illumination, the correct transmission in 2D at several viewing angles. The sample is then reconstructed in 3D by means of a standard Computed Tomography algorithm. To create the adequate illumination, a novel "crossed" structured illumination approach is implemented. In this article, the accuracy and limitation of the method is first evaluated by probing several homogeneous milk solutions at various levels of turbidity. The unique possibility of visualizing an object hidden within such solutions is also demonstrated. Finally the method is applied on two different inhomogeneous scattering spray systems; one transient and one quasi-steady state. (C) 2012 Optical Society of Americ

Quantitative SO2 Detection in Combustion Environments Using Broad Band Ultraviolet Absorption and Laser-Induced Fluorescence

Spectrally resolved ultraviolet (UV) absorption cross sections of SO2 in combustion environments at temperatures from 1120 to 1950 K were measured for the first time in well-controlled conditions through applying broad band UV absorption spectroscopy in specially designed one-dimensional laminar flat flames. The temperature was observed to have a significant effect on the absorption cross-section profiles at wavelength shorter than 260 nm, while at the longer wavelength side, the absorption cross-section profiles have much less dependence on temperature. The absorption cross section at 277.8 nm with a value of 0.68 × 10-18 cm2/molecule was suggested for the evaluation of the SO2 concentration because of the weak dependence on temperature. To make spatially resolved measurements, laser-induced fluorescence (LIF) of SO2 excited by a 266 nm laser was investigated. Spectrally resolved LIF signal was analyzed at different temperatures. The LIF signal showed strong dependence on temperature, which can potentially be used for temperature measurements. At elevated temperatures, spatially resolved LIF SO2 detection up to a few ppm sensitivity was achieved. Combining UV broad band absorption spectroscopy and LIF, highly sensitive and spatially resolved quantitative measurements of SO2 in the combustion environment can be achieved

Extinction coefficient imaging of turbid media using dual structured laser illumination planar imaging

We demonstrate a technique, named dual structured laser illumination planar imaging (SLIPI), capable of acquiring depth-resolved images of the extinction coefficient. This is achieved by first suppressing the multiply scattered light intensity and then measuring the intensity reduction caused by signal attenuation between two laser sheets separated by Delta zmm. Unlike other methods also able to measure this quantity, the presented approach is based solely on side-scattering detection. The main advantages of dual SLIPI is that it accounts for multiple scattering, provides two-dimensional information, and can be applied on inhomogeneous media. (C) 2011 Optical Society of Americ

Development and demonstration of 2D-LIF for studies of mixture preparation in SI engines

Laser-induced fluorescence (LIF) has been developed for visualization of fuel distribution fields in an operating spark-ignition (SI) engine. Since the standard research fuel iso-octane, does not yield a useful LIF signal a fluorescent additive was used. None of the commonly used seeds were found adequate. A seed not commonly used in this context, 3-pentanone, C2H5COC2H5, was chosen due to favorable vaporization characteristics and fluorescent properties. Results from preparatory investigations in the actual engine environment are presented and related laboratory data are discussed. The two-dimensional LIF technique was applied to a spark-ignition engine and the fuel distribution at the ignition time was recorded. The resulting images were processed and converted into fuel/air equivalence ratio using an in situ calibration technique. The processed fuel distribution maps presented a noise level of 10% and a systematic error not exceeding 0.03 fuel/air equivalence units. An increased combustion variability was observed when changing from a homogeneous to an inhomogeneous fuel/air mixture. Correlations of image data to the combustion development indicated that the increased cyclic variability could be largely explained by variations in the mean fuel concentration around the spark gap. The initial flame development therefore seems to be controlled by the average amount of fuel near the spark gap, whereas the actual distribution of the fuel within this volume is of less importance