Structured Laser Illumination Planar Imaging: New horizons for the study of spray dynamics, thermometry and droplet sizing

Thefirst reported applications of laser sheet imaging for the study of atomizing sprays date from the mid 1980th.Since those early tries, blurring effects from the light being scattered multiple times by the surrounding dropletswere already observed and reported. While s trategies in suppressing part of this multiple light scatteringcontribution, were known for transmission imaging (e. spatial Fourier filtering, polarization filtering, time gating)no robust solution was found for laser sheet imaging until the developme nt of Structured Laser Illumination PlanarImaging ( in 2008.Theoriginality of SLIPI resides in using a laser sheet with a spatially modulated light intensity. This light structureencodes the incident illumination beam which can be then decoded a fter image recording. As multiply scatteredphotons have “short memory” they do lose the modulation information while singly scattered photons fullypreserve it. Therefore the two components, single and multiple scattering, can be separated using a demodul ationpost processing algorithm on the recorded images.Thisarticle is a review of the SLIPI technique from its earliest to latest developments. It describes the traditional wayof applying structured illumination using three modulated sub images and its use for averaged imaging of dropletsizes, extinction coefficients and spray thermometry. Recently the technique has been adapted for single shotapplications in order to study spray dynamics and liquid breakups. This is performed using two modulated subimage s instead of three, known as 2p SLIPI, opening new horizons for the study of spray dynamics, single shotdropl et sizing and thermometry, even through optically dense situations

Detailed visualization of spray dynamics using Light Sheet Fluorescence Microscopic imaging

We demonstrate the use of Light Sheet Fluorescence Microscopic (LSFM) imaging for viewing the dynamic of atomizing sprays with high contrast and resolution. The technique presents several advantages: first liquid fluorescence gives a more faithful representation of the structure of liquid bodies, droplets and ligaments than Mie scattering does. The reason for this is that the signal is emitted by the fluorescing dye molecules inside the liquid itself and not generated at the air-liquid interfaces. Second, despite the short depth of field (~0.2 mm) obtained when using the long range microscope, the contribution of out-of-focus light is much smaller on a light sheet than on a line-of-sight configuration providing more clearly sectioned images. Finally by positioning the light sheet on the spray periphery, toward the camera objective, the effects due to multiple light scattering phenomena can be reduced to some extent. All those features provide, for many spray situations, images with high fidelity of the liquid fluid, allowing the extraction of the velocity vectors at the liquid boundaries. Here, double frame images were recorded with a sCMOS camera with a time delay of 5 μs between exposures. A typical pressure-swirl atomizer is used here producing a water hollow-cone spray which was imaged between 20 bars and 100 bars in liquid injection pressure. Such data are important for the validation of CFD models simulating liquid breakups in the near-field spray region

Crossed patterned structured illumination for the analysis and velocimetry of transient turbid media

Imaging through turbid environments is experimentally challenging due to multiple light scattering. Structured laser illumination has proven to be effective to minimize errors arising from this phenomenon, allowing the interior of optically dense media to be observed. However, in order to preserve the image spatial resolution while suppressing the intensity contribution from multiple light scattering, the method relies on multiple acquisitions and thus sequential illumination. These requirements significantly limit the usefulness of structured illumination when imaging highly transient events. Here we present a method for achieving snapshot visualizations using structured illumination, where the spatial frequency domain is increased by a factor of two compared to past structured illumination snapshots. Our approach uses two crossed intensity-modulated patterns, allowing us to expand the spatial frequency response of the extracted data. The snapshot capability of this imaging approach allows tracking single particles and opens up for the extraction of velocity vectors by combining it with standard particle tracking/image velocimetry (PTV or PIV) equipment. In this paper we demonstrate the capabilities of this new method and, for the first time, use structured illumination to extract velocity vectors in 2D in a transient turbid medium, in this case an optically dense atomizing spray

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

Reliable LIF/Mie droplet sizing in sprays using structured laser illumination planar imaging

In this article, Structured Laser Illumination Planar Imaging (SLIPI) is used in combination with the LIF/Mie ratio technique for extracting a reliable two-dimensional mapping of the droplets Sauter Mean Diameter (SMD). We show that even for the case of a fairly dilute spray, where single scattering events are in majority, the conventional LIF/Mie technique still remains largely affected by errors introduced by multiple light scattering. To remove this unwanted light intensity on both the LIF and Mie images SLIPI is used prior to apply the image ratio. For the first time, the SLIPI LIF/Mie results are calibrated and compared with measurement data from Phase Doppler Interferometry (PDI)

Two-pulse structured illumination imaging

Structured illumination (SI), which is an imaging technique that is employed in a variety of fields, permits unique possibilities to suppress unwanted signal contributions that carry misguiding information such as out-of-focus light or multiply scattered light. So far SI has been applied mostly for averaged imaging or for imaging of slowly occurring events because it requires three acquisitions (subimages) to construct the final SI image. This prerequisite puts technological constraints on SI that make "instantaneous" imaging of fast transient processes (occurring on sub-microsecond time scales) very challenging and expensive. Operating SI with fewer subimages generates errors in the form of residual lines that stretch across the image. Here, a new approach that circumvents this limiting factor is presented and experimentally demonstrated. By judiciously choosing the intensity modulation, it is possible to extract an SI image from two subimages only. This development will allow standard double-pulsed lasers and interline transfer CCD or scientific CMOS cameras to be used to acquire temporally frozen SI images of rapidly occurring processes as well as to boost the frame-rate of current SI video systems; a technical advancement that will benefit both macro-and microscopic imaging applications. (C) 2014 Optical Society of Americ

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

High-contrast imaging through scattering media using structured illumination and Fourier filtering

We show in this Letter a novel approach for high-contrast imaging through scattering media by combining structured illumination and Fourier filtering (SIF). To assess the image contrast enhancement at different image spatial frequencies, the modulation transfer function is calculated for four detection schemes: (1) no filtering, (2) Fourier filtering, (3) structured illumination, and (4) SIF filtering. A scattering solution consisting of D = 7.3 μm polystyrene spheres immersed in distilled water and illuminated at λ = 671 nm is used here. We demonstrate the possibility of obtaining, with SIF, an image contrast up to 60% at an optical depth of OD = 10, improving the contrast by a factor of 40 over conventional transmission imaging

3D droplet sizing and 2D optical depth measurements in sprays using SLIPI based techniques

While imaging optically dense media such as atomizing sprays, the multiple light scattering induces image artifacts and blurring effects which limit visibility.Therefore, extracting quantitative spray information such as droplet size and concentration from qualitative images becomes very challenging. However, multiple scattering effects can be efficiently addressed by means of the SLIPI (Structured Laser Illumination Planar Imaging) technique. Recently, using SLIPI in combination with LIF/Mie droplet sizing (ratio of the liquid Laser Induced Fluorescence (LIF) and Mie scattering signals), a mapping of absolute Sauter Mean Diameter (SMD or D32) could be extracted. It was observed that without SLIPI, reliable measurements of SMD could not be achieved. In another work, a 3D map of the droplet extinction-coefficient (µe) in an aerated spray was extracted using the SLIPI-scan technique. In this article, SLIPI-LIF/Mie droplet sizing is performed in combination with SLIPI-scan in order to construct a 3D representation of droplet SMD in the developed spray region and the corresponding optical depth in 2D

Stereoscopic high-speed imaging for 3D tracking of coughed saliva droplets in the context of COVID-19 spreading

Droplets generated by talking and coughing play a major role in the spreading of COVID-19. There is thus a need for accurate measurements of the physical properties of exhaled droplets, including their number, speed and direction.Several challenges are associated with imaging coughed droplets such as high droplet speed near the mouth where both short exposure time to freeze droplet motion and kHz recording rate to resolve their displacement is required. In addition, as a highly non-symmetrical spray system is formed from a cough, three-dimensional visualization is necessary to faithfully capture coughing events. In this work, a 3D, high-speed imaging technique is presented that facilitates such challenging measurements. A laser beam with a probe volume 15 mm thick - 120 mm high is formed and illuminates droplets exiting the mouth imaged using two high-speed cameras. Data has been recorded for four different male subjects where 3D droplet speed and direction has been extracted for 10 coughs each. The maximum speed for a single cough has been estimated to vary between 11 and 45 m/s and the average droplet speed has been found to be in the range 6.5 - 8.7 m/s. These results will be used as input parameters to improve simulation models of droplet transport in the context of virus spreading