Photometric stereo is a three dimensional (3D) imaging technique that uses multiple 2D images, obtained from a fixed camera perspective, with different illumination directions. Compared to other 3D imaging methods such as geometry modeling and 3D-scanning, it comes with a number of advantages, such as having a simple and efficient reconstruction routine. In this work, we describe a low-cost accessory to a commercial digital single-lens reflex (DSLR) camera system allowing fast reconstruction of 3D objects using photometric stereo. The accessory consists of four white LED lights fixed to the lens of a commercial DSLR camera and a USB programmable controller board to sequentially control the illumination. 3D images are derived for different objects with varying geometric complexity and results are presented, showing a typical height error of <3 mm for a 50 mm sized object.
Aggregation of bacteria plays a key role in the formation of many biofilms. The critical first step is cell–cell approach, and yet the ability of bacteria to control the likelihood of aggregation during this primary phase is unknown. Here, we use optical tweezers to measure the force between isolated Bacillus subtilis cells during approach. As we move the bacteria towards each other, cell motility (bacterial swimming) initiates the generation of repulsive forces at bacterial separations of ~3 μm. Moreover, the motile response displays spatial sensitivity with greater cell–cell repulsion evident as inter-bacterial distances decrease. To examine the environmental influence on the inter-bacterial forces, we perform the experiment with bacteria suspended in Tryptic Soy Broth, NaCl solution and deionised water. Our experiments demonstrate that repulsive forces are strongest in systems that inhibit biofilm formation (Tryptic Soy Broth), while attractive forces are weak and rare, even in systems where biofilms develop (NaCl solution). These results reveal that bacteria are able to control the likelihood of aggregation during the approach phase through a discretely modulated motile response. Clearly, the force-generating motility we observe during approach promotes biofilm prevention, rather than biofilm formation.
Abstract:High-speed video stereo-microscopy relies on illumination from two distinct angles to create two views of a sample from different directions. The 3D trajectory of a microscopic object can then be reconstructed using parallax to combine 2D measurements of its position in each image. In this work, we evaluate the accuracy of 3D particle tracking using this technique, by extending the number of views from two to four directions. This allows us to record two independent sets of measurements of the 3D coordinates of tracked objects, and comparison of these enables measurement and minimisation of the tracking error in all dimensions. We demonstrate the method by tracking the motion of an optically trapped micro-sphere of 5 µm in diameter, and find an accuracy of 2-5 nm laterally, and 5-10 nm axially, representing a relative error of less than 2.5% of its range of motion in each dimension. Simpson, "An optically actuated surface scanning probe," Opt. Express 20, 29679-29693 (2012). 12. C. Alpmann, R. Bowman, M. Woerdemann, M. Padgett, and C. Denz, "Mathieu beams as versatile light moulds for 3d micro particle assemblies," Opt. Express 18, 26084-26091 (2010
When a microscopic particle moves through a low Reynolds number fluid, it creates a flow-field which exerts hydrodynamic forces on surrounding particles. In this work we study the 'Lissajous-like' trajectories of an optically trapped 'probe' microsphere as it is subjected to timevarying oscillatory hydrodynamic flow-fields created by a nearby moving particle (the 'actuator'). We show a breaking of time-reversal symmetry in the motion of the probe when the driving motion of the actuator is itself time-reversal symmetric. This symmetry breaking results in a fluidpumping effect, which arises due to the action of both a time-dependent hydrodynamic flow and a position-dependent optical restoring force, which together determine the trajectory of the probe particle. We study this situation experimentally, and show that the form of the trajectories observed is in good agreement with Stokesian dynamics simulations. Our results are related to the techniques of active micro-rheology and flow measurement, and also highlight how the mere presence of an optical trap can perturb the environment it is in place to measure.
<p>Background Prelabour rupture of membranes at term (PROM) is a subject of interest to women and maternity care providers alike. Management of PROM varies internationally, and regionally within New Zealand, despite the presence of interprofessional consensus statements. Northland District Health Board (NDHB) policy differs from most maternity care facilities by enabling expectant management of labour for women at low risk of transmission of Group β Streptococcus to their baby to extend to 96 hours from time of rupture of membranes. This study aimed to explore whether the NDHB policy was applied in practice and safely served the needs of women and babies in this DHB. Methods A retrospective quantitative clinical notes review was conducted of files in a one-year sample. The clinical notes of 123 women who had duration from ROM to birth at term of 18 hours or more were reviewed. Statistical comparisons using percentages, means and odds-ratios were made to a one-year sample of all other births at term at NDHB facilities, and with birth data from the New Zealand College of Midwives Clinical Outcomes Research Database (COMCORD). Findings Variables including demographic data, antenatal and intrapartum care given, time factors and outcomes themselves were reviewed. Few variables impacted outcomes within the PROM sample, though use of intravenous oxytocin was associated with increased epidural use and increased incidence of instrumental birth. Wāhine Māori had an increased incidence of vaginal births but some variables and outcomes highlighted inequities, including reduced antenatal screening, a higher incidence of maternal smoking, severe postpartum haemorrhage, and admission to Special Care Baby Units. Duration splits at 48 and 96 hours were applied to identify whether time increased risks for women or babies, but duration did not appear to be a factor which increased risk. The phenomenon of PROM itself increased risk for women and babies, increasing rates of labour induction, augmentation, epidural use, operative births, postpartum haemorrhage, admission to Special Care Baby Units and decreased exclusive breastfeeding at discharge. Conclusions The NDHB PROM policy appeared to be applied consistently and did not appear to increase risk for women and babies. Risks were increased for all women with PROM, with some inequitable outcomes for wahine Māori and pēpi. This research provides a comparison which is informative for clinical practice, education, and future research, and supports women’s involvement in decision-making.</p>
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