High-precision underwater 3D cameras are required to automate many of the traditional subsea inspection, maintenance and repair (IMR) operations. In this paper we introduce a novel multi-frequency phase stepping (structured light) method for high-precision 3D estimation even in turbid water. We introduce an adaptive phase-unwrapping procedure which uses the phase-uncertainty to determine the highest frequency that can be reliably unwrapped. Light scattering adversely affects the phase estimate. We propose to remove the effect of forward scatter with an unsharp filter and a model-based method to remove the backscatter effect. Tests in varying turbidity show that the scatter correction removes the adverse effect of scatter on the phase estimates. The adaptive frequency unwrapping with scatter correction results in images with higher accuracy and precision and less phase unwrap errors than the Gray-Code Phase Stepping (GCPS) approach.

Raman based gas sensing can be attractive in several industrial applications, due to its multi-gas sensing capabilities and its ability to detect O₂ and N₂. In this article, we have built a Raman gas probe, based on low-cost components, which has shown an estimated detection limit of 0.5 % for 30 second measurements of N₂ and O₂. While this detection limit is higher than that of commercially available equipment, our estimated component cost is approximately one tenth of the price of commercially available equipment. The use of a resonant Fabry-Pérot cavity increases the scattered signal, and hence the sensitivity, by a factor of 50. The cavity is kept in resonance using a piezo-actuated mirror and a photodiode in a feedback loop. The system described in this article was made with minimum-cost components to demonstrate the low-cost principle. However, it is possible to decrease the detection limit using a higher-powered (but still low-cost) laser and improving the collection optics. By applying these improvements, the detection limit and estimated measurement precision will be sufficient for e.g. the monitoring of input gases in combustion processes, such as e.g. (bio-)gas power plants. In these processes, knowledge about gas compositions with 0.1 % (absolute) precision can help regulate and optimize process conditions. The system has the potential to provide a low-cost, industrial Raman sensor that is optimized for specific gas-detection applications.

Although Pb(Zr,Ti)O₃ (PZT) piezoelectric thin films are finding widespread applications on Earth, it is yet unclear if they are suitable for space-related applications. In space, their long-term reliability is a significant concern due to the difficulties of repairing and replacing malfunctioning devices. In this work, PZT thin film micromirrors for compact interferometric 3D imaging systems have been exposed to operating conditions encountered on a space mission and tested according to criteria set by the European Space Agency. Thermal cycling in vacuum, sinusoidal and random mechanical vibrations, and $\gamma$ -radiation with and without bias did not degrade key functional device properties of the micromirror such as angular deflection, resonance frequency, polarization, and permittivity. Apart from $\gamma$ -radiation, stressing the devices enhanced their large-signal angular deflection and improved their electrical lifetime compared to pristine devices. Their dielectric and ferroelectric characteristics remained comparable to that of a lab-scale environment. Simultaneously applying a 10 V field-down bias while $\gamma$ -radiating the micromirrors changed the capacitance-field and polarization-field characteristics and enhanced the electrical imprint. After stress-testing, the median time-to-failure in moderate acceleration conditions of 150 kV/cm and 175 °C ranged from 1.95 to 2.64 h, close to 2.11 h as measured for a reference group. All actuator membranes had shorter electrical lifetimes, smaller voltage acceleration factors, and smaller activation energies, ranging from 2.56 to 2.88 V⁻¹ and 1.03 to 1.09 eV, than simple bonding pads. This work is a device-level report covering a full set of space-relevant tests demonstrating that PZT-based thin film piezomicroelectromechanical systems technology is space-ready.

We have built and characterized a compact, simple and flexible 3D camera based on interferometric fringe projection and stereo reconstruction. The camera uses multi-frame active stereo as basis for 3D reconstruction, providing full-field 3D images with 3D measurement standard deviation of 0.09 mm, 12.5 Hz 3D image capture rate and 3D image resolution of 500 × 500 pixels. Interferometric projection enables a compact, low-power projector that consumes < 1 W of electrical power. The key component in the projector, a movable micromirror, has undergone initial vibration, thermal vacuum cycling (TVAC) and radiation testing, with no observed component degradation. The system's low power, small size and component longevity makes it well suitable for space applications.

In this work, we have investigated the properties of light transport in turbid water for underwater 3D imaging, by comparing literature, simulations, and experiments. The work is motivated by a need for high-quality 3D image data for underwater robotics, also in turbid waters. We present a comparative analysis of how sine wave and line patterns degrade upon propagation through turbid water, and explain the difference in degradation through analysis of the signal response of the water. We find that line patterns provide a sharp signal peak from the un-scattered (ballistic) photons, with a very broad skirt of scattered signal. The detection of the un-scattered peak allows for sub-millimeter 3D reconstruction quality at water turbidities up to 4.8 NTU (nephelometric turbidity units). Using sine waves, 3D precision rapidly drops, as the scattered signal from large parts of the scene causes rapid loss of signal contrast. 3D reconstruction fails at turbidities above 3.1 NTU. In dark parts of the scene, both signal types yield incorrect 3D reconstruction, as forward scattered signal from surrounding brighter regions dominate over the correct signal peak. In line with literature on scattering properties of water, we find that the use of long wavelengths (red signal) gives less scattering and higher signal contrast compared with the use of green and blue, thereby providing lower 3D noise.

A number of common eye diseases can be diagnosed using retina images. A prominent example is diabetic retinopathy, which is a leading cause of vision loss in many developed countries.¹ Other examples include glaucoma,² age-related macular degeneration,³ retinopathy of prematurity,⁴ and malarial retinopathy.⁵ Early screening of these diseases can simplify treatment and prevent complications leading to blindness.⁶ Currently, this type of screening is performed at hospitals or other specialized institutions using expensive fundus camera systems that capture high-resolution retina images. In countries where this type of healthcare service is not available for the majority of the population, such screening is simply not possible. Therefore, a compact, handheld, low-cost, and easy-to-use device that enables early screening, would increase the accessibility of this technology and enable widespread early diagnosis.