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Significance: The use of the transscleral illumination approach has the potential to simplify the optical design of fundus cameras. In particular, this approach could allow the use of smaller and cheaper cameras that are easier to use by non-specialists, thereby facilitating a wider spread of eye disease screening programs. Aim: Our aim was to investigate the suitability of transscleral illumination in a fundus camera system. In particular, we explored the impact of the illumination spectrum and the eye pigmentation on the quality of the image. These factors have never been systematically investigated before in the literature on transscleral illumination. Approach: A fundus camera was constructed using transscleral illumination. We studied the influence of eye pigmentation and choice of illumination spectra on the image quality for a group of 10 individuals with varied skin pigmentation, ranging from pale white (North-European) to darkest brown (African). The influence of the light source spectrum on the image quality was assessed using wavelength filters. Results: There was a difference of a factor of 100 in the signal level of retinal images between individuals with low and high skin pigmentation. The image contrast was highest using illumination wavelengths of 500 to 600 nm. The illumination level can be adjusted to obtain high-quality images for highly pigmented eyes while keeping the system eye-safe. Conclusions: We have demonstrated that a fundus camera with transscleral illumination can provide high-quality images. However, the variations observed in scleral and retinal pigmentation in a practical setting require a system that must be able to adapt illumination and/or exposure to the individual patient. [ABSTRACT FROM AUTHOR]

A correction to the article "Temperature Dependent Ablation Threshold in Silicon Using Ultrashort Laser Pulses" that was published in the November 2012 issue is presented.

We have experimentally investigated the ablation threshold in silicon as a function of temperature when applying ultrashort laser pulses at three wavelengths. By varying the temperature of a silicon substrate from room temperature to 320 °C, we observe that the ablation threshold for a 3 ps pulse using a wavelength of 1030 nm drops from 0.43 J/cm2 to 0.24 J/cm2, a reduction of 43%. For a wavelength of 515 nm, the ablation threshold drops from 0.22 J/cm2 to 0.15 J/cm2, a reduction of 35%. The observed ablation threshold for pulses at 343 nm remains constant with temperature, at 0.10 J/cm2. These results indicate that substrate heating is a useful technique for lowering the ablation threshold in industrial silicon processing using ultrashort laser pulses in the IR or visible wavelength range. In order to investigate and explain the observed trends, we apply the two-temperature model, a thermodynamic model for investigation of the interaction between silicon and ultrashort laser pulses. Applying the two-temperature model implies thermal equilibrium between optical and acoustic phonons. On the time scales encountered herein, this need not be the case. However, as discussed in the article, the two-temperature model provides valuable insight into the physical processes governing the interaction between the laser light and the silicon. The simulations indicate that ablation occurs when the number density of excited electrons reaches the critical electron density, while the lattice remains well below vaporization temperature. The simulated laser fluence required to reach critical electron density is also found to be temperature dependent. The dominant contributor to increased electron density is, in the majority of the investigated cases, the linear absorption coefficient. Two-photon absorption and impact ionization also generate carriers, but to a lesser extent. As the linear absorption coefficient is temperature dependent, we find that the simulated reduction in ablation threshold with increased substrate temperature is linked to the temperature dependence of the linear absorption coefficient. Another factor influencing the ablation threshold is the wavelength dependence of the interaction with the excited electron plasma. This wavelength dependence can explain that we observe experimentally similar ablation thresholds for a wavelength of 1030 nm at 320 °C and for 515 nm at room temperature, even though the linear absorption coefficient in the latter case is much higher. [ABSTRACT FROM AUTHOR]

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 γ -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 γ -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 γ -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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]