Volume 8, Issue 6 - Active Imaging / Guest Editors: Martin Laurenzis, Frank Christnacher

In 2019, the French-German Research Institute of Saint-Louis (ISL) is celebrating its 60th anniversary. Since the beginning, advanced imaging technologies were one of the institute’s flagship areas of research and, from the Cranz-Schardin camera to computational optics, ISL never stopped innovating. Each technological innovation is a testimony to its time, and the research works in visionics make no exception to this rule. Each decade was marked by innovations that made it possible to develop means of vision or visualization, which ensure that our institute remains at the forefront of the research in this field. High-speed cameras, holography, lasers, or active imaging systems developed at ISL are examples of this. The science of photon, photonics, still has a bright future ahead, and there is no doubt that the latest discoveries and technological advances in this field will be applied to systems that will allow our armed forces to maintain their technological superiority and our soldiers to carry out their missions with greater security.

In this article the laser-based technologies for remote sensing that use laser radar principles, but still known under other names like photonics radar, optical coherence tomography, aberrometry, etc., are reviewed A look into some examples of a variety of such nick-named instruments and technologies is proposed assessing the arguments of them, being the members of the laser radar family.

The recent availability of imaging sensors able to work in accumulation mode in the SWIR spectral range allows the realization of new efficient range-gated viewing systems. Such systems relax the illumination constraint so that the energy required to build an image can be distributed over several laser pulses. Semiconductor or fiber lasers can be used instead of high peak power solid state lasers. Such a system was realized in our laboratory, tested, and compared to a more classical flash system under outdoor conditions. In a first step, images of the same scenes recorded in the same weather conditions were compared to those recorded with a classical system working in flash mode. The MTF analysis shows an improvement of up to 40% with the system working in accumulation mode. In order to remove the influence of two different laser sources as well as of two different cameras, a second experiment was conducted. For this purpose, a shorter range and only one system were employed. Both operating modes, the flash and the accumulation mode, were examined. The second experiment confirms that accumulation mode can decrease significantly the value of the scintillation index resulting in a higher resistance to optical perturbations. These results increase the relevance of the accumulation mode for active imaging applications in the SWIR spectral region.

Short-wavelength infrared (SWIR) detection systems are increasingly in demand for surveillance, reconnaissance, and remote sensing applications. Eye-safe SWIR lasers can be utilized for active imaging systems with high image contrast and long detection range. The gated-viewing (GV) technique using short-pulse lasers and fast-gated cameras in the nanosecond range enables utilizing the distance information in addition to the signal intensity of the acquired images. The InGaAs material system is very well suited for the fabrication of high-performance SWIR photodetectors providing a typical cutoff wavelength of 1.7 μm, which covers the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, the usually short integration times needed for GV leads to very small photosignals. We report on the development of SWIR avalanche photodetector (APD) arrays with 640 × 512 pixels and 15 μm pixel pitch based on the InGaAs material system. The InGaAs-APD focal plane arrays have been successfully integrated into SWIR cameras which yield gain values of M  ≈ 10 on camera level at a reverse bias voltage around 21 V and are the first InGaAs-based SWIR cameras worldwide providing a 640 × 512 image format and utilizing avalanche gain for signal amplification. The camera performance is demonstrated by SWIR laser GV sample images.

In the topic of 2D active imaging systems, two technologies exist for image acquisition. The flash mode consists of a very short and intense laser shot, associated with a short time integration of the sensor (range of hundreds of nanoseconds). The second is the cumulative mode which consists of the integration of many low level energy laser pulses over a long-time integration of the sensor (range of tens of milliseconds). Cumulative mode systems have existed for a long time in the near infrared band. But for the past few years, new sensors are available in the short wave infrared (SWIR) band. Cumulative mode in the SWIR band can provide 2D active imaging systems with very low risk considering the eye safety aspects. Moreover, with a similar design, cumulative systems can overcome the range of flash systems, thanks to their ability to average turbulence effects over the sensor integration time. So, in this paper we have proposed a scintillation noise comparison for each mode. First, we exposed the two types of available models, a numerical model, used for image generation. Second one, an analytical model, used for a quick evaluation of the design of a 2D active imaging system. Both models were compared, especially in their area of validity. Then for a specific scenario, we estimated the gain in term of range performance between a cumulative and a flash system.

Underwater laser-based imaging systems and data-processing techniques matured during the past decade. Active imaging systems can, nowadays, be integrated into platforms like remote-operated vehicles (ROV) or autonomous underwater vehicles (AUV). This article gives an overview of different civil and naval applications in underwater imaging with respect to underwater laser scanning (ULS) and laser gated viewing (LGV). Special emphasis has to be given to the environmental conditions, for example, the influence of the local and seasonal dependence of the turbidity with regard to the optical underwater channel. On the basis of tank and sea experiments, advanced techniques for 3D laser oblique scanning (LOS) and possibilities of contrast enhancements for gated viewing are presented.

Reflectors and lenses are common optical components used for image formation and the creation of beam patterns and light distributions. While lenses dominate imaging optics in many applications such as cameras and microscopes, reflectors are widely used in lighting and illumination optics. This article presents an overview of reflectors, including common reflector materials, modern design approaches, and applications. This article does not include information about physical and mathematical properties of reflectors; this information is thoroughly covered in optics industry and academic publications. There will be archetypes and examples discussed in the various sections of this article – neither is there any claim for completeness nor are these meant to be absolute. The intention of this article was to draw a comprehensive horizon around reflector applications and most of all their designs for lighting applications.

The transition between the light and dark areas of the luminous intensity distribution of a headlamp needs to fulfil statutory regulations. For projection headlamps, adjusting the transition is done by adding a scattering structure to the lens surface. The requirements for the transition are pointed out, and typical problems are presented. A procedure to create such scattering structures in computer-aided design is shown. Improvements to the controls of turning machines for manufacturing are discussed. A reverse engineering process using a high-precision cylindrical coordinate measuring instrument with an optical probe for quality assurance is presented.

We proposed a fabrication of nanoimprinted textures on a front glass/transparent conductive oxide interface for dye-sensitized solar cells (DSSCs). These textures were fabricated through polydimethylsiloxane (PDMS) nanoimprint lithography on organosilsesquioxane solution. The texture structures were estimated via optical simulation. Master molds were anodic aluminum oxide templates with nano-texture (N-Tx) and micro-nano double texture (D-Tx). Meanwhile, replicate molds used a hard PDMS. Fluorine-doped tin oxide and titanium dioxide were deposited on textured glass substrates to generate electrodes for DSSCs. Unlike the DSSCs without texture, textured DSSCs realized 11.4% (N-Tx) and 10% (D-Tx) improvement in conversion efficiency.