The use of complementary wavelengths bands in camera systems is a long known principle. The camera system’s spectral range is split in several spectral channels, where each channel possesses its own imaging sensor. Such an optical setup is used, for example, in high-quality 3-sensor colour cameras. A 3-sensor camera is less vulnerable to laser dazzle than a single-sensor camera. However, the separation of the individual channels is not sufficient high enough to suppress crosstalk and thus all three channels will suffer from laser dazzling. To solve that problem, we suggest two different optical designs in which the spectral separation of the channels is significantly increased. The first optical design is a 3-channel camera system, which was already presented earlier. The second design is a 2-channel camera system based on optical multiband elements, which delivers undisturbed colour images even under laser dazzle.
Based on our previous work1, we further investigate the laser-induced damage effects on digital micromirror devices (DMD) in comparison to different electro-optical imaging sensors such as complementary metal-oxide-semiconductors (CMOS) and charge-coupled devices (CCD). In our earlier work, we reported on damage thresholds obtained by pulsed laser radiation of nanosecond pulse width and by continuous-wave laser radiation utilizing irradiation times ranging from 250 milliseconds up to 10 seconds. The main objective of our current work is to fill the gap regarding the time scale of picosecond pulses. In the course of this research, we enhanced the experimental setup and we explicitly describe the achieved improvements in this work. Furthermore, we characterize the damage caused by laser pulse energies exceeding the laser-induced damage threshold (LIDT).
In October 2018, NATO SET-249 performed a common trial at WTD 52, Oberjettenberg, Germany, to study laser dazzle effects in an airborne scenario. The facility is equipped with a cable car and is ideal for slanted path experiments from the base station to the cable car where the sensors were mounted. NATO SET-249’s background is laser threat evaluation and the evaluation of the impact of laser eye dazzle on the visual performance of humans. This work gives an overview on the various measurements performed here: 1. Assessment of dazzle effects originating from light scattering at an aircraft canopy by comparing the images of two cameras: one outside and one inside the canopy. The general findings showed that the canopy, which had been used previously on an aircraft, substantially affected the dazzle pattern in the camera within the canopy as compared to the camera outside. 2. Sensor dazzling: Laser dazzling of complementary metal-oxide-semiconductor (CMOS) cameras in the visible domain and, in addition, laser dazzling of a camera equipped with a fisheye lens, which is commonly present in micro-unmanned aerial vehicles, is demonstrated. The dazzled area in the camera field of view (FoV) grows with increasing laser irradiance, and dazzling is effective at irradiance levels around a few μW/cm². 3. An overview on realistic handheld laser engagement scenarios to test the capabilities of a DSTL-developed Laser Event Recorder (LER) is provided. This technology is able to detect continuous wave (CW) and pulsed lasers, and extract their wavelengths, irradiances, Pulse Repetition Frequency (PRF) and directionality. Applications for this LER include collecting information on aircraft laser exposure events, giving information to assess if engagements are eye safe. 4. Measurements performed on various Fraunhofer IOSB developed sensor systems hardened against laser dazzle: The hardening measure of these systems is based either on the use of spatial light modulators or on the implementation of the principle of complementary wavelength bands. The field trial offered the possibility to generate data of the hardened systems under real life conditions.
We present an optical concept for imaging sensor systems, designed to considerably reduce the sensor’s image information loss in cases of laser dazzle, based on the principle of complementary bands. For this purpose, the sensor system’s spectral range is split in several (at least two) spectral channels, where each channel possesses its own imaging sensor. This long-known principle is applied, for example, in high-quality three-sensor color cameras. However, in such camera systems, the spectral separation between the different spectral bands is too poor to prevent complete sensor saturation when illuminated with intense laser radiation. We increased the channel separation by orders of magnitude by implementing advanced optical elements. Thus, monochromatic radiation of a dazzle laser mainly influences the dedicated transmitting spectral channel. The other (out-of-band) spectral channels are not or—depending on the laser power—only hardly affected. We present our system design as well as a performance evaluation of the sensor concerning laser dazzle.
We present an optical concept for imaging sensor systems, designed to reduce considerably the sensor’s image information loss in cases of laser dazzle, based on the principle of complementary bands. For this purpose, the sensor system’s spectral range is split in several (at least two) spectral channels, where each channel possesses its own imaging sensor. This long-known principle is applied, for example, in high-quality 3-sensor colour cameras. However, in such camera systems, the spectral separation between the different spectral bands is far too poor to prevent complete sensor saturation when illuminated with intense laser radiation. We increased the channel separation by orders of magnitude by implementing advanced optical elements. Thus, monochromatic radiation of a dazzle laser mainly impacts the dedicated transmitting spectral channel. The other (out-ofband) spectral channels are not or – depending on the laser power – only hardly affected. In this paper, we present our system design as well as a performance evaluation of the sensor concerning laser dazzle.
Based on previous work on thermal imager performance analysis at Fraunhofer IOSB using specific scenes and patterns, we present our advances in setting up a testbed for thermal imager characterization with a MIRAGE™ XL infrared scene projector.<p> </p> In the first part, we outline the experimental setup of our testbed. It allows for mimicking infrared imaging of real scenes in a controlled laboratory environment. We describe the process of dynamic infrared scene generation as well as the physical limitations of our scene projection setup.<p> </p> A second part discusses ongoing and future applications. This testbed extends our standard lab measurements for thermal imagers by a image based performance analysis method. Scene based methods are necessary to investigate and assess advanced digital signal processing (ADSP) algorithms which are becoming an integral part of thermal imagers. We use this testbed to look into inferences of unknown proprietary ADSP algorithms by choosing suitable test scenes.<p> </p> Furthermore, we investigate the influence of dazzling on thermal imagers by coupling infrared laser radiation into the projected scene. The studies allow to evaluate the potential and hazards of infrared dazzling and to describe correlated effects. In a future step, we want to transfer our knowledge of VIS/NIR laser protection into the infrared regime.
Fraunhofer IOSB (Ettlingen, Germany) developed and built a measurement system to verify laser threat detection. The system has been given the name MARLA (Maritime Lasermessanlage), eng.: maritime laser measurement system. It is an integral part of an exercise and test range for electronic warfare of the German Navy at Wehrtechnische Dienststelle für Schiffe und Marinewaffen, Maritime Technologie und Forschung WTD 71, Eckernförde, Germany.<p> </p> The system provides realistic simulations of various laser-based threats to ships on sea and allows studies of the efficacy of onboard laser warning receivers. MARLA assists laser counter-measures and enables to include environmental studies (atmospheric transmission, water reflections etc.). Redundant system design ensures laser safety even in public areas.<p> </p> The core of MARLA is a modular laser unit (LU) consisting of five laser modules (LM) and the dedicated laser controllers (LC). The laser modules are mounted on a pan-tilt positioner. MARLA covers the most common laser threats like laser target designator (LTD), laser range finder (LRF), laser beam rider (LBR) and laser dazzler (LD). The individual laser modules are based on commercially available laser sources fitted with multi-stage attenuators to set the laser irradiance within a range of seven orders of magnitude without losing beam quality. By means of a photo detector, the energy of the emitted laser pulses is recorded. An integrated beam shaper enables to vary the beam divergence.<p> </p> The further crucial parts of MARLA are the control and data acquisition system with operating and visualization software and a general laser safety monitoring system. All the subsystems are integrated into a climate-controlled movable 20' sea container. Use of a stand-alone verification system provides reference data to verify the actual on-site irradiation at the test target.
We present our work regarding the evaluation of protection measures against laser dazzling for imaging devices. Different approaches for the evaluation of dazzled sensor images are investigated to estimate the loss of information due to the dazzle spot: (1) counting the number of overexposed pixels, (2) based on triangle orientation discrimination, and (3) using the structural similarity index. The evaluation approaches are applied on experimental data obtained with two different sensors hardened against laser dazzling. The hardening concept of the first sensor is based on the combination of a spatial light modulator and wavelength multiplexing. This protection concept allows spatially and spectrally resolved suppression of laser radiation within the sensor’s field-of-view. The hardening concept of the second sensor utilizes the principle of “complementary bands.” The optical setup resembles a common three-chip camera, with the difference that dedicated filters with steep edges replace the regular spectral band filters. Although this concept does not really represent a “protection measure,” it allows the sensor to provide information even in laser dazzling situations. The data for the performance evaluation were acquired both in a laboratory setup using test charts comprising triangles of different size and orientation as well as in field trials.
The continuous development of laser systems toward more compact and efficient devices constitutes an increasing threat to electro-optical imaging sensors, such as complementary metal–oxide–semiconductors (CMOS) and charge-coupled devices. These types of electronic sensors are used in day-to-day life but also in military or civil security applications. In camera systems dedicated to specific tasks, micro-optoelectromechanical systems, such as a digital micromirror device (DMD), are part of the optical setup. In such systems, the DMD can be located at an intermediate focal plane of the optics and it is also susceptible to laser damage. The goal of our work is to enhance the knowledge of damaging effects on such devices exposed to laser light. The experimental setup for the investigation of laser-induced damage is described in detail. As laser sources, both pulsed lasers and continuous-wave (CW)-lasers are used. The laser-induced damage threshold is determined by the single-shot method by increasing the pulse energy from pulse to pulse or in the case of CW-lasers, by increasing the laser power. Furthermore, we investigate the morphology of laser-induced damage patterns and the dependence of the number of destructive device elements on the laser pulse energy or laser power. In addition to the destruction of single pixels, we observe aftereffects, such as persistent dead columns or rows of pixels in the sensor image.
We present the latest results of our work regarding the evaluation of protection measures against laser dazzling for imaging devices. Three different approaches for the evaluation of dazzled sensor images are investigated and compared to estimate the loss of information due to the dazzle spot by a) counting the number of overexposed pixels, b) based on triangle orientation discrimination (TOD) and c) using the structural similarity (SSIM) index.<p> </p>The performance evaluation approaches are applied on experimental data obtained with two different imaging sensors hardened against laser dazzling. The hardening concept of the first sensor is based on the combination of a spatial light modulator and wavelength multiplexing. This active protection concept allows spatially and spectrally resolved suppression of laser radiation within the sensor's field of view. The hardening concept of the second sensor utilizes the principle of “complementary bands”. The optical setup resembles a common 3-chip camera, with the difference that dedicated filters with steep edges replace the regular spectral band filters. Although this concept does not really represent a “protection measure”, it allows the sensor to provide information even in laser dazzling situations.<p> </p>The data for the performance evaluation was acquired both in a dedicated laboratory setup using test charts comprising triangles of different size and orientation as well as in field trials.
The continuous development of laser systems towards more compact and efficient devices constitutes an increasing threat to electro-optical imaging sensors such as complementary metal-oxide-semiconductors (CMOS) and charge-coupled devices (CCD). These types of electronic sensors are used in day-to-day life but also in military or civil security applications. In camera systems dedicated to specific tasks, also micro-opto-electro-mechanical systems (MOEMS) like a digital micromirror device (DMD) are part of the optical setup. In such systems, the DMD can be located at an intermediate focal plane of the optics and it is also susceptible to laser damage. The goal of our work is to enhance the knowledge of damaging effects on such devices exposed to laser light. <p> </p>The experimental setup for the investigation of laser-induced damage is described in detail. As laser sources both pulsed lasers and continuous-wave (CW) lasers are used. The laser-induced damage threshold (LIDT) is determined by the single-shot method by increasing the pulse energy from pulse to pulse or in the case of CW-lasers, by increasing the laser power. <p> </p>Furthermore, we investigate the morphology of laser-induced damage patterns and the dependence of the number of destructed device elements on the laser pulse energy or laser power. In addition to the destruction of single pixels, we observe aftereffects like persisting dead columns or rows of pixels in the sensor image.
The presented work gives an overview on the efforts of the NATO SET-198 research task group. It comprises nonrestricted
material, which is already published or is to be published in journals. Main topics are the development and
validation of computer models in order to understand the impact of laser dazzling on the detection of objects in a scene
but also on the accomplishment of visual-based tasks. The work includes laboratory and field dazzling tests on sensors
and humans, computer eye-dazzle modeling, automatic character recognition and laboratory observer trials for validation
purposes of the used algorithms. The impact of dazzling is studied in dependence of laser wavelength, laser power and
Electro-optical imaging sensors are widely distributed and used for many different purposes, including civil security and military operations. However, laser irradiation can easily disturb their operational capability. Thus, an adequate protection mechanism for electro-optical sensors against dazzling and damaging is highly desirable. Different protection technologies exist now, but none of them satisfies the operational requirements without any constraints. In order to evaluate the performance of various laser protection measures, we present two different approaches based on triangle orientation discrimination on the one hand and structural similarity on the other hand. For both approaches, image analysis algorithms are applied to images taken of a standard test scene with triangular test patterns which is superimposed by dazzling laser light of various irradiance levels. The evaluation methods are applied to three different sensors: a standard complementary metal oxide semiconductor camera, a high dynamic range camera with a nonlinear response curve, and a sensor hardened against laser dazzling.
Electro-optical imaging sensors are widely distributed and used for many different tasks in military operations and civil
security. However, their operational capability can be easily disturbed by laser radiation. The likeliness of such an
incidence has dramatically increased in the past years due to the free availability of high-power laser pointers. These
laser systems, offering laser powers of several watts, pose an increased risk to the human eye as well as to electro-optical
sensors. An adequate protection of electro-optical sensors against dazzling is highly desirable. Such protection can be
accomplished with different technologies; however, none of the existing technologies can provide a sufficient protection.
All current protection measures possess individual advantages and disadvantages.
We present the results on the performance of two different protection technologies. The evaluation is based on automatic
optical pattern recognition of sensor images taken from a scene containing triangles.
Since the advent of the laser in 1960, the protection of human eyes and sensors against intended or unintended damage by laser radiation is a hot research topic. As long as the parameters of a laser source such as the wavelength and the output power are known, adequate laser safety can be ensured simply by utilizing conventional laser protection filters which are based on absorption or interference effects. This is typically the case in cooperative environments like a laboratory or industrial facilities. A very different situation prevails in military defense or civil security. There, the parameters of encountering laser threats are usually unknown. Protection measures, helping against all types of laser threats, are the long desired objective of countless research activities. The biggest challenge in finding an effective measure arises from single laser pulses of unknown wavelength. The problem demands for a passive protection concept and may be based for example on intensity dependent effects. Moreover, the requested solutions shall comprise add-on possibilities like thin films to be put on existing optics, windshields or glasses. Unfortunately, such an all-embracing solution is still far out of reach.<p> </p>The Fraunhofer IOSB has been working on the evaluation and development of non-conventional laser protection methods for more than 20 years. An overview of the past and present research activities shall be presented, comprising protection measures against laser damaging and laser dazzling.
The optical limiting behaviour of silver nanoparticles with different sizes and shapes is investigated and compared to the optical limiting performances of conventional carbon black suspension. The optical limiting behaviour is characterized by means of nonlinear transmittance and scattered intensity measurements when submitted to a nanosecond pulsed Nd:YAG laser operating at the fundamental or the second harmonic wavelength. We found that the optical limiting effect is strongly particle size dependent, the best performance achieved with the smaller particles. Moreover, it is shown that the surface plasmon resonance is not the main effect responsible for the nonlinear processes. Especially, the particle size and its implication in the backscattering performance is outlined. A stronger backscattered radiation is observed for the 60 nm sized silver particles in comparison with the 580 nm large ones. Alternatively, a stronger scattering in the forward hemisphere is subsequent to nanoparticles whose sizes are significantly greater.
The progress in laser technology leads to very compact but nevertheless powerful laser sources. In the visible and near
infrared spectral region, lasers of any wavelength can be purchased. Especially continuous wave laser sources pose a
serious threat to the human eye and electro-optical sensors due to their high proliferation and easy availability. The
manifold of wavelengths cannot be encountered by conventional safety measures like absorption or interference filters. We present a protection concept for electro-optical sensors to suppress dazzling in the visible spectral region. The key element of the concept is the use of a digital micromirror device (DMD) in combination with wavelength multiplexing. This approach allows selective spectral filtering in defined regions of interest in the scene. The system offers the possibility of automatic attenuation of dazzling laser radiation. An anti-dazzle algorithm comprises the analysis of the laser wavelength and the subsequent activation of the appropriate micromirrors of the DMD.
The European Defence Agency (EDA) launched the Active Imaging (ACTIM) study to investigate the potential of active
imaging, especially that of spectral laser imaging. The work included a literature survey, the identification of promising
military applications, system analyses, a roadmap and recommendations.
Passive multi- and hyper-spectral imaging allows discriminating between materials. But the measured radiance in the
sensor is difficult to relate to spectral reflectance due to the dependence on e.g. solar angle, clouds, shadows... In turn,
active spectral imaging offers a complete control of the illumination, thus eliminating these effects. In addition it allows
observing details at long ranges, seeing through degraded atmospheric conditions, penetrating obscurants (foliage,
camouflage...) or retrieving polarization information. When 3D, it is suited to producing numerical terrain models and to
performing geometry-based identification. Hence fusing the knowledge of ladar and passive spectral imaging will result
in new capabilities.
We have identified three main application areas for active imaging, and for spectral active imaging in particular: (1) long
range observation for identification, (2) mid-range mapping for reconnaissance, (3) shorter range perception for threat
detection. We present the system analyses that have been performed for confirming the interests, limitations and
requirements of spectral active imaging in these three prioritized applications.
The rapid developments in the laser field through the last years led to very compact laser devices with high brightness. In
the visual and near infrared spectral range practically each wavelength is now available. For optronical sensors, laser
radiation states an increasing threat that cannot be encountered just by conventional safety measures like absorption or
We present a concept to protect imaging sensors against laser radiation of any wavelength. The system is based on the
combination of a spatial light modulator and wavelength multiplexing and allows selective spectral filtering in a defined
region of interest in the scene.
Such a system offers the possibility to suppress annoying laser radiation without losing spatial information in the region
of interest. Depending on the used imaging sensor, we discuss different ways to realize a control loop to activate the
appropriate pixels of the spatial light modulator for the attenuation of the laser light.
Laser dazzling of electro-optical sensors states a serious problem. Especially when navigational tasks shall be fulfilled a
disturbance of the sight is inacceptable. Classical protection measures like e.g. laser safety filters usually lack of the
drawback of color distortion and a considerable loss in transmission. Therefore, new protection concepts should be
aspired which are independent of the laser wavelength and do not affect the vision.
In this paper we discuss a novel concept to protect electro-optical sensors against laser dazzling based on spatial light
modulator technology and wavelength multiplexing. In particular, the method is suited as a protection measure against
continuous wave laser sources.
We present our investigation of the optical limiting behaviour of different metal and semiconductor nanoparticles in liquid
and solid media. Nonlinear transmission measurements were performed by the use of nanosecond laser pulses at
532 nm and 1064 nm. For the various nanoparticles in liquid media large differences in the limiting response were found.
Foremost the solid state samples are promising in terms of laser protection devices.
Both the increasing use of laser devices on the battlefield and the misuse of laser devices in civil environments (e.g.
dazzling of pilots during approach) necessitate laser protection devices. Conventional laser protection filters based on
absorption or interference effects only work in narrow wavelength bands and are usually not neutral in colour. Thus, they
are not appropriate for such scenarios. Optical power limiting devices based on nonlinear optical effects (nonlinear
absorption, nonlinear refraction, induced scattering) were proposed to offer broadband laser protection without
disturbing the visual colour impression. Usually, optical nonlinear materials are examined in laboratory setups which are
different from realistic optical systems. We report on the integration of nonlinear laser protection devices into different
optical systems to prove the performance of limiting for commonly used optical systems.
Laser radiation may lead to permanent damage of the human eye when it is exposed to high power irradiation, especially when using magnifying optics such as binoculars, sights or periscopes. Into such an optical system we integrated a novel passive solid-state threshold-triggered Wideband Protection Filter (WPF) that blocks the transmission only if the power exceeds a certain threshold. At input powers below threshold, the filter has high transmission over the whole spectral band. However, when the input power exceeds the threshold power, transmission is decreased dramatically. We demonstrate the WPF integration within a typical optical system and the influence of system parameters on the protection capability of the filter.
We introduce into optical systems, susceptible to be interrupted or damaged from laser, novel passive solid-state
threshold-triggered Wideband Protection Filter (WPF) that blocks the transmission only if the power exceeds a certain
threshold. We present new protection capabilities of our latest filter composed of improved technology. The WPF can be
readily used for protection of detectors, cameras, or eye safety.
The increasing use of IR pilot sight in helicopters calls for a reliable prediction of perception ranges for a variety of
objects, especially those needed for orientation and those posing as a potential hazard, like power poles, masts, isolated
trees etc. Since the visibility of objects in the IR depends mainly on the temperature differences between those objects
and a given background and only marginally on illumination, range prediction techniques used for the visual range or
light-amplified vision are only of very limited use. While range predictions based on the Johnson criterion do offer some
insight into expected ranges, the inherently nominal nature of distance estimates thus obtained hampers their use for an
actual field-deployable pre-flight consulting procedure. In order to overcome those limitations, long-term simultaneous
measurements of relevant objects and background temperatures and weather data were carried out and used for
temperature prediction from prevalent weather conditions. Together with a perception model derived from extensive
observer experiments based on synthetic images of the UH Tiger Pilot Sight Unit we developed a perception range
prediction package which is currently evaluated by the weather service of the Bundeswehr. We will present results from
the observer experiments together with the derived perception models. These are then compared to actual perception
ranges as obtained from flight experiments.