Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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  • Publication
    Laser safety assessments supported by analyses of reflections from metallic targets irradiated by high-power laser light
    ( 2021)
    Henrichsen, Michael
    ;
    Schwarz, Bastian
    ;
    Ritt, Gunnar
    ;
    Azarian, Adrian
    ;
    Eberle, Bernd
    When using kilowatt-class lasers in outdoor environments, ensuring laser safety turns out to be a complex issue due to the large safety areas that must be respected. For the special cases of collimated or focused laser radiation reflected from ideally flat but naturally rough metallic surfaces, the classical laser hazard analysis is deemed insufficient. In order to investigate the corresponding hazard areas for the aforementioned cases, we performed experiments on laser-matter interactions. Using high-power laser radiation, we studied the spatial and temporal reflection characteristics from four different metallic samples. For the evaluation of total reflection characteristics, we performed curve-fitting methods comprising Gaussian-like specular components, diffuse scattering components according to the ABg-scatter model and Lambertian components. For the investigation of occurring caustics, we developed a dedicated model in order to assess the divergence of the contained structures as a function of distance. Our evaluations have shown that the majority of the reflected power is scattered and based on these findings, that resulting nominal optical hazard distance values, even under worst-case assumptions, are significantly smaller than those of the non-reflected laser beam.
  • Publication
    Impact of threshold assessment methods in laser-induced damage measurements using the examples of CCD, CMOS, and DMD
    ( 2021)
    Schwarz, Bastian
    ;
    Ritt, Gunnar
    ;
    Eberle, Bernd
    Based on our earlier investigations, we continued and intensified our effort on the assessment of laser-induced damage effects in the visible range on a digital micromirror device (DMD) in comparison to different electro-optical imaging sensors such as complementary metal-oxide-semiconductors (CMOS) and charge-coupled devices (CCD). The main two objectives of our current work are: i) to fill the gap for the damage threshold regarding the time scale of picosecond pulses (527 nm) for CCD and CMOS devices and ii) evaluate the performance of a new device, the DMD, with both nanosecond pulses (532 nm) and picosecond pulses (527 nm) and compare the results with those of the CCD/CMOS. In the course of this research, we improved the experimental setup. Furthermore, we characterized the damage caused by laser pulse energies exceeding the laser-induced damage threshold (LIDT). For both the CMOS and CCD cameras, we received damage thresholds of about 10mJ/cm2 (picosecond pulses). For the DMD, we obtained LIDT values of 130mJ/cm2 (nanosecond laser pulses) and 1500mJ/cm2 (picosecond laser pulses). In case of the CMOS devices, we additionally compared the appearance of the damage obtained from the output signal of the camera under test and the microscope images of the surface of the camera. The first visible changes on the surface of the sensor occurred at energy densities that are an order of magnitude higher than the threshold values related to the output signal.
  • Publication
    Use of complementary wavelength bands for laser dazzle protection
    ( 2020)
    Ritt, Gunnar
    ;
    Eberle, Bernd
    The use of complementary wavelength bands in camera systems is a long-known principle. The camera system's spectral range is split into several spectral channels, where each channel possesses its own imaging sensor. Such an optical setup is used, for example, in high-quality three-sensor color cameras. A three-sensor camera is less vulnerable to laser dazzle than a single-sensor camera. However, the separation of the individual channels is not high enough to suppress cross talk, 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 three-channel camera system, which was already presented earlier. The second design is a two-channel camera system based on optical multiband elements, which delivers undisturbed color images even under laser dazzle.
  • Publication
    Preventing image information loss of imaging sensors in case of laser dazzle
    ( 2019)
    Ritt, Gunnar
    ;
    Schwarz, Bastian
    ;
    Eberle, Bernd
    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.
  • Publication
    Evaluation of protection measures against laser dazzling for imaging sensors
    ( 2017)
    Ritt, Gunnar
    ;
    Eberle, Bernd
    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.
  • Publication
    Laser-induced damage threshold of camera sensors and micro-optoelectromechanical systems
    ( 2017)
    Schwarz, Bastian
    ;
    Ritt, Gunnar
    ;
    Körber, Michael
    ;
    Eberle, Bernd
    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.
  • Publication
    Protection performance evaluation regarding imaging sensors hardened against laser dazzling
    ( 2015)
    Ritt, Gunnar
    ;
    Koerber, Michael
    ;
    Forster, Daniel
    ;
    Eberle, Bernd
    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.