Estimation of Lens Stray Light with Regard to the Incapacitation of Imaging Sensors-Part 2: Validation.

Recently, we developed a simple theoretical model for the estimation of the irradiance distribution at the focal plane of commercial off-the-shelf (COTS) camera lenses in case of laser illumination. The purpose of such a model is to predict the incapacitation of imaging sensors when irradiated by laser light. The model is based on closed-form equations that comprise mainly standard parameters of the laser dazzle scenario and those of the main devices involved (laser source, camera lens and imaging sensor). However, the model also includes three non-standard parameters, which describe the scattering of light within the camera lens. In previous work, we have performed measurements to derive these typically unknown scatter parameters for a collection of camera lenses of the Double-Gauss type. In this publication, we compare calculations based on our theoretical model and the measured scatter parameters with the outcome of stray light simulations performed with the optical design software FRED in order to validate the reliability of our theoretical model and of the derived scatter parameters.

Impact of threshold assessment methods in laser-induced damage measurements using the examples of CCD, CMOS, and DMD.

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.

Laser safety assessments supported by analyses of reflections from metallic targets irradiated by high-power laser light.

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.

Estimation of Lens Stray Light with Regard to the Incapacitation of Imaging Sensors.

We present our efforts on estimating light scattering characteristics from commercial off-the-shelf (COTS) camera lenses in order to deduce thereof a set of generic scattering parameters valid for a specific lens class (double Gauss lenses). In previous investigations, we developed a simplified theoretical light scattering model to estimate the irradiance distribution in the focal plane of a camera lens. This theoretical model is based on a 3-parameter bidirectional scattering distribution function (BSDF), which describes light scattering from rough surfaces of the optical elements. Ordinarily, the three scatter parameters of the BSDF are not known for COTS camera lenses, which makes it necessary to assess them by own experiments. Besides the experimental setup and the measurement process, we present in detail the subsequent data exploitation. From measurements on seven COTS camera lenses, we deduced a generic set of scatter parameters. For a deeper analysis, the results of our measurements have also been compared with the output of an optical engineering software. Together with our theoretical model, now stray light calculations can be accomplished even then, when specific scatter parameters are not available from elsewhere. In addition, the light scattering analyses also allow considering the glare vulnerability of optical systems in terms of laser safety.

Automatic laser glare suppression in electro-optical sensors.

Progress in laser technology has led to very compact but nevertheless powerful laser sources. In the visible and near infrared spectral region, lasers of any wavelength can be purchased. Continuous wave laser sources pose an especially serious threat to the human eye and electro-optical sensors due to their high proliferation and easy availability. The manifold of available wavelengths cannot be covered 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.

Size and shape effects on the nonlinear optical behavior of silver nanoparticles for power limiters.

The optical limiting behavior of silver nanoparticles with different sizes and shapes is investigated and compared to the optical limiting performance of conventional carbon black suspension (CBS). The optical limiting behavior is characterized by means of nonlinear transmittance and scattered intensity measurements when submitted to nanosecond pulsed Nd:YAG lasers operating at the fundamental or the second harmonic wavelength. We found that the optical limiting effect is strongly particle size dependent and the best performance is achieved with the smaller particles. Moreover, it is shown that the surface plasmon resonance is not the main effect responsible for the nonlinear processes. A theoretical model based on the computation of the Mie scattering functions is exposed, and it is shown that the experimental results can be well explained from the calculations.

Investigations on the Nonlinear Optical Properties of 0D, 1D, and 2D Boron Nitride Nanomaterials in the Visible Spectral Region.

In recent years, boron nitride nanomaterials have attracted increasing attention due to their unique properties such as high temperature stability and high thermal conductivity. They are structurally analogous to carbon nanomaterials and can also be generated as zero-dimensional nanoparticles and fullerenes, one-dimensional nanotubes and nanoribbons, and two-dimensional nanosheets or platelets. In contrast to carbon-based nanomaterials, which have been extensively studied during recent years, the optical limiting properties of boron nitride nanomaterials have hardly been analysed so far. This work summarises a comprehensive study on the nonlinear optical response of dispersed boron nitride nanotubes, boron nitride nanoplatelets, and boron nitride nanoparticles using nanosecond laser pulses at 532 nm. Their optical limiting behaviour is characterised by means of nonlinear transmittance and scattered energy measurements and a beam profiling camera is used to analyse the beam characteristics of the transmitted laser radiation. Our results show that nonlinear scattering dominates the OL performance of all measured boron nitride nanomaterials. Boron nitride nanotubes show a large optical limiting effect, much stronger than the benchmark material, multi-walled carbon nanotubes, which makes them promising for laser protection applications.

Automatic suppression of intense monochromatic light in electro-optical sensors.

Electro-optical imaging sensors are widely distributed and used for many different tasks. Due to technical improvements, their pixel size has been steadily decreasing, resulting in a reduced saturation capacity. As a consequence, this progress makes them susceptible to intense point light sources. Developments in laser technology have led to very compact and powerful laser sources of any wavelength in the visible and near infrared spectral region, offered as laser pointers. The manifold of wavelengths makes it difficult to encounter sensor saturation over the complete operating waveband by conventional measures like absorption or interference filters. We present a concept for electro-optical sensors to suppress overexposure in the visible spectral region. The key element of the concept is a spatial light modulator in combination with wavelength multiplexing. This approach allows spectral filtering within a localized area in the field of view of the sensor. The system offers the possibility of automatic reduction of overexposure by monochromatic laser radiation.

Electro-optical sensor with spatial and spectral filtering capability.

We describe a new concept of an electro-optical sensor with the capability of simultaneous spatial and spectral filtering. It is based on a spatial light modulator, and in combination with the technique of wavelength multiplexing, it enables one to manipulate the spectral content of an indicated spot within the field of view of the sensor. This new concept allows the attenuation of monochromatic light of undetermined wavelengths in particular and is of worth for imaging vision systems to suppress unwanted detector overexposure.