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Laser safety for electro-optical imaging systems: exposure limits and hazard distances
Laser safety is an important topic. Everybody working with lasers has to follow the long-established occupational safety rules to prevent people from eye damage by accidental irradiation. These rules comprise, for example, the calculation of the Maximum Permissible Exposure (MPE), as well as the corresponding laser hazard distance, the so-called Nominal Ocular Hazard Distance (NOHD). At exposure levels below the MPE, laser eye dazzling may occur and is described by a quite new concept, leading to definitions such as the Maximum Dazzle Exposure (MDE) and to its corresponding Nominal Ocular Dazzle Distance (NODD). In earlier work, we defined exposure limits for sensors corresponding to those for the human eye: The Maximum Permissible Exposure for a Sensor, MPES, and the Maximum Dazzle Exposure for a Sensor, MDES. In this publication, we report on our continuative work concerning the laser hazard distances arising from these exposure limits. In contrast to the human eye, unexpected results occur for electro-optical imaging systems: For laser irradiances exceeding the exposure limit, MPES, it can happen that the laser hazard zone does not extend directly from the laser source, but only from a specific distance to it. This means that some scenarios are possible where an electro-optical imaging sensor may be in danger of getting damaged within a certain distance to the laser source but is safe from damage when located close to the laser source. This is in contrast to laser eye safety, where it is assumed that the laser hazard zone always extends directly from the laser source. Furthermore, we provide closed-form equations in order to estimate laser hazard distances related to the damaging and dazzling of the electro-optical imaging systems.
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.
Laser-induced camera damage thresholds were measured for several sensors of three different sensor architectures using a Q-switched Nd:YAG laser in order to determine their pulsed laser-induced damage thresholds. Charge coupled device (CCD), front-side illuminated complimentary metal-oxide semiconductor (FSI CMOS), and back-side illuminated (BSI) CMOS sensors were assessed under laboratory and outdoor environments by increasing the focused laser intensity onto the sensors and recording the sensor output. The damage sites were classified qualitatively into damage types, and pixel counting methods were applied to quantitatively plot damage scale against laser intensity. Probit-fits were applied to find the intensity values where a 95% probability of damage would occur (FD95) and showed that FD95 was approximately the same under laboratory conditions for CCD, FSI CMOS, and BSI CMOS sensors (mean 532 nm FD95 of 0.077 ± 0.01 Jcm⁻²). BSI CMOS sensors were the most robust to large-scale damage effects—BSI sensor kill was found at approximately 10³ Jcm⁻², compared to 10 Jcm⁻² for FSI CMOS, and between ~1.6 and 2.7 Jcm⁻² for CCDs.
The damage threshold of silicon-based cameras to laser irradiation is measured for continuous wave lasers at both in-band and out-of-band wavelengths. Clarifications about the various kinds of damage reported in the literature are also presented and explained through various tests. For increasing laser intensities, the sequence of laser effects on cameras usually starts from dazzling, to a decrease of the pixel response up to the complete neutralization of its pixel matrix. However, the intensity range for the complete sequence of these laser effects can strongly depend on the laser wavelengths and the tested cameras, particularly for out-of-band laser wavelengths. The proposed definitions of damage thresholds presented in this work offer a basis for the future comparison of results between different studies.
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 (picosecond pulses). For the DMD, we obtained LIDT values of (nanosecond laser pulses) and (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.
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.
This publication presents an approach to adapt the well-known classical eye-related concept of laser safety calculations on camera sensors as general as possible. The difficulty in this approach is that sensors, in contrast to the human eye, consist of a variety of combinations of optics and detectors. Laser safety calculations related to the human eye target terms like Maximum Permissible Exposure (MPE) and Nominal Ocular Hazard Distance (NOHD). The MPE describes the maximum allowed level of irradiation at the cornea of the eye to keep the eye safe from damage. The hazard distance corresponding to the MPE is called NOHD. Recently, a laser safety framework regarding the case of human eye dazzling was suggested. For laser eye dazzle, the quantities Maximum Dazzle Exposure (MDE) and the corresponding hazard distance Nominal Ocular Dazzle Distance (NODD) were introduced. Here, an approach is presented to extend laser safety calculations to camera sensors in an analogous way. The main objective thereby was to establish closed-form equations that are as simple as possible to allow also non-expert users to perform such calculations. This is the first time that such investigations have been carried out for this purpose.
A simple safety framework for laser eye dazzle, based on a complex model developed from human subject experiments, is proposed to address the urgent need for guidance within international laser safety standards. Maximum Dazzle Exposure (MDE) safety limits are derived that set the laser irradiance at the eye above which an object cannot be visually detected. A newly defined concept of dazzle level accounts for the extent of visual obscuration, and different ambient light levels are accommodated by determining safety limits for night, dusk/dawn, and day conditions. The resulting table of MDE values allows dazzle effects to be quantified in simple safety calculations across a wide range of scenarios. This safety framework is intended to empower the laser safety community to understand and quantify the impacts of laser eye dazzle, specify protection measures for those at risk, and assure the safety and effectiveness of laser dazzle devices.
A safety framework for laser eye dazzle has been constructed to address the urgent need for dazzle advice within international laser safety standards. Simple calculations are presented to permit dazzle effects to be quantified, based upon the new concepts of Dazzle Level (DL), Maximum Dazzle Exposure (MDE) and Nominal Ocular Dazzle Distance (NODD).
A simple model for laser eye dazzle is presented together with calculations for laser safety applications based on the newly defined Maximum Dazzle Exposure (MDE) and Nominal Ocular Dazzle Distance (NODD). A validated intraocular scatter model has been combined with a contrast threshold target detection model to quantify the impact of laser eye dazzle on human performance. This allows the calculation of the MDE, the threshold laser irradiance below which a target can be detected, and the NODD, the minimum distance for the visual detection of a target in the presence of laser dazzle. The model is suitable for non-expert use to give an estimate of anticipated laser eye dazzle effects in a range of civilian and military scenarios.
We have measured the functional degradation of silicon CCD photodetector
arrays when subjected to Nd:YAG laser irradiation at 1.06 micrometers by
10 ns pulses. Operational tests such as dark leakage,
point-spread-function, and modulation-transfer-function have been
developed for testing individual pixels and have been applied to the
testing of locally laser-damaged CCD arrays. The tests were carried out
on polysilicon gate devices of 2048 X 64 pixels manufactured by EG&G
Reticon (RA-2048J). Testing revealed that the primary failure mechanism
involved spreading of the PSF along the direction of clocked charge
motion due to a decreased depth of potential wells within the
laser-damage spot. Lesser degradation was observed for pixels near the
damage spot that were served by the clock lines that traversed the
damage spot. This damage behavior was correlated with decreased
breakdown voltage and increased leakage current between adjacent clock
lines. Functional damage occurred at locations where two clock lines
were very close to each other and the thin oxide layer separating them
was subjected to high temperatures due to laser-heating of the adjacent
polysilicon.
Laser-induced morphological and electrical changes to silicon CCD devices have been studied. The devices were poly-silicon gate Time Delay Integrating (TDI) CCD arrays of 2048x96 elements. The laser source for these experiments was a Q-switched Nd:YAG laser at 1.06 μm with 10 ns pulses at a 10 Hz repetition rate focused to an approximately 400 μm spot radius. Single pulse and multiple pulse damage behavior was studied. Both CCD arrays and diagnostic structures from the wafer periphery were tested. The additional diagnostic structures included poly-Si resistors and MOS-FET gates. Of the measurements made, it was found that drain-to-substrate and drain-to-source leakage currents and transconductance in FETs were the most sensitive parameters to laser-induced change. The onset of electrical parameter changes was observed as low as 0.2 J/cm2. Severe electrical parameter changes began at 0.5 J/cm2 and continued up to the onset of severe morphological damage at 1.0 J/cm2. Above this fluence, both poly-Si and aluminum interconnect lines were melted and broken.
Simple polynomial formulas to calculate the FWHM and full width at 1/e ² intensity diffraction spot size and the depth of focus at a Strehl ratio of 0.8 and 0.5 as a function of a Gaussian beam truncation ratio and a system f-number are presented. Formulas are obtained by use of the numerical integration of a Huygens-Fresnel diffraction integral and can be used to calculate the number of resolvable spots, the modulation transfer function, and the defocus tolerance of optical systems that employ laser beams. I also derived analytical formulas for the diffraction ring intensity as a function of the Gaussian beam truncation ratio and the system f-number. Such formulas can be used to estimate the diffraction-limited contrast of display and imaging systems.
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. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
A simple model has been developed and implemented in Matlab code, predicting the over-exposed pixel area of cameras caused by laser dazzling. Inputs of this model are the laser irradiance on the front optics of the camera, the Point Spread Function (PSF) of the used optics, the integration time of the camera, and camera sensor specifications like pixel size, quantum efficiency and full well capacity. Effects of the read-out circuit of the camera are not incorporated. The model was evaluated with laser dazzle experiments on CCD cameras using a 532 nm CW laser dazzler and shows good agreement. For relatively low laser irradiance the model predicts the over-exposed laser spot area quite accurately and shows the cube root dependency of spot diameter on laser irradiance, caused by the PSF as demonstrated before for IR cameras. For higher laser power levels the laser induced spot diameter increases more rapidly than predicted, which probably can be attributed to scatter effects in the camera. Some first attempts to model scatter contributions, using a simple scatter power function f(θ), show good resemblance with experiments. Using this model, a tool is available which can assess the performance of observation sensor systems while being subjected to laser countermeasures.
The design, analysis, and performance of a small -angle scatterometer are presented. The effects of the diffraction background, geometrical aberrations and system scatter at the small-angles are separated. Graphs are provided that quantify their contribution. The far-field irradiance distributions of weakly truncated and untruncated Gaussian beams are compared. The envelope of diffraction ringing is shown to decrease proportionately with the level of truncation in the pupil. Spherical aberration and defocus are shown to have little effect on the higher-order diffraction rings of Gaussian apertures and as such will have a negligible effect on most scatter measurements. A method is presented for determining the scattered irradiance level for a given BRDF in relation to the peak irradiance of the point spread function. A method of Gaussian apodization is presented and tested that allows the level of diffraction ringing to become a design parameter. Upon sufficient reduction of the diffraction background, the scattered light from the scatterometers' primary mirror is seen to be the limiting component of the small-angle instrument profile. The scatterometer described was able to make a meaningful measurement close enough to the specular direction at 0.6328mum in order to observe the characteristic height and width of the scatter function. This allowed the rms roughness and autocorrelation length of the surface to be determined from the scatter data at this wavelength. The inferred rms roughness agreed well with an independent optical profilometer measurement of the surface. The BRDF of the samples were also measured at 10.6mum. The rms roughness inferred from this scatter data did not agree with the other measurements. The BRDF did not scale in accordance with the scaler diffraction theory of microrough surfaces. The scattering in the visible was dominated by the effects of surface roughness whereas the scattering in the far-infrared was apparently dominated by the effects of contaminants and surface defects. The model for the surface statistics is investigated. A K_0 (modified Bessel function) autocorrelation function is shown to predict the scattered light distribution of these samples much better than the conventional negative -exponential function. Additionally, a sampling theory is developed that addresses the negative-exponentially correlated output of lock-in amplifiers, detectors, and electronic circuits in general. It is shown that the optimum sampling rate is approximately one sample per time constant and at this rate the improvement in SNR is sqrt {N/2} where N is the number of measurements.
Light that is scattered from lenses and mirrors in an optical system produces a halo of stray light around bright objects within the field of view. The angular distribution of scattered light from any one component is usually described by the Harvey model. This paper presents analytic expressions for the scattered irradiance at a focal plane from optical components that scatter light in accordance with the Harvey model. It is found that the irradiance is independent of the location of an optical element within the system, provided the element is not located at or near an intermediate image plane. It is also found that the irradiance has little or no dependence on the size of the element.