Laser radar (ladar) systems are effective measurement systems with favourite applications like collision control or reconnaissance, etc.
A new pixel oriented simulation for scannerless, imaging 3D ladar has been developed. For the first time, any kind of 3D target scenes can be investigated by importing 3D data with the help of VRML.
Now, using the simulation, it is possible to easily investigate the influence of laser spot profiles, different target scenes and reflectivities or system configurations.
By changing its parameters, the simulation can be adapted to a wide range of systems. Hence, comparison of systems or performance estimation is possible. The simulation proofed as a useful tool for system design, testing of applicability or verification and evaluation of algorithms. The function of the simulation was verified by measurement data.
In this paper, the use of simulation is presented. By showing different applications, future development is discussed.
Eye-safe laser radar systems based on gated viewing use narrow infrared laser pulses to illuminate a whole scene for direct (incoherent) detection. Due to the time of flight principle and a very fast shutter with precisely controlled delay time, only light reflected in a certain range slice is detected. Due to the lack of off-shelf components, the development of such systems is difficult. Also, comparison of different systems is complicated, since atmospheric transmission, target reflectivity, polarization, and different noise effects have great influence on the system performance. The laser radar equation is used to estimate range performance in a a general manner. In this paper we discuss improvements in the system modelling of our laser radar system. Pixel wise simulation in combination with three dimensional scenes, generated with Virtual Reality Markup Language (VRML) is used to generate realistic range images. Changing to a pixel oriented approach and three dimensional modelled scenes, we are now able to study the system response for targets with arbitrary form and even different reflectivity. Also, we take into account the gaussian nature of the illuminating laser spot and different noise sources. Hence it is possible to simulate gray value images and calculate range images.
The development of eye-safe, imaging, scannerless laser radar systems based on gated viewing with a range of some hundred meters is difficult due to the lack of fast and amplifiable detector arrays for wavelengths in the near infrared (NIR). Nevertheless, one basic approach is to gate an InGaAs-FPA-camera with an electro-optical modulator (EOM) in the range of about 30 ns to achieve a sufficient resolution of depth. The laser radar works with a Nd:YAG-OPO laser of 1574 nm wavelength, 7 ns pulse length, and 25 Hz pulse frequency. Because of the EOM in conjunction with an adapted lens design and the resolution of 128 by 128 pixels FPA, the main interest is object detection. This requires imaging with optimized system performance to reach further target distances.
In this paper, we present the simulation of the current system. Comparison of simulation data with indoor measurements is shown by the calculation of range images from a sequence of range slices. We discuss advantages of this simulation for range estimation, system comparison and design process.
Laser radar based on gated viewing uses narrow laser pulses to illuminate a whole scene for direct (incoherent) detection. Due to the time of flight principle and a very fast shutter with precisely controlled delay time, only light reflected in the range R (range slice ΔR) is detected by a camera. Scattered light which reaches the shutter outside a given exposure time (gate) is suppressed. Hence, it is possible to "look" along the optical axis through changing atmospheric transmissions (rain, haze, fog, snow). For each laser pulse, the grey value image ES(x,y) of the camera is captured by a framegrabber for subsequent evaluation. Image sequences from these laser radar systems are ideally suited to recognize objects, because of the automatic contrast generation of the technology. Difficult object recognition problems, detection, target tracking, or obstacle avoidance at bad weather conditions are favorite applications.
In this paper we discuss improvements in the system modelling and simulation of our laser radar system. Formerly the system performance was calculated for the whole system using the signal-to-noise ratio (SNR), leading to a general estimation of the maximum range of target detection. Changing to a pixel oriented approach, we are now able to study the system response for targets with arbitrary two and even three dimensional form. We take into account different kinds of target reflectivity and the Gaussian nature of the illuminating laser spot. Hence it is possible to simulate gray value images (range slices) and calculate range images. This will lead to a modulation transfer function for the system in future. Finally, the theoretical
considerations are compared with experimental results from indoor measurements.
Due to the lack of fast switchable and sufficiently sensitive detectors for the near infrared (NIR) wavelength, the development of eye-safe, imaging, scannerless laser radar systems based on gated viewing is challenging. However, one basic approach is to use a pulsed Nd:YAG-OPO laser with 1574nm wavelength as transmitter and to gate an InGaAs-FPA camera with an electro-optical modulator (EOM) as receiver. With the EOM (a Pockels cell) the original integration time of the camera of 16ms is trimmed to the minimal exposure time (gate width) of 18ns. It is implemented in an adapted optics with focal length of 400mm. To guide the light with low divergence through the crystal of the EOM with a diameter of 5mm and a length of 80mm, the f-number was chosen to 26. Since the aperture of the EOM causes optical limitations, it is a decisive factor for the optical performance of the system. Additionally, because of the NIR laser light in conjunction with different target surfaces and the particularities of the receiver optics, artifacts in the laser radar images arise. In this paper we discuss the combination of target surface properties (e.g. reflectivity) and speckles, which cause a degradation of the quality of the range gated intensity images of the presented system. Since this leads to a reduction of the possible achievable contrast ratio, it affects directly the possible distance and depth resolution. The correlation of the target surface structure and the system performance will be analyzed, which is necessary to improve the current system design leading to enhanced system performance.
The development of eye-safe, imaging, scannerless laser radar systems based on gated viewing with a range of some hundred meters is difficult due to the lack of fast switchable and sufficiently sensitive detectors for NIR wavelengths. Nevertheless, one basic approach is to gate an InGaAs-FPA-camera with an electro-optical modulator (EOM) in the range of about 30ns to achieve a sufficiently high resolution of depth. In this paper, we present field measurements of a system with a Nd:YAG-OPO laser of 1574nm wavelength, 7ns pulse length, 25Hz pulse frequency, and 80mJ pulse energy. Because of the special optical behavior of the EOM in conjunction with an adapted lens design and a resolution of a 128 by 128 pixel FPA the main interest is object detection, which leads to imaging with improved system performance. The experimental data will be compared with the theoretical performance model and a second system (at a wavelength of 532nm), concerning imaging under certain weather conditions. Based on the current system design we discuss problems of contrast modulation and speckle effects leading to an outlook for improvements of the system design.
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