Peter Lutzmann

Advanced short-wavelength infrared range-gated imaging for ground applications in monostatic and bistatic configurations

Some advanced concepts for gated viewing are presented, including spectral diversity illumination techniques, non-line-of-sight imaging, indirect scene illumination, and in particular setups in bistatic configurations. By using a multiple-wavelength illumination source target speckles could be substantially reduced, leading to an improved image quality and enhanced range accuracy. In non-line-of-sight imaging experiments we observed the scenery through the reflections in a window plane. The scene was illuminated indirectly as well by a diffuse reflection of the laser beam at different nearby objects. In this setup several targets could be spotted, which, e.g., offers the capability to look around the corner in urban situations. In the presented measuring campaigns the advantages of bistatic setups in comparison with common monostatic configurations are discussed. The appearance of shadows or local contrast enhancements as well as the mitigation of retroreflections supports the human observer in interpreting the scene. Furthermore a bistatic configuration contributes to a reduced dazzling risk and to observer convertness.

Lidar frequency modulation vibrometry in the presence of speckle.

We report laboratory target vibration measurements that use an easily aligned and adjusted fiber-based 1.5-microm heterodyne lidar. The targets are simple spherically curved retroreflectors with well-controlled vibration frequencies and amplitudes. A rotating ground-glass screen creates Gaussian speckle. We wish to understand the modulated and fast-fading lidar returns seen from real target. We frequency demodulated the recorded laboratory data by phase differencing to provide estimates of dphi/dt, where phi is the phase of the received carrier-plus-noise phasor. Experimental results for signal strength and signal-to-noise ratio, for specific target modulation parameters, agree well with our recently developed dphi/dt correlation-function theory.

Range accuracy of a gated-viewing system compared to a 3D flash LADAR under different turbulence conditions

While a Gated-Viewing system primarily provides the intensity values of the captured laser radiation, it is also possible to determine range information in a static scenario by the sliding gates method. In this paper, we compare this method to a time-of-flight based 3-D Flash LADAR technique in terms of range accuracy under moderate and strong turbulence conditions. The first method requires several Gated-Viewing images (several laser pulses) with stepwise increased gate delay times. For a 3-D Flash LADAR system, one laser pulse is sufficient because for each pixel the range is determined by the time-of-flight method. We have combined a Gated-Viewing camera (640 × 480 pixels) as well as a 3-D Flash LADAR camera (128 × 128 pixels) with a pulsed 1.57 μm laser source. The maximal laser pulse energy was 67 mJ. We have conducted field measurements at different times of day. Two reflectance panels and a vehicle at a distance of 2 km were recorded. The plates were positioned diagonal to the line of sight with an angle of about 45 degrees in order to determine range accuracies. In addition, a laser scintillometer provided atmospheric turbulence strength along the propagation path.

Range accuracy of a gated-viewing system as a function of the number of averaged images

Primarily, a Gated-Viewing (GV) system provides range gated imagery. By increasing the camera delay time from frame to frame, a so-called sliding gates sequence is obtained by which 3-D reconstruction is possible. Scintillation caused by atmospheric turbulence degrades each Gated-Viewing image and thus, the range accuracy that can be achieved with the sliding gates method. By averaging a certain number of images per range, this degradation can be reduced. In this paper we have studied the influence of the number of averaged images on the resulting range accuracy. Therefore, we have combined the Intevac Gated-Viewing detector M506 with a pulsed 1.57 μm laser source. The maximal laser pulse energy was 65 mJ. The target was a 1-m2-plate at a distance of 500 m. The plate was laminated with a Spectralon layer having Lambertian reflection behavior with a homogeneous reflectance of 93 %. It was orientated diagonally to the line of sight of the sensor in order to provide a depth scenario. We have considered different combinations of the four parameters »detector binning mode« (1x1, 2x2), »optics« (f = 250 mm, f/2.1; f = 500 mm, f/3.3; f = 2032 mm, f/10), »gate length« (13.5 m, 23.25 m, 33 m) and »signal-to-noise ratio« (SNR) (1 dB, 2 dB,…, 9 dB). For each considered set of parameters, a sliding gates sequence of the target was recorded. Per range, 20 frames were collected. Finally, the range accuracies were derived as a function of the number of averaged frames per range.

ACTIM: an EDA initiated study on spectral active imaging

This paper will describe ongoing work from an EDA initiated study on Active Imaging with emphasis of using multi or broadband spectral lasers and receivers. Present laser based imaging and mapping systems are mostly based on a fixed frequency lasers. On the other hand great progress has recently occurred in passive multi- and hyperspectral imaging with applications ranging from environmental monitoring and geology to mapping, military surveillance, and reconnaissance. Data bases on spectral signatures allow the possibility to discriminate between different materials in the scene. Present multi- and hyperspectral sensors mainly operate in the visible and short wavelength region (0.4-2.5 μm) and rely on the solar radiation giving shortcoming due to shadows, clouds, illumination angles and lack of night operation. Active spectral imaging however will largely overcome these difficulties by a complete control of the illumination. Active illumination enables spectral night and low-light operation beside a robust way of obtaining polarization and high resolution 2D/3D information. Recent development of broadband lasers and advanced imaging 3D focal plane arrays has led to new opportunities for advanced spectral and polarization imaging with high range resolution. Fusing the knowledge of ladar and passive spectral imaging will result in new capabilities in the field of EO-sensing to be shown in the study. We will present an overview of technology, systems and applications for active spectral imaging and propose future activities in connection with some prioritized applications.

Super-resolution depth information from a short-wave infrared laser gated-viewing system by using correlated double sampling

Primarily, a laser gated-viewing (GV) system provides range-gated 2D images without any range resolution within the range gate. By combining two GV images with slightly different gate positions, 3D information within a part of the range gate can be obtained. The depth resolution is higher (super-resolution) than the minimal gate shift step size in a tomographic sequence of the scene. For a state-of-the-art system with a typical frame rate of 20 Hz, the time difference between the two required GV images is 50 ms which may be too long in a dynamic scenario with moving objects. Therefore, we have applied this approach to the reset and signal level images of a new short-wave infrared (SWIR) GV camera whose read-out integrated circuit supports correlated double sampling (CDS) actually intended for the reduction of kTC noise (reset noise). These images are extracted from only one single laser pulse with a marginal time difference in between. The SWIR GV camera consists of 640 x 512 avalanche photodiodes based on mercury cadmium telluride with a pixel pitch of 15 μm. A Q-switched, flash lamp pumped solid-state laser with 1.57 μm wavelength (OPO), 52 mJ pulse energy after beam shaping, 7 ns pulse length and 20 Hz pulse repetition frequency is used for flash illumination. In this paper, the experimental set-up is described and the operating principle of CDS is explained. The method of deriving super-resolution depth information from a GV system by using CDS is introduced and optimized. Further, the range accuracy is estimated from measured image data.

Laser vibration sensing at Fraunhofer IOSB: review and applications

Abstract. Laser vibrometry based on coherent detection allows noncontact measurements of small-amplitude vibration characteristics of objects. This technique, commonly using the Doppler effect, offers high potential for short-range civil applications and for medium- or long-range applications in defense and security. Most commercially available laser Doppler vibrometers are for short ranges (up to a few tens of meters) and use a single beam from a low-power HeNe laser source (λ=633  nm). Medium- or long-range applications need higher laser output power, and thus, appropriate vibrometers typically operate at 1.5, 2, or 10.6  μm to meet the laser safety regulations. Spatially resolved vibrational information can be obtained from an object by using scanning laser vibrometers. To reduce measuring time and to measure transient object movements and vibrational mode structures of objects, several approaches to multibeam laser Doppler vibrometry have been developed, and some of them are already commercially available for short ranges. We focus on applications in the field of defense and security, such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines. Examples of civil medium-range applications are also given.

Comparison of high speed imaging technique to laser vibrometry for detection of vibration information from objects

The development of camera technology in recent years has made high speed imaging a reliable method in vibration and dynamic measurements. The passive recovery of vibration information from high speed video recordings was reported in several recent papers. A highly developed technique, involving decomposition of the input video into spatial subframes to compute local motion signals, allowed an accurate sound reconstruction. A simpler technique based on image matching for vibration measurement was also reported as efficient in extracting audio information from a silent high speed video. In this paper we investigate and discuss the sensitivity and the limitations of the high speed imaging technique for vibration detection in comparison to the well-established Doppler vibrometry technique. Experiments on the extension of the high speed imaging method to longer range applications are presented.

Mono- and bi-static SWIR range-gated imaging experiments for ground applications

FOI (Sweden) and FGAN-FOM (Germany) have carried out common field trials using range gating imaging at 1.5 μm. Some examples showing the potential of bi-static vs. the common mono-static configurations are given. In addition the experiments include a comparison between active SWIR and passive EO-IR imaging. The paper discusses the results with respect to suitability for defence and security applications.

Detection of object vibrations from high speed infrared images

Remote detection of vibrational features from an object is important for many short range civil applications, but it is also of interest for long range applications in the defense and security area. The well-established laser Doppler vibrometry technique is widely used as a high-sensitivity, non-contact method. The development of camera technology in recent years made image-based methods reliable passive alternatives for vibration and dynamic measurements. Very sensitive applications have been demonstrated using high speed cameras in the visual spectral range. However, for long range applications, where turbulence becomes a limiting factor, image acquisition in the short- to mid-wave IR region would be desirable, as the atmospheric effects attenuate at longer wavelength. In this paper, we investigate experimentally the vibration detection from short- and mid-wave IR image sequences using high speed imaging technique. Experiments on the extraction of vibration signature under strong local turbulence conditions are presented.