Benjamin Göhler

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.

Laser vibration sensing: overview and applications

Laser vibrometry based on coherent detection allows non-contact measurements of small-amplitude vibration characteristics of objects. This technique, commonly using the Doppler effect, offers much potential for short-range civil applications and for long-range applications in defence and security. Most commercially available laser vibrometers are for short ranges (up to a few tens of metres) and use a single beam from a low power HeNe laser source (λ = 632 nm). Long-range applications need higher laser output power, and thus appropriate vibrometers typically operate at 1.5 μm, 2 μm 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 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. In this paper we focus on applications in the field of defence and security such as target classification and identification, including camouflaged or partly concealed targets, and the detection of buried land mines. Some examples of civil medium-range applications are given also.

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 gate shift step size

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. An important parameter of a sliding gates sequence is the step size by which the gate is shifted. In order to reduce the total number of required images, this step size should be as large as possible without significantly degrading the range accuracy. In this paper we have studied the influence of the gate shift step size on the resulting range accuracy. Therefore, we have combined the Intevac Gated-Viewing detector M506 with a pulsed 1.57 μm illumination laser. The maximal laser pulse energy is 65 mJ. The target is a one-square-meter-plate at a distance of 500 m. The plate is laminated with a Spectralon layer having Lambertian reflection behavior with a homogeneous reflectance of 93 %. For the measurements, this plate 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 two parameters »gate length« (13.5 m, 23.25 m, 33 m) and »signal-to-noise ratio« (SNR) (2 dB, 3 dB, 5 dB, 6 dB, 7 dB, 8 dB). For each considered set of parameters, a sliding gates sequence of the target was recorded. Per range, 20 frames were collected. The gate shift step size was set to the minimal possible value, 75 cm. By skipping certain ranges, a sliding gates sequence with a larger gate shift step size is obtained. For example, skipping the ranges 2, 3, 5, 6, 8, 9,… (equivalent: taking the ranges 1, 4, 7, …) results in a gate shift step size of 2.25 m. Finally, the range accuracies were derived as a function of the gate shift step size. Additionally, the influence of frame averaging on these functions was studied.

Review on short-wavelength infrared laser gated-viewing at Fraunhofer IOSB

Abstract. This paper reviews the work that has been done at Fraunhofer IOSB (and its predecessor institutes) in the past ten years in the area of laser gated-viewing (GV) in the short-wavelength infrared (SWIR) band. Experimental system demonstrators in various configurations have been built up to show the potential for different applications and to investigate specific topics. The wavelength of the pulsed illumination laser is 1.57  μm and lies in the invisible, retina-safe region allowing much higher pulse energies than for wavelengths in the visible or near-infrared band concerning eye safety. All systems built up, consist of gated Intevac LIVAR® cameras based on EBCCD/EBCMOS detectors sensitive in the SWIR band. This review comprises military and civilian applications in maritime and land domain—in particular vision enhancement in bad visibility, long-range applications, silhouette imaging, 3-D imaging by sliding gates and slope method, bistatic GV imaging, and looking through windows. In addition, theoretical studies that were conducted—e.g., estimating 3-D accuracy or modeling range performance—are presented. Finally, an outlook for future work in the area of SWIR laser GV at Fraunhofer IOSB is given.

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.

Measurements and analysis of active/passive multispectral imaging

This paper describes a data collection on passive and active imaging and the preliminary analysis. It is part of an ongoing work on active and passive imaging for target identification using different wavelength bands. We focus on data collection at NIR-SWIR wavelengths but we also include the visible and the thermal region. Active imaging in NIRSWIR will support the passive imaging by eliminating shadows during day-time and allow night operation. Among the applications that are most likely for active multispectral imaging, we focus on long range human target identification. We also study the combination of active and passive sensing. The target scenarios of interest include persons carrying different objects and their associated activities. We investigated laser imaging for target detection and classification up to 1 km assuming that another cueing sensor – passive EO and/or radar – is available for target acquisition and detection. Broadband or multispectral operation will reduce the effects of target speckle and atmospheric turbulence. Longer wavelengths will improve performance in low visibility conditions due to haze, clouds and fog. We are currently performing indoor and outdoor tests to further investigate the target/background phenomena that are emphasized in these wavelengths. We also investigate how these effects can be used for target identification and image fusion. Performed field tests and the results of preliminary data analysis are reported.

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.