Kjell Karlsson

Laser imaging of small surface vessels and people at sea

The development of new asymmetric threats to civilian and naval ships has been a relatively recent occurrence. The bombing of the USS Cole is one example and the pirate activities outside Somalia another. There is a need to recognize targets at long ranges and possibly also their intentions to prepare for counteractions. Eye safe laser imaging at 1.5 μm offers target recognition at long ranges during day and night. The 1.5 μm wavelength is suitable for observing small targets at the sea surface such as boats and swimmers due to the low reflectivity of water compared to potential targets. Turbulence and haze limits the sensor performance and their influence is estimated for some cases of operational interest. For comparison, passive EO images have been recorded with the same camera to investigate the difference between sun illuminated and laser illuminated images. Examples of laser images will be given for a variety of targets and external conditions.Image segmentation for future automated recognition development is described and examplified. Examples of relevant 1.5 μm laser reflectivities of small naval targets are also presented. Finally a discussion of system aspects is made.

Single- and double-path 10.6-μm laser link measurements over sea water

In order to evaluate the performance of laser-based applications in maritime environments, laser beam propagation studies over the Baltic Sea during the period of about half a year have been carried out. A slightly slanted path with the laser system located 18 m above the sea level and corner cube targets and separate receivers placed on islands at 2.5-, 5.5-, and 16.5-km distance were used. From the registered laser signals irradiance fluctuation parameters for different beam offsets relative to the beams center, temporal, and amplitude signal distributions, we derived the probability and mean time of fade. Results and statistics from single and double propagation paths are compared.

Coordinated multi-wavelength laser system propagation experiments

Atmospheric propagation degradation effects including attenuation, aerosol scattering and turbulence have a great impact on the performance of optical systems. Relevant military optical systems include active and passive imaging for target recognition, free-space optical communication and DIRCM/EOCM. This paper will report on experimental work including measurement of retro signals at 1.5 and in the 3-5 μm wavelength regions for evaluation of retro communication links and DIRCM performance. Imaging experiments using a range-gated system both in the active and passive mode at 1.5 μm, will also be carried along the same paths. A dedicated target box and test targets have been fabricated for mounting on a mast at 8 km from our laboratory. The box contains reflectors and receivers in different slots each of which can be opened by a telephone call. A heated target on top simulates a point target in the IR region. The test targets are aimed for the range-gated imaging system. Preliminary experimental data will be presented and discussed.

Laser propagation through turbulence over land and sea (Invited Paper)

This paper will describe single and double path laser link measurements over land and sea. The laser system consisted of a CO2 laser, a pointing and tracking head, a quadrant laser receiver, a 3-5 μm IR-camera, a TV camera and a laser range finder. For the naval scenario the laser system was placed in a building 18 m above water and corner cube targets and a single path receiver were placed on islands at 2.5, 5.5 and 16.5 km. For the land scenario the laser system was placed in a building about 13 meters above ground looking at targets and a single path receiver at 2 km range. Together with the laser registrations, separate instruments such as a scintillometer and a weather station were recording the meteorological parameters. The analysis contains evaluation of temporal and amplitude signal distributions, probability and mean time of fade and tracking performance. Results from single ended and double ended paths will be compared. Different ways of using this database for turbulence simulation and laser system performance predictions also in other wavelength regions will be discussed as well as the impact on applications including laser imaging, free-space laser communications and directed infrared countermeasures.

Multi-wavelength laser propagation experiments

Atmospheric propagation experiments for active and passive EO systems were performed over a 2 and 8 km path. Single and double path propagation effects were studied using retroreflectors and hot point targets. The systems used include laser systems at 1.5 and 3.5 μm wavelength as well as imaging systems in the visible, 1-2 and 8-9 μm regions. A scintillometer operating at 0.8 μm wavelength was also used for the shorter path. Experimental data will be presented and evaluated concerning statistics. The results will be discussed mainly from a laser countermeasure point of view.

Slant path range gated imaging of static and moving targets

This paper will report experiments and analysis of slant path imaging using 1.5 μm and 0.8 μm gated imaging. The investigation is a follow up on the measurement reported last year at the laser radar conference at SPIE Orlando. The sensor, a SWIR camera was collecting both passive and active images along a 2 km long path over an airfield. The sensor was elevated by a lift in steps from 1.6-13.5 meters. Targets were resolution charts and also human targets. The human target was holding various items and also performing certain tasks some of high of relevance in defence and security. One of the main purposes with this investigation was to compare the recognition of these human targets and their activities with the resolution information obtained from conventional resolution charts. The data collection of human targets was also made from out roof top laboratory at about 13 m height above ground. The turbulence was measured along the path with anemometers and scintillometers. The camera was collecting both passive and active images in the SWIR region. We also included the Obzerv camera working at 0.8 μm in some tests. The paper will present images for both passive and active modes obtained at different elevations and discuss the results from both technical and system perspectives.

Slant path 1.5 μm range gated imaging close to ground

This paper will report experiments, analysis and simulations of slant path imaging using 1.5 μm gated imaging. The measurements were taking place at a former airfield along a 2 km path. The sensor was elevated by a lift in steps from 2-12.5 meters. Targets were resolution charts. The turbulence was measured along the path with a scintillometer. Turbulence information was also obtained at various path positions including the elevated cage using anemometers. The camera was collecting both passive and active images in the SWIR region. In the passive mode (using solar illumination) the noise due to speckles are eliminated and the influence by scintillation limited. In the active mode on the other hand these noise sources are present to a varying degree depending on stabilized frame averaging and on the sensor elevation. A trend is that the image quality is improved for elevated sensor positions. Two light sources in the camera FOV (head lights from a car) gave independent turbulence level estimates. The paper will present evaluated images for both passive and active modes obtained at different elevations and the result will be compared with theory including image simulation.

Predicting laser beam propagation in a naval environment

Directed infrared countermeasure (DIRCM) systems are being developed in several configurations ranging from open loop, closed loop (CLIRCM) and lately for preemptive use (PIRCM). Critical performance parameters, from an atmospheric propagation point of view, include attenuation as well as turbulence effects on target detection, tracking and the capability to deliver enough power to the target (power in bucket). In order to evaluate the performance of DIRCM and other laser applications in a naval environment we have performed laser beam propagation studies over the Baltic Sea during June to October 2003. The experimental data were used to derive atmospheric attenuation, irradiance fluctuation parameters for different beam offsets relative to the beam centre, temporal and amplitude signal distributions, probability and mean time of fade and tracking performance. Results from simultaneous single and double-ended paths were obtained. This paper will shortly summarize the results from this campaign, and discuss and exemplify how the results can be used for prediction of DIRCM system performance.

Green light laser exposure at 532 nm near the exposure limit during a human volunteer vehicle driving task does not alter structure or function in the visual system

This study aimed to verify nonexistence of clinically important pathological effects to the visual system after exposure to 532 nm green laser light close to the exposure limit. The present medical surveillance of vision and visual health reported in this paper is the conjunction with a study of driver performance in the presence of 532 nm laser induced glare. The driving time varied between 25 and 55 s, depending on background luminance. The laser was on during the complete test drive. The peak corneal irradiance typically was 3.5 W·m−2 in one test drive. Considering a typical test drive, the typical time integrated corneal radiant exposure for one test drive was estimated to be 53 J·m−2. The number of test drives varied among drivers but was typically 50, thus resulting in a cumulative corneal exposure dose of approximately 2.7 kJ·m−2. Altogether, ten subjects were recruited according to inclusion and exclusion criteria. All ten subjects were examined for visual acuity, intraocular pressure, contrast se...

Modeling of 1.5 μm range gated imaging for small surface vessel identification

Within the framework of the NATO group (NATO SET-132/RTG-72) on imaging ladars, a test was performed to collect simultaneous multi-mode LADAR signatures of maritime objects entering and leaving San Diego Harbor. Beside ladars, passive sensors were also employed during the test which occurred during April 2009 from Point Loma and the harbor in San Diego. This paper will report on 1.5 μm gated imaging on a number of small civilian surface vessels with the aim to present human perception experimental results and comparisons with sensor performance models developed by US Army RDECOM CERDEC NVESD. We use controlled human perception tests to measure target identification performance and compare the experimental results with model predictions.