Rolf Persson

Evaluation of biological aerosol stand-off detection at a field trial

We have performed a field trial to evaluate technologies for stand-off detection of biological aerosols, both in daytime and at night. Several lidar (light detection and ranging) systems were tested in parallel. We present the results from three different lidar systems; one system for detection and localization of aerosol clouds using elastic backscattering at 1.57 μm, and two systems for detection and classification of aerosol using spectral detection of ultraviolet laser-induced fluorescence (UV LIF) excited at 355 nm. The UV lidar systems were utilizing different technologies for the spectral detection, a photomultiplier tube (PMT) array and an intensified charge-coupled device (ICCD), respectively. During the first week of the field trial, the lidar systems were measuring towards a semi-closed chamber at a distance of 230 m. The chamber was built from two docked standard 20-feet containers with air curtains in the short sides to contain the aerosol inside the chamber. Aerosol was generated inside the semi-closed chamber and monitored by reference equipments, e.g. slit sampler and particle counters. Signatures from several biological warfare agent simulants and interferents were measured at different aerosol concentrations. During the second week the aerosol was released in the air and the reference equipments were located in the centre of the test site. The lidar systems were measuring towards the test site centre at distances of either 230 m or approximately 1 km. In this paper we are presenting results and some preliminary signal processing for discrimination between different types of simulants and interference aerosols.

Aerosol extinction models based on measurements at two sites in Sweden

Two aerosol extinction models have been developed using statistical analysis of long-term optical transmission measurements in Sweden performed at two locations from July 1977 to June 1982. The aerosol volume extinction coefficient for infrared (IR) radiation is calculated by the models with visibility, temperature, and air pressure as input parameters. As in the MODTRAN model, the IR extinction coefficient is proportional to the coefficient at 550 nm, which depends on the visibility. In the new models, the wavelength dependence of the extinction also depends on the visibility. The models predict significantly higher attenuation in the IR than does the Rural aerosol model from MODTRAN, which is commonly used. Comparison with the Maritime model shows that the new models predict lower extinction values in the 3-5 microm region and higher values in the 8-12 microm region. The uncertainties in terms of variance levels are calculated by the models. The properties of aerosols, and thereby the extinction coefficient, are partly correlated to local meteorological parameters, which enables the calculation of a mean predicted value. A substantial part of the variation is, however, caused by conditions in the source area and along the trajectory path of the aerosols. They are not correlated to the local meteorological parameters and therefore cause the variance in the models.

Using an eyesafe military laser range finder for atmospheric sensing

Laser Rangefinders are well established components in various electro-optical fire control systems. Recent range finders are often operating at eye safe wavelengths around 1.5 μm which extend their utility. One such extension is the use of the sensor for atmospheric sensing based on the measured backscatter signal. The present paper investigates the use of an eye-safe laser rangefinder at 1.5 μm to obtain information on atmospheric attenuation at various paths in the atmosphere. This knowledge can in turn be used in combination with atmospheric and target/background models to estimate the performance of other EO sensors like TV and thermal imagers beside the laser range finder itself. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along a variable atmosphere especially in darkness. The paper will describe the experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths. The backscatter curve is used to evaluate the extinction. This extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at 2 ranges from the sensor. The results indicated a good correspondence between these results. Finally the results are illustrated in a system perspective by estimating the performance for thermal IR and other EO sensors.

Using an eye-safe laser rangefinder to assist active and passive electro-optical sensor performance prediction in low visibility conditions

Abstract. Laser rangefinders are used in various electro-optical (EO) fire control systems. They often operate at eye-safe wavelengths around 1.55  μm, which extends their utility. The paper investigates the use of a modified eye-safe laser rangefinder at 1.55  μm to obtain information on atmospheric attenuation and couple that information to the performance of active and passive EO sensors with an emphasis of lower visibility conditions. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along variable and slant atmospheric paths, especially in darkness. The experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths are presented. The backscatter curve is used to evaluate the extinction. These extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at two ranges from the sensor. TV and IR images of test targets along a 1.8 km path close to sea surface in the Baltic Sea were collected in parallel with the lidar. A weather station and a scintillometer collected weather and turbulence parameters. Results correlating the lidar attenuation with the imaging performance will be given.

Laser dazzling impacts on car driver performance

A growing problem for the Police and Security Forces has been to prevent potentially hostile individuals to pass a checkpoint, without using lethatl violence. Therefore the question has been if there is a laser or any other strong light source that could be used as a warning and dazzling device, without lethal or long term effects. To investigate the possibilities a field trial has been performed at a motor-racing track. A green CW laser with an irradiance on the eye of maximum 0.5 MPE, as defined by ICNIRP [1] and the ANZI standard [2], was used as a dazzle source. Ten drivers have been driving with dipped headlights through a course of three lines with orange cones. In every line there has been only one gate wide enough to pass without hitting the cones. The time through the course, the choice of gates and the number of cones hit have been measured. For every second trial drive through the track, the driver was exposed to the laser dazzler. The background illuminances ranged from a thousand lux in daylight to about ten millilux in darkness. The protective effect of the sun-visor of the car was investigated. The drives visual system was carefully examined before and after experimental driving and a few weeks after the experimental driving to verify that no pathological effects, that could potentially be induced by the laser exposure, pre-existed or occurred after the laser exposure. An analysis of variance for a within subjects design has been used for evaluation. It was found that green laser light can have an obvious warning effect in daylight. Dazzling does reduce the drivers ability to make judgments and manouver the car in twilight and darkness. A sun-visor can reduce the glare and give the driver an improved control, but that perception can be unjustified. No damage to the visual system was observed.

Infrared transmission measurement in the atmosphere

With the purpose of establishing a model for the IR extinction of atmospheric aerosols in relation to weather parameters, we have performed measurements with a transmissometer and meteorological sensors. A CO2-lidar is used to measure aerosol extinction at 106 microns in slant paths in order to investigate the altitude dependence of the extinction. The beam from a pulsed TEA-laser is emitted into the atmosphere and backscattered radiation is detected. The signal is digitized and used as input in a computer program to solve the lidar equation. The extinction coefficient vs range or altitude is obtained. The principle and characteristic properties of the two instruments are described and examples of obtained results are presented.

Lidar measurement as support to the ocular hazard distance calculation using atmospheric attenuation

The reduction of the laser hazard distance range using atmospheric attenuation has been tested with series of lidar measurements accomplished at the Vidsel Test Range, Vidsel, Sweden. The objective was to find situations with low level of aerosol backscatter during this campaign, with the implications of low extinction coefficient, since the lowest atmospheric attenuation gives the highest ocular hazards. The work included building a ground based backscatter lidar, performing a series of measurements and analyzing the results. The measurements were performed during the period June to November, 2014. The results of lidar measurements showed at several occasions’ very low atmospheric attenuation as a function of height to an altitude of at least 10 km. The lowest limit of aerosol backscatter coefficient possible to measure with this instrument is less than 0.3•10-7 m-1 sr-1. Assuming an aerosol lidar ratio between 30 – 100 sr this leads to an aerosol extinction coefficient of about 0.9 - 3•10-6 m-1. Using a designator laser as an example with wavelength 1064 nm, power 0.180 W, pulse length 15 ns, PRF 11.5 Hz, exposure time of 10 sec and beam divergence of 0.08 mrad, it will have a NOHD of 48 km. With the measured aerosol attenuation and by assuming a molecule extinction coefficient to be 5•10-6 m-1 (calculated using MODTRAN (Ontar Corp.) assuming no aerosol) the laser hazard distance will be reduced with 51 - 58 %, depending on the lidar ratio assumption. The conclusion from the work is; reducing of the laser hazard distance using atmospheric attenuation within the NOHD calculations is possible but should be combined with measurements of the attenuation.

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...

Passive and active EO sensing close to the sea surface

The present paper investigates the use of an eye-safe laser rangefinder at 1.5 μm and TV/IR imaging to obtain information on atmospheric properties at various paths close to the sea surface. On one day active/passive imaging NIR and SWIR systems were also used. The paper will describe the experimental equipment and the results from measurements of atmospheric backscatter as well as TV and IR images of test targets along a 1.8 km path close to the Baltic Sea. The site also contained a weather station and a scintillometer for logging weather and turbulence parameters. Results correlating the lidar attenuation with the imaging performance will be given and compared with models.

The use of measured RF power signals to evaluate feasibility of inverse methods to retrieve refractivity parameters

Radio wave propagation over sea paths is influenced by the local meteorological condition at the atmospheric layer near the surface, especially during ducts. Duct condition can be determined by measurements of local meteorological parameters, by weather forecast models or by using inverse methods. In order to evaluate the feasibility of using inverse methods to retrieve the refractivity profiles a measurement of RF signals and meteorological parameters were carried out at a test site in the Baltic. During the measurements, signal power from two broadcast antennas, one at Visby and one at Vastervik, were received at Musko, an island south of Stockholm. The measurements were performed during the summer 2005 and the data was used to test the software package for inversion methods, SAGA (Seismo Acoustic inversion using Genetic Algorithms, by Peter Gerstoft UCSD, US). Refractivity profiles retrieved by SAGA were compared with the refractivity profiles calculated from measured parameters, during parts of the experiment, from rocket sounding, radio sounding, local meteorological measurements using bulk model calculations, and also obtained by the Swedish operational weather forecast model HIRLAM. Surface based duct height are predicted in relative many situations even though the number of frequencies or antennas height has to be increased to diminish the ambiguous of the refractive index profile.