Leslie Salem Balfour

Contribution Of Oxygen To Attenuation In The Solar Blind UV Spectral Region

The Solar Blind Ultraviolet (SBUV) spectral region covers the interval between 230 nm and 290 nm. The lower limit of this interval is given by the edge of the Schumann-Runge band and the upper limit is determined by solar radiation penetrating the stratospheric ozone shield. The SBUV region is interesting from the experimental point of view, since the lack of solar background is favorable in such applications as lidar, atmospheric communication and remote sensing. The present models (LOWTRAN-6) include as atmospheric attenuators in this region ozone absorption, aerosol and molecular scattering. New theoretical calculations of the Herzberg I oxygen band predict significant absorption by 02. This prediction is confirmed experimentally in the present study. Field measurements at 252, 255 and 264 nm are reported over optical paths of up to 2750 m. Results show that LOWTRAN-6 is inadequate in the SBUV region, as indicated by the present extinction measurements.

Statistical analysis of measurements of atmospheric turbulence in different climates

Atmospheric turbulence may have a strong impact on the imaging quality of long range warning sensors and other electro-optical systems. Major effects are beam broadening, intensity fluctuations (or scintillation) and angle-of- arrival fluctuations. The structure constant of refractive index fluctuations, Cn2, is the parameter most commonly used to describe the strength of atmospheric turbulence. FGAN-FOM measured Cn2 values in two different climates, moderate climate in mid-Europe, Germany and arid climate in Israel. The measurements in arid climate were carried out in cooperation with the EORD (Electro-Optics Research & Development Foundation Ltd.), TECHNION, Haifa, Israel. The measurements were performed with identical laser scintillometers along a horizontal optical path of about 100 m, above grassland in mid-Europe, and above stony ground without vegetation in Israel. The data were collected continuously for a time period of at least one year at a time resolution of 5 minutes. For both climates examples of the diurnal cycle of Cn2 are given. Since Cn2 usually changes as a function of time-of-day and of season its influence on electro-optical systems can only be expressed in a statistical way. Therefore the cumulative frequencies of occurrence of Cn2 were calculated for a time period of one month for both climates. These results were used to calculate the corresponding turbulence modulation transfer function (MTF) and point spread function (PSF) for a typical IR sensor with a Cadmium Mercury Telluride detector (CMT) and a UV sensor.

Polarization properties of targets and backgrounds in the infrared

Modern infrared (IR) imaging systems are sensitive enough to detect weak targets, but background clutter makes the detection difficult. The introduction of an IR polarizer into thermal imaging systems is one of the techniques to improve this low target-to-clutter ratio. The use of polarized IR energy helps to detect man-made objects in complex natural backgrounds. Over the past 4 years, we have investigated the polarization properties of thermal IR radiation (8 - 12 micrometers ). In the course of our work, we have built an infrared imaging polarimeter and participated in field and laboratory experiments. This paper summarizes the results of our work. It includes a brief theoretical background, description of the equipment, and a comparison of our empirical findings with a theoretical model and with results of other researchers.

Contribution of oxygen to attenuation in the solar blind UV spectral region

The solar blind ultraviolet (SBUV) spectral region covers the interval between 230 and 290 nm. The lower limit of this interval is given by the edge of the Schumann-Runge band and the upper limit is determined by solar radiation penetrating the stratospheric ozone shield. The SBUV region is interesting from the experimental point of view, since the lack of solar background is favorable in such applications as lidar, atmospheric communications, and remote sensing. The present models (LOWTRAN-6) include as atmospheric attenuators in this region ozone absorption and aerosol and molecular scattering. New theoretical calculations of the Herzberg I oxygen band predict significant absorption by O(2). This prediction is confirmed experimentally in this study. Field measurements at 252, 255, and 264 nm are reported over optical paths of up to 2750 m. Results show that LOWTRAN-6 is inadequate in the SBUV region, as indicated by the present extinction measurements.

Semi-empirical model-based approach for IR scene simulation

This paper describes a novel approach of generating radiometric scenes of natural backgrounds that will serve as an input for simulating E-O sensor scene outputs in the thermal band. The methodology is based on segmentation of a measured scene (in any spectral band) into elements that have similar thermal behavior. The thermal radiance value for each thermal element is calculated using a set of four semi empirically determined coefficients that relate the surface temperature to the local meteorological parameters such as solar radiation, longwave sky radiation, air temperature and wind speed. The thermal coefficients are determined using a theoretical model and an experimental data base. The diurnal variations of the scene are thus easily predicted by knowing the meteorological parameters and the individual set of thermal coefficients for the various thermal elements of the scene. Since the approach is based on a real scene image and an experimental database the generated images have a realistic appearance including realistic clutter properties. The generated thermal scene will serve as the input to a sensor model that will calculate the expected image of a thermal camera viewing the scene. The paper describes the methodology of the scene generation, the sensor model and demonstrates the approach by giving some examples.

Different aspects of backgrounds in various spectral bands

Both the theoretical and the experimental problems of backgrounds are examined. The authors show why the current definitions of correlation length should be used with care, with attention paid to the intensity histogram of a scene. Different effects of the sub-pixel features in a measured scene on the clutter for imaging and scanning systems are also explained. The two- dimensional polarization of a scene is measured and found to compare favorably with the theoretical predictions. Finally, the authors show how to simulate backgrounds whose power spectrum is given, together with constraints on the image proper. This is achieved by iteratively transforming between the image plane and its Fourier conjugate, while imposing the appropriate constraints in both planes.

Quantitative evaluation of errors in remote measurements using a thermal imager

This work presents quantitative evaluation of errors in measurement that arise in using a thermal imager as a radiometer. The analysis will mainly deal with the 7 13 micron spectral region . The main sources contributing to errors in spatial radiometric measurements can be divided into three categories: 1) inaccuracies in determining atmospheric transmittance and path radiance, 2) calibration errors and dynamic range problems due to nonlinearities of instrument response, 3) errors due to spatial response arising from the finite point spread function and non uniformities of the field of view. The contribution of each of these factors to the final cumulative error in the measured radiometric quantity will be analysed and the sensitivity to the individual factors shown. This analysis is done with respect to an existing measurement system in use, namely the AGEMA 780 Dual Band Thermovision Imager.

Determination of infrared contrast transmittance through an obscuring atmosphere

This work describes a new technique that can be used to determine the IR transmittance and path radiance of an obscuring atmosphere. The method is based on alternate measurements of contrast through a clear and obscuring atmosphere respectively. An advantage of this technique is that it utilizes existing thermal imagers and does not require an additional transmissometer in the field. The technique was tested using an AGEI4A 780 Thermovision camera operating in the 7.7-13.2 micron spectral region. A good agreement between theory and the experimental results was obtained.

Polarization signatures of man-made objects in the SW infrared spectral band

Detectability of man made objects in natural environment is strongly affected by the clutter properties of the background. To improve the ability to detect and recognize targets by human observers or by machine vision (ATR algorithm) additional information obtained from polarization properties can be exploited. This paper presents an experimental set up for the measurements of the polarization signature of man made objects based on a commercial FPA IR camera and an externally mounted linear polarizer. Some typical results obtained by this system are presented.

Progress in radiometry: measurement techniques and analysis methods

Remote sensing is based on the ability to measure accurately the spectral radiance of remote objects in the object plane. This ability is limited by the measuring system (resolution and sensitivity) and by the atmospheric transmittance, especially when long distances are involved. As a result, the need to enhance S/N led us to develop new measurements techniques and analysis methods. This presentation deals with two different techniques of modern radiometry -- point spectroradiometry with moderate spectral resolution and spatial radiometry (imaging systems) with low spectral resolution. This presentation will address three issues related to advanced analysis methods of radiometric measurements: (1) The effect of the exact shape of the slit- function of the point radiometer on the results of the spectral analysis, (2) the optimal calculation of a signature from radiometric imager, and (3) the correcting factor that must be introduced into the analysis of a spatial picture of point target which is much smaller than the IFOV of the imaging system (star detection). The experience and knowledge gained by IMOD and EORD in the area of radiometric analysis was implemented in a user friendly software (TIRAS) that is used for the radiometric (and not temperature) analysis of various spatial radiometers. The radiometric data was measured for various applications of IMOD such as data bases of targets and backgrounds, and study of radiometric behavior of IR scene elements.