Results are reported from a study of propagation near-ground within the solar blind ultraviolet wavelength range, including measurements of ultraviolet transmission along a horizontal path. In addition a discussion is presented about ozone data measured at 41 stations spread across Sweden, collected and stored by the Swedish Meteorological and Hydrological Institute (SMHI) and the Swedish Environmental Protection Agency (SEPA). In connection with this a simple statistical analysis of occurrence of the ground ozone is done.
The transmission measurements were made along slightly inclined path of 1830 m during the period from mid- November 2018 to mid-January 2019. The transmission values were collected together with weather data including measurement of ozone content in the atmosphere.
A simple model for ultraviolet transmission that takes into account both visibility and measured ozone content has been developed. This model shows a difference to the model found in the Modtran program, often used to predict atmospheric transmissions in several wavelength bands including ultraviolet wavelengths. Overall, the new model shows a weaker connection between ultraviolet extinction, ozone content and visibility compared to Modtran.
Data from the countrywide ozone measurements as well as data collected in connection with transmission measurement showed variations in the atmospheric ground-level ozone content that are not easily explained from weather parameters alone. Reports, see for instance , state that it is volatile hydrocarbons and other molecules, both natural and anthropogenic, along with solar radiation that influence the amount of ozone. The variation of both volatile hydrocarbons and solar radiation together with a variation in wind direction and speed have an impact of varying transmission.
In practice all measured ozone values during conditions with low relative humidity were relatively high. There was also a high correlation between high ozone values and high wind velocities. The relation between ultraviolet extinction deduced from measurements and the corresponding values for the visual wavelength range and corresponding values calculated using Modtran show different relationships. The transmission measurements in the ultraviolet wavelengths range showed that the amount of aerosols has no major significance for the ultraviolet extinction at visual ranges above 15 km.
Due to the short period of data collection, mostly fall-like weather conditions, the conclusions that can be drawn from the measurements are limited. Furthermore, the ground-level horizontal measurements make it very hard to draw conclusions about the ultraviolet transmission at higher air layers.
As the sensor technology for polarimetric imaging is advancing into more robust commercial systems such sensors could soon be expected for, e.g., military surveillance and reconnaissance applications in addition to more conventional sensor systems. Thus, there might be an upcoming need to understand limitations on present camouflage systems to meet this new sensor threat. Some of the reasons why polarimetric imaging has drawn attention is the ability to achieve a higher contrast for artificial surfaces against natural backgrounds, by analyzing the degree of linear polarization, which in this work has been analyzed for different types of surfaces as a function of wavelength. We also compare with the polarimetric vision of horse-flies and other aquatic insects via the polarization properties of different colors of horse coat hair in order to give some further insight into polarimetric vision techniques developed by nature. In this work we have used different measurement techniques, such as angle dependent polarimetric spectral directional hemispherical reflectance and polarimetric imaging.
Polarimetric imaging sensors in the electro-optical region, already military and commercially available in both the visual and infrared, show enhanced capabilities for advanced target detection and recognition. The capabilities arise due to the ability to discriminate between man-made and natural background surfaces using the polarization information of light. In the development of materials for signature management in the visible and infrared wavelength regions, different criteria need to be met to fulfil the requirements for a good camouflage against modern sensors. In conventional camouflage design, the aimed design of the surface properties of an object is to spectrally match or adapt it to a background and thereby minimizing the contrast given by a specific threat sensor. Examples will be shown from measurements of some relevant materials and how they in different ways affect the polarimetric signature. Dimensioning properties relevant in an optical camouflage from a polarimetric perspective, such as degree of polarization, the viewing or incident angle, and amount of diffuse reflection, mainly in the infrared region, will be discussed.
Polarimetric information has been shown to provide means for potentially enhancing the capacity of electro-optical sensors in areas such as target detection, recognition and identification. The potential benefit must be weighed against the added complexity of the sensor and the occurrence and robustness of polarimetric signatures. While progress in the design of novel systems for snapshot polarimetry may result in compact and lightweight polarimetric sensors, the aim of this work is to report on the design, characterization and performance of a polarimetric imager, primarily designed for polarimetric signature assessment of static scenes in the long wave thermal infrared. The system utilizes the division-of-time principle and is based on an uncooled microbolometer camera and a rotating polarizing filter. Methods for radiometric and polarimetric calibrations are discussed. A significant intrinsic polarization dependency of the microbolometer camera is demonstrated and it is shown that the ability to characterize, model and compensate for various instrument effects play a crucial role for polarimetric signature assessment.
Radiometric calibration and non-uniformity correction are key operations in a signature measurement, which is a
special challenge in the infrared range where a large number of parameters need to be characterized. The radiation
hitting the camera's pixels will not only be due to the target, but will also depend on the atmosphere and optical
components like lens and spectral filters. To obtain broadband radiative properties of a target, the spectral properties
of these components must be accurately characterized. In addition to signal contributions from the incident radiation,
the pixel’s digital numbers will also depend on the individual responses of the pixels. Results are presented for an
infrared camera of the following examined parameters: stabilization period, dynamic response, dynamic range,
ambient temperature dependence and non-uniformity. In radiometric calibrations using area blackbody sources, an
estimate of the sensor signal is obtained by pixel averaging (which reduces the influence of non-uniformity), and the
spectral distributions of the sources are known (via the Planck distribution). These conditions do not normally apply
for signature measurements e.g. of small hot spots involving only a few pixels. The measurement uncertainty is
compared between calibrations based on mean values and pixel-wise calibration. It is shown that the pixel-by-pixel
variation should be included in an analysis of the measurement uncertainty. A discussion is given of the effects of
unknown spectral distributions on the measurement uncertainty.