|Short Wave Infrared Technology: a standardized irradiance measurement and compatibility model to evaluate reflective band systems
|Sensors & Electronics Technology|
airglow, infrared, irradiance, models, night sky, nightglow, Shortwave, SWIR illumination
The state-of-the-art shortwave infrared (SWIR) imaging technology, performance and maturity has emerged to a point where its application in military is now expanding. For instance, SWIR cameras have the potential to replace traditional night vision goggles (NVGs) by providing some key advantages, namely digital format, new detection band for day and night operation. There are numerous other applications where SWIR imaging capability is playing an important role. However, a clear understanding of highly variable SWIR illumination levels and performance assessment is lacking. The night illumination levels for the visible to near infrared are standardized and excellent models to estimate irradiance at different conditions, i.e. full moon, starlight etc. exists. There is no such standard available in SWIR band and there is no standard method by which comparison between SWIR cameras with other systems (ex. NVG) operating in reflective band is available. Therefore, any meaningful sensitivity comparisons between cameras should also incorporate standard illumination data in the SWIR band. Accurate measurement and estimation of SWIR irradiance components and understanding the total illumination in space, time and varied environmental conditions is necessary to evaluate and optimally design a SWIR system specific to mission and applications. The earth’s atmosphere scatters and absorbs incident radiation thus modifies its spectral content, transmission properties, reflectivity and decreases the intensity at the earth surface and scene objects. Additionally, photons from various natural sources as well as from urban lighting interact with the higher earth atmosphere to re-radiate in the form of so called “Nightglow” or “Airglow”. The nightglow phenomenon is particularly interesting in SWIR band. For example, during moonless nights the SWIR irradiance is an order of magnitude higher than the visible band. The nightglow intensity typically peaks in the 1.4 to 1.8 um range well within the SWIR band (0.9-2.8um). SWIR radiation reaching the Earth’s surface is significantly modulated by direct and indirect multiple interactions with clouds and other atmospheric factors that are highly variable in space and time. SWIR band also has several transmission windows that include 0.9-1.3; 1.4-1.8 and 2-2.8 micrometers. Understanding irradiance in these bands will open doors for potential new applications. A focused effort to measure SWIR irradiance, standardization and to develop a model for accurate estimation of total SWIR illumination levels will be essential. Such understanding will be needed to exploit SWIR potential role in meeting and exceeding operational requirements for NATO relevant military systems. Assessment, modeling and data collection efforts would benefit from joint NATO cooperation.
The primary objectives are joint activities to provide common methodology for measuring and modeling SWIR night sky irradiance for imaging sensors and developing techniques on how to apply them to assess and compare reflective band imaging systems. Research areas include field data collection, laboratory data assessment, field performance assessment, performance modeling, and developing a standardized set of tools. Anticipated tools include standard sets of data, models as well as configurations for end-to-end sensor performance characterization and comparison in the laboratory and field. Efforts will be made to exchange data, models and techniques to establish a common basis for research.
The effort will cover modeling, laboratory assessment, and field data collection of SWIR irradiance to understand the data and develop a model similar to visible – near infrared band. The work will cover development of a common framework for SWIR system design, evaluation and a method to compare different systems operating in the reflective band. Collected field and laboratory data will be provided to participating nations for the assessment of developed techniques. Models developed will be made available. A report consisting of recommendations for performance assessment criterion will be provided along with the detailed study results as part of the deliverables. Information may be exchanged with other NATO RTGs such as SET-217 on sensor fusion – SWIR-LWIR