MWIR and LWIR spectral signatures of water and associated materials

Abstract

Spectral infrared emissivity measurements have been made of a variety of materials both with and without surface water. The surface water was either natural, in the form of dew or residual rainwater, or artificially introduced by manual wetting. Materials naturally high in water content were also measured. Despite the rather diverse spectral population of the underlying materials, they exhibited very similar, featureless, water-like spectra; spectrally flat with a very high magnitude across the emissive infrared region. The implication to exploitation personnel that may use emissive infrared hyperspectral image data is that in areas where condensation is likely (e.g. high humidity) or in areas populated with high water content background materials (e.g. highly vegetated areas), discrimination may prove an intractable problem with hyperspectral infrared sensing for ambient temperature targets. A target that exhibits a temperature either below or above ambient temperature may be detectable, but not identified, and may be more economically pursued with a far simpler, single-band midwave or longwave sensor.

Effect of perspiration on skin temperature measurements by infrared thermography and contact thermometry during aerobic cycling

“…Additionally, moistening of the skin surface due to perspiration may lead to difficulties in evaluating the temperature of the skin. Although sweat emissivity has not been determined, a coating of dew presented emissivity values as high as human skin (35), and hence, a low potential to modify skin surface emissivity could be expected. Nevertheless, Ammer suggested that a water coating on the skin due to "profuse" sweating might act as a filter for the emitted infrared radiation from the skin surface (36) , even if the water layer is just a few microns as determined for other types of surfaces (35).…”

Thermal-to-visible face recognition using partial least squares

“…Significant perspiration induced by strenuous exercise is expected to dramatically alter the thermal facial signature, because water is a strong absorber of thermal radiation and dominates IR measurements. Mitchell et al showed that a layer of water only 10 microns in depth will be optically thick in the IR spectrum [35]. Furthermore, the non-uniform distribution of pores in the face is expected to lead to significant nonlinear distortions of the thermal facial signature with strenuous exercise.…”

Spectral reflectance and emissivity of man-made surfaces contaminated with environmental effects

“…Such pristine databases are appropriate for material identification or spectral unmixing applications, but they fail to capture the variability in reflectance that can arise in outdoor natural settings, and is necessary for realism in synthetic image generation applications. There has been some work in characterizing the effects of disturbances on soils 17 and measurements of the effects of thin layers of water, 18 but no previous work could be found that conducted a systematic study of the impact on spectra of environmental contaminants for several man-made surfaces.…”