Thermal imaging in nature

InfraMation 2018 Application Paper Submission

Michael Vollmer, Klaus-Peter Möllmann 

Brandenburg University of Applied Sciences, Germany

ABSTRACT 

Thermography is mostly used for specialized applications, e.g. in industry or for security, however - as  every thermographer knows – it usually offers a multitude of additional fascinating sights in nature as well.  The paper describes some typical situations in nature where infrared thermal imaging provides  fascinating views which also contain a lot of interesting physics.

INTRODUCTION 

Although thermography is primarily used with a specific purpose in mind, dealing e.g. with problems in  industry, science, surveillance etc., it can also offer many fascinating sights in nature as well. Before or  after outdoor inspections one may just turn the camera from the object under study to the sky, sun or  moon, or just to natural thermal phenomena happening everywhere in the world around us. The paper gives a short overview of some potential studies of infrared imaging in nature which besides their  aesthetics also contain interesting physics as well. This will explain why what we actually measure is  sometimes not what we expect to measure and why sometimes we do not even know what to expect. Many more details on all phenomena can be found in the second edition of the well-known textbook  Infrared Thermal Imaging – Fundamentals, Research and Applications [1].

EXAMPLE INVESTIGATIONS IN NATURE WITH IR CAMERAS

Often working outside, probably many thermographers cannot resist the temptation to point the camera  towards the Moon or the Sun, the sky or clouds. If one is not only interested in nice colorful imagery, but  rather also in quantitative understanding of these images, one may sometimes get caught by surprise.  Think e.g. of the Moon. What kind of surface temperatures are expected and is it possible to measure  these temperatures from, say, sea level locations on Earth? The answers to these questions are not easy. 

In principle, one can estimate the surface temperatures on the lunar surface according to the exact  position on the Moon, however, what is usually measured by a commercial camera is far off the expected  values and only a very detailed analysis of what happens with the lunar IR radiation in the Earth  atmosphere can resolve the discrepancy [2-4], see Fig. 1. The presentation will give some hints to critical  issues in the analysis.

Similarly interesting though for most camera users by far more difficult is to measure surface  temperatures of the sun using a handheld IR camera. Quantitative analysis is still possible [2], however,  one must carefully adjust attenuation of the solar radiation to avoid any camera sensor damage. There  are however, situations on Earth where it is possible without risk to pointing the camera directly towards  the Sun in a clear sky. The last time this happened in the US was August 21st, 2017 during the total solar  eclipse. Fig. 2 depicts an image recorded with an InSb MW IR camera during totality [5]. It nicely depicts  the solar corona which is illuminated by solar radiation and therefore scatters residual IR radiation  towards the Earth.

Besides Moon and Sun, one may of course easily point the IR camera towards the sky or clouds. Initially one may be surprised about the temperature readings, because clear skies usually have very low  readings. On second thought, however, the temperatures are well above what one may expect if  anticipating a transparent atmosphere and looking at the cold background of the universe. The solution is  threefold. First, most commercial cameras are at their lower detection limit and signals get buried in noise,  which means the detected temperatures may resemble the noise level. Second, the atmosphere is not  entirely transparent and residual absorption also means residual IR emission in the detected IR spectral  range. Third, depending on elevation angle of the observation direction above the horizon the camera  sees different amounts of atmospheric thickness, a phenomenon described by the so-called air mass [6].  Any analysis must account for this geometrical effect. Fig. 3 shows an example of uncorrected  temperature readings of sky recorded with a LW IR camera directed towards the horizon sky. For  reference reasons, the setting moon was included in the image. The apparent temperature of the sky  correlates with the elevation above the horizon, which is obviously an artifact as mentioned above.

Sky detection is easier if there is contrast to clouds. The contrast between clear skies and thin clouds is  increased in the IR range. As a matter of fact, IR imaging has been used to detect clouds in the arctic  regions [1,7], which is important for the modelling of cloud cover in climate change models. 

There are many fascinating optical phenomena of the atmosphere in the visual spectral range such as  rainbows, mirages, halos, glories, and many more. The obvious question is whether it is also possible to  detect such phenomena in the IR spectral range. The answer is: it depends on the IR wavelength range!  Nearly all of the mentioned visual atmospheric phenomena can be observed equally well in the IR range  [1, 8-11]. We have recorded various of the phenomena in at least 4 different spectral ranges: the very  near IR (NIR) from 0.8 to 1.1µm wavelength [8-10], then the so called SW IR range from 0.9 to 1.7 µm  [1,9], the MW IR from 3-5 µm and the LW IR from 8 to 14µm [1,11]. Fig. 4 depicts an example of a NIR  glory, recorded between 0.8 and 1.1µm. 

Fig. 5 shows a LW IR mirage, recorded at Bozeman airport. The visual mirage on top nicely shows the  mirror image on the air strip. The corresponding LW IR mirage image is similar but also shows pronounced differences to the visual one. For example, in the visible mirage, bright objects such as the  white hull of the plane and the headlights are readily seen. In the IR mirage image, however, the upper  sides of the wings show cold sky reflections, the headlights are not visible, instead the tires which  warmed up upon touchdown and nicely show up. 

We note that there are many other pronounced differences between visual observations of atmospheric  optical phenomena and those in the adjacent IR wavelength ranges [10].

IR imaging in nature is also obviously useful whenever thermal features are observed. These include for  example all kinds of wildlife [12] or wildfires. In addition, some of the most fascinating phenomena involve  really hot objects, think for example of volcanic eruptions. Active volcanos can be dangerous places, and  

it is a little safer to study geothermal features, e.g. hot springs and pools and geysers wherever  geothermally active regions are located, e.g. in Island, New Zealand or Yellowstone National Park in the  US. Thinking of the newly available smartphone accessory IR cameras, such features can be easily  studied during private visits. However, a safe distance to sceneries usually requires better spatial  resolution. Fig. 6 depicts as an example a spectacular infrared view of Grand Prismatic Spring, recorded  from a nearby hill. We have recorded and discussed many thermal IR features in Yellowstone park with  their physics context [13-15].

SUMMARY 

We have presented some examples for IR imaging in nature. The oral presentation will show many more  examples. More details on all these phenomena can be found in our comprehensive textbook on IR  imaging [1] and the respective original publications [2-15]. 

REFERENCES

[1] M. Vollmer, K.-P. Möllmann, Infrared Thermal Imaging – Fundamentals, Research and  Applications, Wiley (2018), Second Edition, ISBN 978-3-527-41351-5 

[2] Measurements of sun and moon with IR cameras: effects of air mass, M. Vollmer, F. Pinno,  K.-P. Möllmann, Inframation 2010, Proc. Vol 11, p. 57-74 

[3] Surface temperatures of the Moon: measurements with commercial infrared cameras, M.  Vollmer, K.-P. Möllmann, Eur. J. Phys. 33, 1703-1719 (2012) 

[4] Infrared moon imaging for remote sensing of atmospheric smoke layers, J.A. Shaw, P.W.  Nugent, M. Vollmer, Applied Optics 54/4, B64-B75 (2015)  

[5] Measurements of SWIR backgrounds using the swux unit of measure, A. Richards, M.  Hübner, M. Vollmer, Proc. SPIE 10625, Infrared Imaging Systems: Design, Analysis,  Modeling, and Testing XXIX, 106250P (2018) 

[6] Colors of the sun and moon: the role of the optical air mass, M. Vollmer, S. Gedzelman, Eur.  J. Phys. 27 299-309 (2006) 

[7] Physics principles in radiometric infrared imaging of clouds in the atmosphere, J.A Shaw,  P.W Nugent, Eur. J. Phys. 34 S111–S121 (2013) 

[8] The Physics of Near-Infrared Photography, K. Mangold, J.A. Shaw, M. Vollmer, Eur. J. Phys.  34/6, S51-71 (2013) 

[9] NIR photography and NIR thermal cameras, M. Vollmer, K.-P. Möllmann, Inframation 2016  Proceedings, 2016-039 

[10] Atmospheric Optics in the Near Infrared, J.A. Shaw, M. Vollmer, Applied Optics, 56/19,  G145 (2017) 

[11] Visible and invisible mirages: Comparing inferior mirages in the visible and thermal infrared,  M. Vollmer, J.A. Shaw, P.W. Nugent, Applied Optics 54/4, B76-B84 (2015)  

[12] Energetic costs of mange in wolves estimated from infrared thermography, P. Cross, et al, Ecology 97 (8), 1938-1948 (2016) 

[13] Infrared Yellowstone, J.A. Shaw, P.W. Nugent, W. Harris, M. Vollmer, Optics and Photonics  News 28 (6), 37-43 (2017) 

[14] Photonics in Nature: Yellowstone National Park in IR, M. Vollmer, J.A. Shaw, P.W. Nugent,  W. Harris, K. Gillis, W. Weiss, L. Carpenter, A. Carpenter, B. Scherrer, in Education and  Training in Optics and Photonics (ETOP) 2017, edited by Xu Liu and Xi-Cheng Zhang, Proc.  of SPIE Vol. 10452, 104521B-1 

[15] Colors of thermal pools at Yellowstone National Park, P.W. Nugent, J.A. Shaw, M. Vollmer,  Applied Optics 54/4, B128-B139 (2015)  

ACKNOWLEDGEMENTS

The authors wish to thank Joe Shaw and Paul Nugent for the fruitful collaborations on IR imaging of the  Moon, LW mirages, and of Yellowstone National Park (Figs. 1,5,6) as well as Austin Richards for  providing the MW IR image during the total solar eclipse (Fig. 2).

ABOUT THE AUTHOR 

Michael Vollmer has been a Professor of Physics at the University of Applied Sciences in  Brandenburg, Germany since 1994. He received his PhD degree for studies of clusters on surfaces, and  his habilitation on optical properties of metal clusters from the University of Heidelberg, Germany. Later  assignments were with the University of Kassel, Germany, the university of California in Berkeley, USA,  as well as with various institutions in the United States and Asia during sabbaticals. His research  interests include atmospheric optics, spectroscopy, infrared thermal imaging, and the didactics of  physics. Professor Vollmer has authored one science book and co-authored two scientific monographs  and about 250 scientific and didactical papers. 

K.-P. Möllmann has been a Professor of Physics at the University of Applied Sciences, in Brandenburg,  Germany since 1994. He received his PhD from the Humboldt University of Berlin, Germany, studying  strongly doped narrow band semiconductors at low temperatures and later, for his habilitation, MCT  infrared photo detectors. Professor Möllmann subsequently held positions with the Humboldt University  and with several businesses in industry. His research interests include MEMS technology, infrared  thermal imaging, and spectroscopy. He is the co-author of about 150 scientific and didactical papers.