1. Fermat’s Principle and the Laws of Reflection and Refraction, http://scipp.ucsc.edu/~haber/ph5B/fermat09.pdf
2. M. Born, E. Wolf, Principle of Optics, 7th edn. (Pergamon, Oxford, UK, 2007)
3. W. Singer, M. Totzek, H. Gross, Physical Image Formation (Wiley, 2005)
4. Beer-Lambert law, https://en.wikipedia.org/Beer-Lambert_law
5. M. Vollmer, K.-P. Mollmann, Infrared Thermal Imaging: Fundamentals, Research and Applications (Wiley-VCH Verlag GmbH & Co. KGaA, Germany, 2010)