A number of technological processes and natural phenomena are accompanied by heat transfer concerned with a propagation of thermal radiation in disperse systems. Generally, thermal radiation is thought to be relevant only at high temperatures. This widespread error is easily overcome if we remember, for example, that the weather and climate on our planet are mainly determined by thermal radiation of cloudy atmosphere and the Earth surface. Few people are aware that quality of ordinary sleeping-bag is connected with changing of radiation transfer conditions in fibrous material.

In examples mentioned, as well as in many other cases, thermal radiation emission, absorption, and scattering takes place in a medium containing numerous particles of size comparable with the radiation wavelength. Such media are customary called the disperse systems. One has to solve radiation heat transfer problems for disperse systems in highly different applications such as heat transfer in solid propellant rocket engines and solar chemical reactors, characterization of advanced composite coatings and highly porous thermal insulations, microwave remote sensing of the ocean surface with breaking waves, and spacecraft thermal control by use of liquid droplet radiator. The geometrical scales of particles, bubbles, and pores in the above mentioned thermal radiation problems and in many other problems may vary in a very wide range – from nanometers in some advanced materials to several millimeters or even greater in the microwave applications. As a result, both experimental technique and theoretical modeling should be based on a general physical analysis of electromagnetic waves interaction with single particles and adequate description of the radiation propagation in complex disperse systems. It goes without saying that direct simulation of the radiation emission, absorption, and scattering based on the first principles is impractical at the moment and one should find alternative engineering approaches by using the known solutions to some simplified problems. In several articles following from the present introductory article, both theoretical models and experimental methods are described in some details.

The well-known Mie theory is usually considered as a basis of theoretical modeling of spectral radiative properties of single particles. One can recommend the excellent books by Van de Hulst (1957) and Bohren and Huffman (1983) for studying the rigorous theory for spherical and cylindrical particles and some particular theories which are applicable in important limiting cases. The present-day possibilities in theoretical analysis of absorption and scattering characteristics of complex-shape single particles and clusters or aggregates of particles are well presented in the book by Mishchenko et al. (2006), the numerical methods can be found in review by Kahnert (2003). To find key knowledge on the theoretical modeling and the radiative properties of single particles (especially those which are often encountered in the engineering problems) one can be addressed to the book by Dombrovsky (1996) or to the article “Radiative properties of particles and fibers (theoretical analysis)” and a set of related articles of the Radiation Area of Thermopedia.

The Mie solution for spherical particles is employed in analysis of so many present-day problems (not only in heat transfer theory) that I would like to recommend to study and use this general tool for all students and engineering. Even in the case if you radically change a direction of your research work, the Mie theory could help you. As an example, please look at recent papers by Costello et al. (2007) on predicting light scattering from particles observed in human age-related cataracts and Serebrennikova et al. (2008) on quantitative interpretation of reflectance spectra of blood. Both studies are based on the rigorous Mie theory.

Of course, it is often insufficient to know the properties of single particles to predict the radiative properties of complex disperse systems. A good example is concerned with very complex properties of advanced thermal insulations. In this case, the experimental identification provides the knowledge of real material properties. As an introduction to the problem of the experimental identification of spectral radiative properties of dispersed materials, one can recommend a review by Baillis and Sacadura (2000). Some more recent references will be given in particular articles following from the article “Experimental study and theoretical modeling of spectral radiative properties of dispersed materials”. It is important to note that theoretical and experimental studies of the radiative properties of dispersed materials are closely related with each other. A long-time experience of the work in this field has showed that the most interesting results of practical importance can be obtained on the basis of a reasonable combination of both theoretical knowledge and the experimental skill.

#### REFERENCES

Baillis, D. and Sacadura, J.-F. (2000) Thermal Radiation Properties of Dispersed Media: Theoretical Prediction and Experimental Characterization, *J. Quant. Spectr. Radiat. Transfer*, 67(5): 327-363.

Bohren, C.F. and Huffman, D.R. (1983) *Absorption and Scattering of Light by Small Particles*New York: Wiley.

Costello, M.J., Johnsen, S., Gilliland, K.O., Freel, C.D., and Fowler, W.C. (2007) Predicted light scattering from particles observed in human age-related cataracts using Mie scattering, Invest. Ophtalmol. Vis. Sci., 48(1): 303-312.

Dombrovsky, L.A. (1996) *Radiation Heat Transfer in Disperse Systems*, New York: Begell House.

Kahnert, F.M. (2003) Numerical Methods in Electromagnetic Scattering Theory, *J. Quant. Spectr. Radiat. Transfer*, 79-80: 775-824.

Mishchenko, M.I., Travis, L.D., and Lacis, A.A. (2006) *Multiple Scattering of Light by Particles: Radiative Transfer and Coherent Backscattering*, New York: Cambridge Univ. Press.

Serebrennikova, Y.M., Smith, J.M., Huffman, D.E., Leparc G.F., and Garcia-Rubio, L.H. (2008) Quantitative interprewtation of Visible–NIR reflectance spectra of blood, Opt. Express, 16(22): 18215–18229.

Van de Hulst, H.C. (1957) *Light Scattering by Small Particles*, New York: Wiley (also Dover Publ., 1981).