Conclusion
Thermal radiation from opaque materials

Today, for a growing number of practical applications, it is becoming essential to take radiative transfer into account, whether at low temperatures (a few Kelvin) or at high temperatures (3,000 to 4,000 K).

An example of a situation where knowledge of the radiative flux exchanged is required is when determining temperature by thermography. This non-destructive measurement technique is becoming increasingly widespread. However, an infrared camera is a flux sensor, and temperature-flux conversion requires not only knowledge of the emissivities of the various points in the scene being viewed, but also evaluation of the fluxes reflected by elements in the scene, which come from sources outside the field of view and reach the camera