Heat Radiation

Since we feel the sun's heat on Earth, but there is no solid matter between the Earth and the Sun that could transport the heat to us, there must be another mechanism for transferring heat. As early as 1879, Josef Stephan had shown that a black body emits thermal radiation. He found that power increases proportionally to the fourth power of temperature. He also found a proportionality between the power and the size of the radiating surface. The necessary constant of proportionality $\sigma$ is called the Stephan-Boltzmann constant since a few years later Ludwig Boltzmann derived this relation theoretically. It has also been found that shiny materials and/or fabrics that are not completely black emit less power. The so-called Stephan-Boltzmann law must therefore be multiplied by a temperature-dependent correction factor, the degree of emission $\varepsilon$. Overall, this results in the complete formulation of \begin{equation} \boxed{P = \varepsilon\sigma AT^4} \end{equation} The Stefan-Boltzmann constant has value here \begin{equation} \sigma = 5.670374419 \cdot 10^{-8}\,\frac{\mathrm{W}}{\mathrm{m}^2\cdot K^4} \end{equation} This is one of the few laws in which a physical quantity depends so strongly on changes in a parameter.

Due to the law discussed, people generally emit a few kilowatts of power. Still, they also absorb a great deal of power from their surroundings, so that an overall balance is established. In winter, starry nights are significantly colder because clouds absorb radiation from the earth and accordingly send thermal radiation back to the earth's surface, resulting in a higher temperature.

The emissivity is largely constant for most substances over a large temperature range. Typical values ​​of artificially produced and naturally occurring substances range from around 0.9 for dark and rough surfaces to very small values ​​of around 0.01 for highly polished and shiny metal surfaces. According to Kirchhoff's radiation law, good absorbers of radiation are also good emitters and vice versa. This can be justified qualitatively by the fact that all bodies in the universe constantly absorb radiation from their surroundings, such as the walls of the room. If the absorption behavior were better than the emission behavior, heat would flow from the colder to the warmer body, which would violate the second law of thermodynamics.

In non-contact thermometers (so-called pyrometers), which usually also have a laser device for aiming, the heat radiation from objects is recorded. To focus the radiation, a lens is required that is permeable to far-infrared. Lenses made of polyethylene (PE) are usually used here at a low cost. The thermal radiation is then converted into an electrical voltage by heating thermopiles connected in series, which can be directly converted into a temperature. An important prerequisite here is knowledge of the emission coefficient, which can usually be set via an attached control panel. For thermometers to measure fever in patients, this is usually already preset for temperature measurement on the forehead or in the ear. There is also the option of analyzing the environment using thermal image cameras In most cases, high-quality lenses made of germanium are used, which are also permeable to infrared radiation. The sensor consists of a matrix of sensors that, like the pyrometer, have the task of converting thermal radiation into electrical signals.