## 2 Technical Fundamentals of IRT  
Infrared thermography is a touch-free imaging technique that has the ability to map the isotherms of the surface of a selected area or an entire object, owing to the detection and registration of infrared emissions [8]. Importantly, in contrast to other imaging techniques in medicine, IRT is a completely passive (non-invasive and non-radiating) technique. The possibility of creating an image on the screen plane of the temperature distribution of a body with a temperature higher than absolute zero, as observed from a certain distance, results from several laws of physics, which are the basis for the operation of thermal imaging cameras: [PERSON]'s law; [PERSON]'s law; [PERSON]'s law; and [PERSON]'s law. Those interested in these processes are referred to theoretical studies [9; 10; 11] for the details. All living objects consist of matter in random motion, containing heat from kinetic energy and having surface temperatures above absolute zero (0 K; \(-\)273.15 \({}^{\circ}\)C). They emit electromagnetic radiation characterized by two features: its wavelength (\(\lambda\)) and intensity (Q). Both the intensity and the wavelength at which the radiation is most intense depend on the surface temperature of the emitting body. Thus, objects with a certain temperature emit radiation of different wavelengths. Though the spectral range of visibility changes from person to person, the human eye is sensitive to a fraction of the electromagnetic spectrum, ranging approximately from 400 to 760 nm [12], but it cannot detect the IR radiation emitted by human skin. The infrared emissions from human skin at 27 \({}^{\circ}\)C lie within the wavelength range of 2-20 \(\upmu\)m, and they peak around 10 \(\upmu\)m. Based on [PERSON]'s law, roughly 90% of the infrared radiation emitted by humans is of a longer wavelength (8-15 \(\upmu\)m) [13]. For medical applications, a very narrow wavelength band corresponding to the range 8-12 \(\upmu\)m, termed as body infrared rays, is in general use [14]. The human body's surface temperature can be calculated based on the total radiant power per surface area M (Wm\({}^{-2}\)). The principle of the operation of thermal imaging cameras is to measure and convert the energy of the radiation emitted by the body into a temperature value. The total amount of energy consists of the object's emissions, the emissions reflected from the source of the environment, and the emissions from the atmosphere. The detectors of an IR camera convert the incoming power of the electromagnetic rays emitted by the body into electronic video signals that are sequentially amplified and transmitted to monitors. Thermal images generated by IR cameras are called thermograms, where each image pixel corresponds to a digital value proportional to the amount of received energy. By reason of the achieved time resolutions of 30 frames per second (fps), it is possible to analyze dynamic changes in temperature. To calculate the surface temperature correctly based on the emitted radiation, it is necessary to know the emissivity of the object [15]. Regarding the spectral region, human skin is a black-body radiator with an emissivity factor of 0.97-0.99 [16] and is therefore a perfect emitter of infrared radiation at room temperature. The skin microcirculatory flow determines infrared irradiation from the human surface and, in that way, also skin temperature (Tsk). There is a complex relationship between skin, the metabolism of internal tissues, and the functions of local blood vessels. The skin and skeletal muscles have a relatively low resting metabolism, therefore postganglionic neurons of the sympathetic nervous system (SNS) extend to the target organs, and also to the blood vessels and the sweat glands with the peripheral nerves or blood vessels playing a dominant role in regulating the flow in its microcirculation. For this reason, SNS is the most important way by which human body temperature is regulated through thermoregulatory mechanisms of heat exchange between organs, tissues, and skin [17].