## 3 Fundamentals of IRT  
IRT is currently widely used in many fields, and its importance is also irreplaceable in human and veterinary medicine [30; 31]. It is a noninvasive technique in which physical contact with the surface to be measured is not necessary [32]. IRT is based on the fundamental laws of radiation--Stefan-Boltzmann law, [PERSON]'s displacement law, and [PERSON]'s law [33]. The principle of IRT is the property of all bodies with a surface temperature above absolute zero to emit electromagnetic radiation [32]. The portion of the electromagnetic radiation spectrum with wavelengths ranging from 780 nm to 1 mm is defined as infrared radiation and can be used to measure the surface temperature of a body [34]. Infrared radiation is absorbed by a thermal imaging camera and is converted into electrical pulses that are processed by software and displayed with a thermogram in pseudocolors that represent different temperatures in the infrared range [35; 36; 37; 38; 39]. A thermal image, a thermogram, is an image taken with a thermal imaging camera. In accordance with the amount of information captured, thermograms can be divided into nonradiometric and radiometric [40]. A nonradiometric thermogram is a simple image of temperature distribution [41]. A radiometric thermogram contains information about the surface temperature of an object and the surface properties affecting the ability to emit infrared radiation [42]. These parameters include emissivity \(\epsilon\), information about the surrounding atmosphere--apparent reflected temperature, relative humidity, and distance from the object being measured [43]. Emissivity is the ratio of the intensity of emission of a real body to that of an absolute blackbody with the same temperature. Emissivity determines the ability of a body to radiate heat. Emissivity is not constant for a given surface but is a function of a number of parameters: the angle of deviation from the surface normal, the temperature of the object, the wavelength, the surface color, the surface texture and its contamination, etc. [44]. The atmospheric temperature, relative atmospheric humidity and the distance between the thermal imaging camera and the surface of the object to be measured are adjusted to correct for the influence of the atmosphere. The atmosphere attenuates the thermal radiation from the surface to be measured. Attenuation depends mainly on the relative humidity and the distance. The atmosphere itself is a source of thermal radiation. The intensity of thermal radiation of the atmosphere depends mainly on its temperature, but also on its composition. The apparent reflected temperature is the ambient thermal radiation reflected from the surface of the object to be measured which then hits the sensor of the thermal camera. The camera does not distinguish whether it is the thermal radiation of a body or reflected thermal radiation. The effect of the reflected apparent temperature is higher the greater the reflectivity of the surface. The reflectivity of the surface is smaller the greater the emissivity. The distance from the object to be measured affects the field of view of the camera, the smallest detectable object of the camera, and the smallest measurable object of the camera. The field of view of the camera is the area visible to the sensor. The smallest detectable object of the camera corresponds to the size of one pixel depending on the distance. In general, the influence of the atmosphere is greater the more distant the objects being measured are [38]. A thermogram consists of individual pixels and provides information about the surface distribution of apparent temperature on the surface of the object(s) being measured [45]. Infrared cameras do not measure temperature directly, but rather its output signal, which is proportional to the intensity of the incident radiation. Pseudocolors (false colors) are colors that are different from what the human eye or sensors capturing light at the same wavelengths as the eye would pick up.