## 2 Materials and Methods
### Data Collection  
This study lasted from October 2021 to February 2023. We performed measurements in periods when extremely critical climatic conditions did not occur (approximately for level 40 < THI < 72). The monitored parameters were as follows: the dairy cow's rectal temperature (RT), the dairy cow's eye temperature (ET), the air temperature (AT), the relative humidity (RH), and the temperature-humidity index (THI). Sufficient acclimatization of the animal in the room where tests are performed is important. The adaptation time should be extended in case of a significant temperature difference in the rooms. This knowledge is confirmed by the research of [PERSON] et al. [11] in tracking enzootic bronchopneumonia in calves (BRDC--Bovine Respiratory Disease Complex) using thermal imaging. We were aware of the findings reported by [PERSON] et al. [37], that the body temperature of cattle varies regularly depending on the reproductive cycle and disease status. [PERSON] et al. [18] points out the need to ensure stable atmospheric conditions when using thermography. As the author further states, even slight fluctuations in temperature (<0.8 \({}^{\circ}\)C) on the surface of the animal's body can lead to incorrect diagnosis. When making thermographic images, it is important to immobilize the animal and maintain the correct distance of the thermal imaging camera from the animal, and the distance should not be less than 1.0 to 1.2 m. During our experiment, we tried to apply the knowledge of the cited authors. When making a thermal image of the eye, the time that has passed since the lid is closed (blinking) is important, because by closing the lid, the temperature of the eye harmonizes with the body temperature. Only clinically healthy dairy cows after the first to sixth lactation, which were at its beginning, were selected for the experiment. The dairy cows were in a barn with free housing on deep litter (straw); after milking, the dairy cows were herded into a selected area outside the housing facility where they were fixed with barriers on the stand. The dairy cows were allowed to acclimatize, and rectal temperature and eye temperature were subsequently measured using thermal imaging. Data of 271 measured dairy cows were included into the study. Only dairy cows that did not show signs of a stress reaction during physical contact were used in the experiment. The temperature of the left eye was measured with the help of an infrared camera; thus, we followed up on the previous experience of experts [41; 42]. The distance of the infrared camera from the cattle's eye was approximately 1 m, as recommended by [PERSON] et al. [18]. Emissivity as a sensitive parameter, the setting of which affects the result part of the measurements, ranges from 0 to 1. This depends on what surfaces will be measured and how these materials absorb or reflect the emitted radiation. When measuring the body surface of dairy cows, a value from 0.93 to 0.98 was used in previous studies [43; 44; 45; 46]. For the measurements in our research, this value was set according to previous experience to \(\varepsilon=0.95\) for all animals. In the place of the experiment, before the start of individual measurements, a one-time measurement of microclimatic conditions was carried out on each day, namely:  
- Air temperature in the measured object [\({}^{\circ}\)C];  
- Relative humidity [\(\%\)];  
- Air speed [m\(\cdot\)s\({}^{-1}\)]. Data were selected for the considered file where the speed of the air flow when scanning the cow did not exceed the value v = 0.2 m\(\cdot\)s\({}^{-1}\). To measure RT, we used a Thermoval Kids Flex (Thermoval systems s.r.o., Zlin, the Czech Republic) digital thermometer with a flexible tip and memory to store the last measured values. The thermographs were recorded using an FLIR SC660 (FLIR, Teledyne FLIR LLC, Wilsonville, OR, USA). The resolution of the camera was 640 \(\times\) 480 px.