## 1 Introduction  
Skin blood flow plays an important role in thermoregulation processes of the human body as a whole [1, 2], allowing for the maintenance of a constant core temperature in spite of changes in the temperature of either body or environment. Notably, the thermoreceptors in the skin respond to temperature as well as to the rate of change of temperature [3]. The complex of thermoregulatory processes in the body provide thermal comfort [4]. Despite a large number of methods for assessing thermal sensation, an active search is underway for new methods that could clarify the mechanisms of thermoregulation [3]. The importance of considering vascular reactivity in response to temperature effects has been confirmed in a number of clinical studies, described in detail in respective reviews. Thus, the importance of a cold vasoconstrticr test (the implementation of which is closely related to the state of endothelial function and activation of the autonomic nervous system) for the prognosis of patients with cardiovascular diseases is underlined in reference [5]. [PERSON]'s syndrome, which develops in various rheumatological and vascular diseases, presents a separate problem. It is manifested by the increased reactivity of microvessels in response to cold stress. However, preclinical assessment of vascular reactivity is possible only with the help of the nailfold capillaroscopy [6]. Diseases such as type 2 diabetes mellitus (DM) or [PERSON]'s disease cause a disturbance of blood microcirculation or local neural activity, which leads to impaired thermoregulation and thermal sensation, both locally (mainly in the limbs) and generally, thus increasing the risk of heat exhaustion or heat stroke [7]. The relationship between the reaction of microvessels in response to heating and the European scale of global risk SCORE in patients with hypertension was demonstrated in [8]. The study of microvascular reactivity to temperature changes is even more clinically important in diabetic patients [9]. Therefore, the temperature reactivity of microvasculature is an actual pathophysiological component of many processes, but its study in a clinical environment is limited by the lack of reliable hardware assessment methods. The reaction of the microvasculature to temperature effects is often assessed by using indirect methods, such as measuring the dynamics of blood pressure or neurohormones. Despite the variety of non-invasive methods to investigate the skin microcirculation including capillaroscopy, optical coherence tomography, laser Doppler flowmetry (LDF), photoplethysmography (PPG), thermography and near-infrared spectroscopy [10], only a few of them have been used to assess the temperature reactivity, and experiments have usually been carried out in laboratory conditions. Particularly, it was shown that changes in skin blood flow measured by LDF and PPG are related to changes in the skin temperature [11]. Recently, the possibility of quantifying the change in blood microcirculation with local temperature effects on the human hand has been demonstrated by using the imaging photoplethysmography (IPPG) system [12]. It is worth noting that the PPG waveform was measured by [PERSON] et al. [12] only afterwards the thermal impact. However, monitoring of the processes before, during, and after temperature changes would provide more information about the vascular processes. In addition, most studies of the relationship between local skin temperature and microcirculation have been carried out on the subjects' fingers. It is well known that the temperature impact on different bodys' regions leads to different reactions of the circulatory system. For example, immersing one's hand in cold water or applying ice to the chest can cause systemic vasoconstriction and tachycardia, whereas cooling the face is often accompanied by bradycardia. Differences in the local regulation of microvascular tone are also possible.