## 1 Introduction  
Human skin tissue that serves as a barrier to protect the underlying organs or tissues from environmental pollution, chemicals, mechanical shocks, temperature changes, and microorganisms consists of three main layers--the epidermis, dermis, and hypodermis--and accounts for 16% of a person's total body weight. Figure 1 shows the anatomy of the skin consisting of the three main layers. The epidermis, the outermost layer, is made up of many sublayers known as the stratum corneum (also known as a horny layer), stratum lucidum, stratum granulosum, stratum spinosum, stratum basale, and the basement membrane, and is of the order of thickness of around 70 to 100 \(\upmu\)m [1]. In contrast, the dermis is composed of two connective tissues, the papillary dermis and the reticular dermis, and is mainly made up of dermal fibroblasts, collagen/elastin fibers, ground substances such as glycosaminoglycans (GAGs), and blood/lymphatic vessels. This layer is responsible for giving flexibility, elasticity, and firmness to the skin while providing physical support to the epidermis, vascular networks, nerves, and appendages. The hypodermis (also known as the subcutaneous fat layer) mainly consists of fat, blood vessels, and nerves, providing a reservoir of progenitor cells, energy from fat, and insulation to the body. Further, it protects the skin from mechanical impact, links the dermis to the various organs, and is associated with significant endocrine and paracrine signaling [2] (refer to [2] and references therein). With the primary function of the skin being to protect the body from external heat, under heat stress, the blood vessels within the skin contract and dilate in response to changes in external temperature [3]. The contraction and dilation of blood vessels happens due to their elastic nature, which regulates body temperature in response to changes in external temperature, such as heating and cooling [4]. Under normal conditions, when the body temperature rises, blood vessels dilate, which has the effect of decreasing body temperature. However, with aging [3; 5], there are significant changes in both structure and function. Moreover, diseases such as chronic heart failure [6] and diabetes [7] can cause the gradual loss of vascular elasticity, making vascular regulation difficult. In order to evaluate the cutaneous microvasculature, several noninvasive in vivo optical techniques exist. Cutaneous blood flow is measured by confocal microscopy, venous occlusion plethysmography, Doppler ultrasound, and laser Doppler flowmetry [8]. All of these techniques have limitations such as limited depth range, no direct visualization, and issues with respect to spatial localization. Furthermore, because the nature of observation involves measurement from the surface, the localization of signals to be from a specific depth is almost impossible. In contrast, optical coherence tomography (OCT) is an optical imaging technique where light from a low coherent source is focused onto the tissue,  
Figure 1: Skin anatomy showing the main layers present in skin (credits: [PERSON], National Institutes of Health, US). and the reflectivity of the internal microstructures at different depths is measured by an interferometer [9; 10]. Optical coherence tomography (OCT) is an optical imaging modality that enables noninvasive 3D imaging at a depth resolution of 1-20 \(\upmu\)m [11] and is capable of visualizing the skin microarchitecture. Based on the reflectance measurements using low-power non-ionizing near-infrared light, OCT has been used to visualize the microvasculature in a range of dermatological applications, such as diabetes [10], age-related macular degeneration, and basal cell carcinoma [12]. There exist studies that used OCT to measure skin before and after heating or ischemia [9]. In relation to heat stress, [PERSON] et al. [11] employed optical coherence tomography (OCT) to assess acute changes in the density and size of cutaneous microvessels.