## 4 Discussion  
Our primary finding is that the temperature drop across the skin depends on the temperature gradient between the core body temperature and ambient air. Thus, the ambient temperature is a crucial factor in any thermographic measurement. For example, according to Equation (29), the change in ambient temperature from 22 to 27 \(\mathrm{\SIUnitSymbolCels}\) will result in  
Figure 4: The dependence of the temperature drop across the skin (_Delta_) on the perfusion rate for several ambient temperatures: 25 \(\mathrm{\SIUnitSymbolCels}\) (green dot-dash line), 20 \(\mathrm{\SIUnitSymbolCels}\) (solid red line), and 10 \(\mathrm{\SIUnitSymbolCels}\) (blue dashed line). The black dotted line outlines the broadly reported value for _Delta_ (3.5 \(\mathrm{\SIUnitSymbolCels}\)). **Inser**: calculated perfusion rate (solid blue line) to keep _Delta_ at 3.5 \(\mathrm{\SIUnitSymbolCels}\) as a function of the ambient temperature, \(T_{\infty}\). a one-third decrease in _Delta_. So, any thermographic measurement needs to be considered in the context of the ambient temperature. To normalize results on the ambient temperature, \(Delta/(T_{c}-T_{\infty})\) ratio can be used instead. The difference between the core body temperature and the skin temperature, _Delta_, can be decomposed into the sum of three factors:  
\[Delta = T_{c}-T_{p}+\Delta T_{d}+\Delta T_{e} \tag{33}\]  
Here \(\Delta T_{d}\) and \(\Delta T_{e}\) are the temperature drop in the dermis and epidermis, respectively. The temperature drop in the epidermis, \(\Delta T_{e}\), is relatively small (0.1 \({}^{\circ}\)C) for most body parts (non-glabrous skin). However, it can be on a scale of 0.5-1 \({}^{\circ}\)C for glabrous skin and 1.5-2 \({}^{\circ}\)C or even higher for calluses. These results are in line with clinical observations [22], see Figure 5. In this particular case of a routine foot examination in a wound care clinic, the callus-related skin temperature drop is on the scale of 4 \({}^{\circ}\)C.  
These findings can have practical applications in podiatry and wound care. While in many cases, calluses are harmless, if not removed timely, they may lead to skin ulceration or infection, which is of particular importance for patients with diabetes. Thus, the removal of calluses is an essential part of surgical debridement. However, in many cases, healthcare professionals experience problems with their identification. While corms are typically clearly visible, calluses are often not clearly visible. Consequently, some areas of dead skin can be missed during debridement. Thermal imaging can help identify these areas. The bulk heat generation in the dermis leads to minimal effects on the scale of 0.03 \({}^{\circ}\)C. Due to high thermal conductivity and low metabolic heat production in the dermis, the temperature drop in the dermis, \(\Delta T_{d}\), is relatively small and can be ignored in most practical applications. This observation also supports our assumption that the fine structure of the dermis can be omitted from a thermophysical perspective. The superficial vascular plexus and blood moving through capillaries do not change the thermal distribution significantly. The primary player is the deep vascular (reticular) plexus, the largest skin blood reservoir. The main temperature drop occurs at the deep vascular plexus, and its temperature is regulated primarily through perfusion. Thus, the heat exchange in the venous plexus is the primary driver of the difference between the core body and skin temperatures. We also investigated the impact of skin wetness on skin temperature. The skin wetness significantly increases the skin's heat transfer, resulting in a lower temperature of the  
Figure 5: White light and thermographic images of the foot: (**panel a**) Anatomical (white light) image; (**panel b**) Absolute temperature color map, (**panel c**) Thermal gradient—relative temperature color map. A thermal gradient was generated with respect to the reference point displayed on the absolute temperature map.