## 1 Introduction
### Previous Research on Skin-Cooling Techniques for Dynamic Infrared Thermography  
In laser dermatology, to protect the epidermis in laser dermatology applications, the two most popular methods for skin cooling are spray cooling with tetrafluoroethane (boiling point \(-\)26 \({}^{\circ}\)C) and contact cooling (\(-\)27 \({}^{\circ}\)C sapphire plate). With the spray and contact cooling, the basal skin layer is cooled down to be between \(-\)5 and 5 \({}^{\circ}\)C. These cooling methods must be carefully controlled to prevent irreversible skin damage caused by local skin freezing [39]. Spray and contact cooling provide efficient skin cooling for laser dermatology applications, but due to the possible serious skin damage, especially on critical skin lesions, it is not suited for dynamic infrared thermography on skin cancer lesions. [PERSON] and [PERSON] [40] performed a computational analysis of different skin-cooling techniques on a human skin model based on the Pennes bioheat equation. The skin lesions in this study are defined by an invasion depth of less than 4 mm. The cooling methods they considered are constant temperature cooling, water-soaked cotton and convective cooling. The constant temperature cooling method is an idealized case of conduction cooling: the temperature of the coolant remains constant during cooling, which is difficult to achieve in practice. The second cooling method studied in their work is cooling with a cotton patch soaked in water. [PERSON] modeled the water-soaked cotton patch as a 2 mm-thick layer of water in direct and ideal contact with the skin. Their third cooling case is based on convective cooling by air, varying the convective heat transfer coefficient and the temperature of the air. The thermal penetration depth of the three cooling cases are compared for a cooling temperature of 4 \({}^{\circ}\)C, 12 \({}^{\circ}\)C and 20 \({}^{\circ}\)C. [PERSON] and [PERSON] concluded that cooling at 20 \({}^{\circ}\)C is a suitable cooling temperature that allows the cooling effect to reach a lesion depth of 4 mm within 2 min. [PERSON] et al. [41] also experimented with different cooling methods to find the most convenient cooling method for infrared thermography on skin diseases. In their experiments, they tested different methods: cooling by gel, metal medal with 6 s of skin contact or compressed air. The application of compressed air showed that the temperature is not controllable because the compressed air is not conditioned properly. Cooling by gel is possible if the disease covers a small area. The application of the cooling gel will interfere with the accuracy of the thermographic readings due to the change in emissivity. The use of the metal block is most suitable for their experiments, but the field of view during cooling is obstructed. [PERSON] et al. [42] used a conductive cooling method with cooled gel packs. An area of 10 cm by 10 cm is cooled to 20 \({}^{\circ}\)C. They mentioned that the recording protocol needs improved standardization due to the patient-individual variation of the acquisition conditions. [PERSON] et al. [43] use immersion in water at 15 \({}^{\circ}\)C for 2 min. When immersion in water was not possible, forced evaporation with an alcohol spray and fan currents was used. They concluded that a non-contact system using cold air currents was necessary to collect qualitative data without damaging the skin or causing changes in skin permeability. [PERSON] et al. [44] introduced cooling with a stream of cold air from a vortex tube for a period of one minute. Compressed air (from an air tank or air compressor supply lines) must be present for the vortex tube to operate. An area with a diameter of 5 cm is cooled by the outlet of the vortex tube, but the cooling temperature is not mentioned in their work. Controlling the outlet temperature of a Ranque-Hilsch vortex tube is difficult to achieve; the outlet temperature is pressure, flow and ambient temperature dependent.