It was shown in [15] that the optical properties of TOTFC depend on the volume fraction of TiO\({}_{2}\) microspheres (k) in it, which can be changed in the converter design. In the same paper, it was demonstrated that TOTFC with 0.15 \(\leq\) k \(\leq\) 0.26 (at the wavelength of 980 nm) and 0.15 \(\leq\) k \(\leq\) 0.2 (at the wavelength of 1470 nm) and the diameter of d \(>\) 0.67 mm are optimal for EVLT since it is in these ranges of k and d that TOTFC absorbs laser radiation very effectively. Heating of the vein wall that occurs when exposed to TOTFC laser radiation with wavelengths ranging from 532 nm to 2100 nm was studied and showed that in this range the TOTFC absorbs from 82% to 86% of laser radiation [16]. The temperatures on the inner surface of the vein wall achieved by using varying wavelengths from the above range of wavelength do not differ much, which corresponds to the results presented in [17]. It has also been shown that under the action of laser radiation with the wavelength of 980 nm, TOTFC heats the vein wall to the temperature required for its coagulation (80 \({}^{\circ}\)C) slower than the mentioned above carbonized layer, which minimizes the risk of perforation of the vein wall [14]. Thus, TOTFC is very promising for use in the process of endovascular laser treatment. The temperature of the vein wall is of great importance for a successful EVLT. If the temperature on the vein wall is too low (less than 80 \({}^{\circ}\)C), it will not be enough to coagulate the wall. If it is too high (more than 100 \({}^{\circ}\)C), this can lead to perforation of the wall, damage to the surrounding tissues, and an increase in the rehabilitation time. During the EVLT, the temperature on the vein wall can be controlled by changing the average power of laser radiation and the speed of pulling (traction) of the fiber. In clinical practice, when performing the EVLT, the optical fiber is moved inside the vein manually or automatically using an automated moving device, and the laser radiation power is left constant at a predetermined level [18]. With manual traction, the surgeon usually monitors the EVLT process using an ultrasound device and manually adjusts the speed of fiber pulling accordingly. In this case, the outcome of the operation depends on the surgeon's experience, the accuracy of the surgeon's subjective assessment of the course of the operation and may be unpredictable since the surgeon cannot always manually maintain the traction speed necessary for uniform coagulation of the vein wall. Thus, to achieve uniform coagulation of the vein wall during manual traction, it is necessary for the temperature of the vein wall to remain constant and at a level sufficient for coagulation, regardless of the traction speed. This means changing the laser radiation power so that when the speed changes, the temperature of the vein wall is constant. With automatic traction, the pulling speed is usually set constant, but in this case, the outcome of the operation is determined by the fluctuation of the vein diameter along its length, which ultimately also excludes uniform coagulation of the vein wall. Thus, with automatic traction, it is also necessary to adjust the power of the laser radiation so that when the vein diameter varies, the temperature of the vein wall is constant. However, this is extremely difficult because, in the EVLT, there is no possibility of measuring the temperature of the vein wall. In this regard, the search for vein wall temperature measurement methods and algorithms for controlling laser radiation parameters during EVLT is a very urgent task. As shown earlier [14], the temperature of the vein wall depends on the temperature of the carbonized layer or the optothermal converter. In this regard, the temperature control of the optothermal converter allows controlling the temperature of the vein wall during the EVLT. As known when a body is heated, thermal radiation is known to occur [19]. This is a well-studied effect.