Monitoring the surface temperature of in-vessel components is part of machine protection. The surface temperature itself and the resulting temperature of the interface to the cooling structure have to be taken into account. The tolerated surface temperature is not a fixed quantity but depends on the heat load scenario. The interface temperature can be calculated by solving the heat diffusion equation and determining the temperature profile inside the target. Surface effects and parasitic radiation falsify the estimated temperature, which is higher than the real bulk temperature. From the machine protection point of view, the contributions are inherently safe. They might result in an early alarm, not justified by the target temperature itself reducing the tolerated operation range. Real-time characterization and quantification can be done by considering the temporal evolution of the measured surface temperature. This is recommended to be done by heat load calculation. Infrared (IR) systems under development allow one to calculate the heat load from the measured photon flux in real time. The impact of surface effects and parasitic radiation on the calculated temperature is dependent on wavelength. A suitable compromise for an IR system is a mid-wave IR system. It should be combined with a near-IR system for temperature validation at higher temperatures.