## 1 Introduction  
The use of infrared (IR) thermography as a radiometric technique is of great interest thanks to the capability to be a full-field, real-time, and contactless investigation method. Using radiation techniques for quantitative temperature measurements can be difficult due to the dependency of the surface radiation on the emissivity, \(\varepsilon_{obj}\) (a surface property that changes with several parameters such as the direction of radiation, temperature, and wavelength, and it is also strictly dependent on the state of the surface [1]. During the plasma wind tunnel (PWT) tests, the thermal protection system (TPS) articles undergo thermal stresses, leading to high-temperature values (up to 2500 \({}^{\circ}\)C). Contactless, non-intrusive techniques are needed, and in order to predict information about the distribution of temperature over surface objects that are subject to the hypersonic plasma, proper tools have to be developed capable of solving problems such as the emissivity knowledge that, in most cases, is mainly due to the of the chemical feature variations occurring during the test and the innovativeness of the used materials. In the last years, a wide variety of techniques have been developed in order to obtain accurate and precise temperature measurements. Intrusive techniques, including the use of thermocouples,gardon gauges, pressure sensors, and non-intrusive techniques through infra-red (IR) pyrometers, spectroscopy [2], and IR cameras [3] have been used for the monitoring of the phenomena in the hypersonic motion conditions. The use of thermocouples needs direct contact with the material for which the temperature is measured [3; 4]. Moreover, the application requires a priori knowledge of the spots of interest and the detection of high temperatures. Moreover, the mechanical features of the test article can be affected by the thermocouple's application and for this reason, they cannot always be used because they are often placed under the surfaces of the material exposed to the flow jet. Methodologies based on thermal radiation belong to two main methods--pyrometry and thermography. The first is able to provide point-like information in terms of the object's thermal state, while thermography can be used to detect a 2D (two-dimensional) surface map of temperature. The attention in recent years is focused on IR thermography due to features such as being a contactless, full-field, and real-time inspection method [1]. Nevertheless, for the temperature measurement of test articles in PWT, pyrometry is well known and often used, but it is characterized by the limitation to provide information relative to a limited number of target surface points. This is mainly due both to its intrinsic limitations and to the limited number of the PWT optical accesses. So, if an operator wanted to build a 2D map of the temperature of the target, by interpolating the pyrometer measurements associated with several spots, a huge number of pyrometers should be used and the result could hardly be compared in terms of spatial resolution to the ones obtained through the modern thermographic devices characterized by a resolution up to 640 \(\times\) 512 pixels. This study is a concise review paper regarding the 2D free emissivity dual-color IR methodology with the novelty to expand the technique to the unsteady conditions for different emissivity classes, in a very distinctive environment as in the aerospace field. To our knowledge, this technique has never been developed to get 2D thermographic images and is also never applied in the aerospace field. So, a state of the art of diagnostic methodology that is applied to thermal conditions compatible with the ones connected to the atmospheric re-entry has been summarized and compacted in several IR spectral ranges (medium [5; 6; 7], long [6], and near [7] wavelength ranges) for a deeper understanding of the readers.