are equally affected by 'dirty' surfaces or optics. Temperature dependence Several observations are pertinent to the figure on the right: Oxysulfide materials exhibit several different emission lines, each having a different temperature dependence. Substituting one rare-earth for another, in this instance changing La to Gd, shifts the temperature dependence. The YAG:Cr material (Y3Al5O12:Cr3+) shows less sensitivity but covers a wider temperature range than the more sensitive materials. Sometime decay times are constant over a wide range before becoming temperature dependent at some threshold value. This is illustrated for the YVO4:Dy curve; it also holds for several other materials (not shown in the figure). Manufacturers sometimes add a second rare earth as a sensitizer. This may enhance the emission and alter the nature of the temperature dependence. Also, gallium is sometimes substituted for some of the aluminium in YAG, also altering the temperature dependence. The emission decay of dysprosium (Dy) phosphors is sometimes non-exponential with time. Consequently, the value assigned to decay time will depend on the analysis method chosen. This non-exponential character often becomes more pronounced as the dopant concentration increases. In the high-temperature part, the two lutetium phosphate samples are single crystals rather than powders. This has minor effect on decay time and its temperature dependence though. However, the decay time of a given phosphor depends on the particle size, especially below one micrometer. There are further parameters influencing the luminescence of thermographic phosphors, e.g. the excitation energy, the dopant concentration or the composition or the absolute pressure of the surrounding gas phase. Therefore, care has to be taken in order to keep constant these parameters for all measurements. Thermographic phosphor application in a thermal barrier coating A thermal barrier coating (TBC) allows gas turbine components to survive higher temperatures in the hot section of engines, while having acceptable life times. These coatings are thin ceramic coatings (several hundred micrometers) usually based on oxide materials. Early works considered the integration of luminescent materials as erosion sensors in TBCs. The notion of a "thermal barrier sensor coating" (sensor TBC) for temperature detection was introduced in 1998. Instead of applying a phosphor layer on the surface where the temperature needs to be measured, it was proposed to locally modify the composition of the TBC so that it acts as a thermographic phosphor as well as a protective thermal barrier. This dual functional material enables surface temperature measurement but also could provide