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==History==
The first mention of temperature measurement utilizing a phosphor is in two patents originally filed in 1932 by Paul Neubert.<ref>Allison, S. W. (2019). A brief history of phosphor thermometry. Measurement Science and Technology, 30(7), 072001.</ref>
==Time dependence of luminescence==
▲[[Image:Phase diff demo.JPG|400px|thumb|Phase difference between LED output and luminescence.]]
Typically a short duration [[ultraviolet]] lamp or [[laser]] source illuminates the phosphor coating which in turn luminesces visibly. When the illuminating source ceases, the luminescence will persist for a characteristic time, steadily decreasing. The time required for the brightness to decrease to [[e (mathematical constant)|1/e]] of its original value is known as the decay time or lifetime and signified as <math>\tau</math>. It is a function of temperature, T.
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A temperature sensor based on direct decay time measurement has been shown to reach a temperature from 1000 to as high as 1,600 °C.<ref>J.L. Kennedy and N. Djeu (2002), "Operation of Yb:YAG fiber optic temperature sensor up to 1,600°C", Sensors and Actuators A <u>100</u>, 187-191.</ref> In that work, a doped YAG phosphor was grown onto an undoped YAG fiber to form a monolithic structure for the probe, and a laser was used as the excitation source. Subsequently, other versions using LEDs as the excitation source were realized. These devices can measure temperature up to 1,000 °C, and are used in microwave and plasma processing applications.<ref>Commercialized by MicroMaterials, Inc. under US Patents 6,045,259 and 9,599,518 B2.</ref>
If the excitation source is periodic rather than pulsed, then the time response of the luminescence is correspondingly different. For instance, there is a phase difference between a sinusoidally varying [[light
<math>\!\, \phi=tan(2 {\pi} f {\tau})</math>
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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 (Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:Cr<sup>3+</sup>) 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 YVO<sub>4</sub>: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 [[Yttrium aluminium garnet|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.
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==See also==
{{div col|colwidth=22em}}
* [[Fluorescence]]
* [[Luminescence]]
* [[Photoluminescence]]
* [[Thermometer]]
* [[Thermometry]]
{{div col end}}
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