Phosphor thermometry
Phosphor thermometry is an optical
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 method for surface temperature measurement. The method exploits luminescence
Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...

 emitted by phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...

 material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light. Certain characteristics of the emitted light change with temperature, including brightness, color, and afterglow duration. The latter is most commonly used for temperature measurement.

Time dependence of luminescence

Typically a short duration ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 lamp or laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 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 1/e
E (mathematical constant)
The mathematical constant ' is the unique real number such that the value of the derivative of the function at the point is equal to 1. The function so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base...

 of its original value is known as the decay time or lifetime and signified as . It is a function of temperature, T.

The intensity
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...

, I of the luminescence commonly decays exponentially
Exponential function
In mathematics, the exponential function is the function ex, where e is the number such that the function ex is its own derivative. The exponential function is used to model a relationship in which a constant change in the independent variable gives the same proportional change In mathematics,...


Where I0 is the initial intensity (or amplitude).

The method is also referred to as fluorescence thermometry since it is also the case that similar materials in the form of glass, crystals, or even optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

s will fluoresce and may be used as temperature sensors. Fiberoptic amplifiers are based on optical fibers doped with rare earths. Such fibers are useful for temperature measurement.

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 emitting diode (LED) signal of frequency f and the fluorescence that results (see figure). The phase difference varies with decay time and hence temperature as:

Temperature dependence of emission lines: intensity ratio

The second method of temperature detection is based on intensity ratios of two separate emission lines; the change in coating temperature is reflected by the change of the phosphorescence spectrum. This method enables surface temperature distributions to be measured. The intensity ratio method has the advantage that polluted optics has little effect on the measurement as it compares ratios between emission lines. The emission lines are equally affected by 'dirty' surfaces or optics.

Temperature dependence

Several observation 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
    Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...

     is sometimes substituted for some of the aluminium
    Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

     in YAG
    Yttrium aluminium garnet
    Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite . YAG is commonly used as a host material in various solid-state...

    , also altering the temperature dependence.

  • The emission decay of dysprosium
    Dysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...

     (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 a means to measure temperature within the TBC and at the metal/topcoat interface, hence enabling the manufacturing of an integrated heat flux gauge. First results on yttria-stabilized zirconia
Yttria-stabilized zirconia
Yttria-stabilized zirconia is a zirconium-oxide based ceramic, in which the particular crystal structure of zirconium oxide is made stable at room temperature by an addition of yttrium oxide...

 co-doped with europia (YSZ:Eu) powders were published in 2000. They also demonstrated sub-surface measurements looking through a 50 μm undoped YSZ layer and detecting the phosphorescence of a thin (10 µm) YSZ:Eu layer (bi-layer system) underneath using the ESAVD technique to produce the coating. The first results on electron beam physical vapour deposition of TBCs were published in 2001. The coating tested was a monolayer coating of standard YSZ co-doped with dysprosia (YSZ:Dy). First work on industrial atmospheric plasma sprayed (APS) sensor coating systems commenced around 2002 and was published in 2005. They demonstrated the capabilities of APS sensor coatings for in-situ two-dimensional temperature measurements in burner rigs using a high speed camera system. Further, temperature measurement capabilities of APS sensor coatings were demonstrated beyond 1400 ºC. Results on multilayer sensing TBCs, enabling simultaneous temperature measurements below and on the surface of the coating, were reported. Such a multilayer coating could also be used as a heat flux gauge in order to monitor the thermal gradient and also to determine the heat flux through the thickness of the TBC under realistic service conditions.

Applications for thermographic phosphors in TBCs

While the previously mentioned methods are focusing on the temperature detection, the inclusion of phosphorescent materials into the thermal barrier coating can also work as a micro probe to detect the aging mechanisms or changes to other physical parameters that affect the local atomic surroundings of the optical active ion. Detection was demonstrated of hot corrosion processes in YSZ due to vanadium attack.

Videos: application of thermographic phosphors

Phosphorescence sensor coating for online temperature detection

See also

  • Fluorescence
    Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

  • Luminescence
    Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...

  • Photoluminescence
    Photoluminescence is a process in which a substance absorbs photons and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon...

  • Thermometer
    Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...

  • Thermometry
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