Phosphor thermometry

Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor 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. Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor 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. 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 1/e of its original value is known as the decay time or lifetime and signified as τ {displaystyle au } . It is a function of temperature, T. The intensity, I of the luminescence commonly decays exponentially as: I = I o e − t τ {displaystyle !,I=I_{o}e^{frac {-t}{ au }}} 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 fibers 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: ϕ = t a n ( 2 π f τ ) {displaystyle !,phi =tan(2{pi }f{ au })} 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.