A proposed thermographic technique would exploit the unique fluorescence characteristics of yttrium vanadate doped with dysprosium (YVO4:Dy3+). The particular aspect of the fluorescence characteristics that would be exploited in this technique is the relative intensity of emitted light as a function of the temperature and of the wavelength of the light used to excite the fluorescence (see figure).
An object to be thermographed would be coated with YVO4:Dy3+ and would be imaged by use of a high-speed framing camera with timing circuitry that could be used to obtain exposure times shorter than 1 μs. The YVO4:Dy3+-coated surface would be illuminated in succession by two laser pulses: one at a wavelength between 275 and 310 nm, the other at a wavelength between 340 and 355 nm. The first-mentioned wavelength band contains the peaks of the curves shown in the figure and is the spectral region wherein the relative intensity of emitted light decreases with increasing temperature. In the second-mentioned wavelength band, the relative intensity of emitted light increases with increasing temperature.
The operation of the camera and lasers would be synchronized, with suitable triggering delays. The camera would be made to acquire an image a few tens of microseconds after the laser pulse (allowing for fluorescence rise time) at the first wavelength. This image would be digitized. Then, similarly, the camera would be made to acquire an image after the laser pulse at the second wavelength and that image would also be digitized. To make it possible to distinguish between fluorescence excited by the two laser pulses, the delay between the pulses would be made a multiple (or at least a significant fraction) of the fluorescence decay time (which is of the order of 160 μs or less).
The data from digitization of the images would be processed to extract the temperature of each pixel from the relative intensities for that pixel in the two images. The processing would involve inversion of the excitation-wavelength and temperature dependences.
This work was done by Gregory M. Buck of Langley Research Center. For further information, contact the Langley Commercial Technology Office at (757) 864-6005.