Thermographic phosphor digital image correlation
The invention advances the ratio method of phosphor thermography using two machine vision cameras for full-field temperature measurements of a solid surface. As an example, algorithms from digital image correlation (DIC) can be used to determine the stereoscopic imaging system intrinsic and extrinsic parameters, and accurately register material points on the sample to subpixel locations in each image with 0.07 px or better accuracy. A phosphor calibration sample fabricated using aerosol deposition can be used for in situ determination of the temperature-versus-intensity ratio relationship. The full calibration methodology and several improvements on two-color phosphor thermography open the door for full-field temperature measurements in dynamic tests with deforming test specimens. In particular, thermographic phosphors can be combined with stereo digital image correlation in a novel diagnostic, TP+DIC, to measure time-resolved full-field surface strains and temperatures simultaneously, thereby enabling testing in combined thermo-mechanical environments.
1. A ratio method for phosphor thermography, comprising
providing a specimen coated with a thermographic phosphor,
illuminating the specimen with incident light having a wavelength within an absorption feature of the thermographic phosphor,
collecting and spectrally filtering an emission of the thermographic phosphor for an exposure time into a first spectral band image of a first camera and a second spectral band image of a second camera of a stereoscopic imaging system,
correcting the first and second cameras for non-linear behavior so that an image intensity of each pixel is linearly proportional to a number of photons from the incident light impinging upon each pixel during the collection step,
spatially registering the first and second spectral band images using stereo digital image correlation,
calculating ratios of the intensities of the first and second spectral bands for each pixel of the spatially registered spectral band images, and
inferring a spatially resolved temperature image of the specimen from the spatially resolved intensity ratios using a temperature-vs-intensity ratio calibration curve, thereby providing a time-resolved, full-field thermographic image of the specimen.
2. The ratio method of claim 1 wherein the temperature-vs-intensity ratio calibration curve is generated pixel-wise by an in situ calibration procedure.
3. The ratio method of claim 1 , wherein the collecting, calculating, and inferring steps are repeated for a plurality of times to provide a plurality of time-resolved, full-field thermographic images of the specimen.
4. The ratio method of claim 1 , wherein the thermographic phosphor coating is patterned on the substrate, and wherein the invention further comprises imaging the pattern and tracking the pattern image with stereo digital image correlation as the specimen is deformed.
5. The ratio method of claim 4 , wherein time-resolved, full-field strain images are calculated from the tracked pattern images.
6. The ratio method of claim 1 , further comprising collecting temperature-vs-intensity ratio data for each of the pixels in an image of a constant temperature calibration sample and fitting a curve on a pixel-by-pixel basis to the temperature-vs-intensity ratio data to eliminate collection optics-dependent spatial non-uniformities in the spatially resolved temperature image.