A spatially resolved in-situ calibration applied to infrared thermography

When using thermography at elevated ambient temperature levels to determine the surface
temperature of test specimen, radiation reflected on the test surfaces can lead to a large
measurement error. Calibration methods accounting for this amount of radiation are available
in the open literature. Those methods, however, only account for a scalar calibration
parameter. With new, complex test rigs and inhomogeneous reflected radiation distribution, the
need for a spatially resolved calibration arises. Therefore, this paper presents a new correction
method accounting for a spatially varying reflected radiation. By computing a geometrical raytracing, a spatially resolved correction factor is determined. An extended calibration technique
based on an in situ approach is proposed, allowing a local correction of reflected radiation.
This method is applied to a test case with defined boundary conditions. The results are
compared to a well-known in situ calibration method. A major improvement in measurement
accuracy is achieved: the error in calibrated temperature can be reduced from over 10% to
well below 2.5%. This reduction in error is especially prominent when the test surfaces are
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colder than the hot ambient, which is the case in many cooling applications, e.g. in gas turbine
cooling research