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 ray-tracing, 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 colder than the hot ambient, which is the case in many cooling applications, e.g. in gas turbine cooling research.