A multi-color technique for surface temperature measurements in the presence of reflected radiation

Infrared pyrometry is used as a non-intrusive technique to assess surface temperatures when conventional temperature measurements are not feasible. Turbine engine internal hot part surface temperatures are important in understanding many aspects of the design and operation of aircraft engines such as structural integrity, life, and IR signatures. The problem of assessing engine internal temperatures is similar to the problem of measuring surface temperatures in a non-isothermal cavity. Radiometric measurements made of surfaces within such a cavity include reflected radiation incident to the surface of interest. This incident radiation can cause errors in temperatures determined from the radio-metric measurements.

A three-color radiance measurement technique is described for determining the surface temperature of a heated target that is reflecting radiation from an adjacent surface. Radiance measurements are made in three wavelength bands. If the surface behaves as a graybody, the three-color technique can correct for the errors induced by reflected radiation. An experiment was performed to demonstrate the three-color technique for a simple case, and the results are reported.

The three-color technique was shown to be highly sensitive to non-gray behavior. Application of the three-color technique with corrections for non-gray behavior resulted in temperature measurement errors less than 2 percent in the presence of reflected radiation. Conventional ratio pyrometry (two colors) resulted in temperature measurement errors greater than 10 percent in the presence of reflected radiation.