A Proposal for Spectroscopic Infrared Thermography and Its Estimation

Infrared thermography is a convenient procedure by which to visualize and estimate various temperature fields of objective surfaces nondestructively by sensing energy emitted from them as electromagnetic waves. This technique allows a two-dimensional temperature field to be evaluated quantitatively, nondestructively and simultaneously at every picture element even though the object has a complicated shape. This increasingly recognized applicability has led to development of remote-sensing diagnoses for various engineering applications. However, the emitted energy transferred to an infrared sensor, which is usually used as a signal for determining temperature, will always include energy reflected from the surroundings as noise. In particular, it is difficult to apply infrared thermography directly to quantitative temperature measurement of glossy metal surfaces under near-ambient conditions, where the influence of the reflected energy becomes marked, and the measurement error is large. Distinguishing the signal for determining temperature from a signal detected with the infrared sensor is difficult in any case. For this reason, despite some creative research on infrared thermography, the present accuracy of measurement is still not satisfactory. The infrared sensing techniques proposed previously by the authors, two-colored and three-colored techniques, are further discussed for developing a more general nondestructive method of quantitative temperature measurement under near-ambient conditions. The three-colored technique for quantitative temperature measurement is modified by applying three kinds of newly-developed infrared filters simultaneously in the present study. A new hypothesis is also introduced. Each filter has a selective wavelength band of several mu m in width in the range of 7-14 mu m. The applicability was confirmed by empirical investigation allowing a parametric study of how the result varies for various surface conditions. We investigated the measurement error experimentally, and the effect of surround disturbance and directivity on the present technique. The modified method allows quantitative temperature measurement for target surfaces at each picture element without presuming any emissivity, reflectivity and ambient conditions, so that it may be useful across various medical, chemical, physical and engineering disciplines.