Background fluctuation limit of infrared detection of thermal waves at high temperatures

Analysis of multifunctional oxycarbide and oxynitride thin films by modulated IR radiometry

“…This may not always be the case, e.g. in the case of rather thin coatings, for which the relative extremum should be measured at very high modulation frequencies where the thermal wave detection [7] may already be affected by IR background fluctuations (Comp. Sect.5, Fig.…”

“…In principle, the radiation incident on the IR detector is affected by two types of fluctuations: -Fluctuations of the thermal wave signal and fluctuations of the IR background radiation. Using a Lock-in amplifier, fluctuations of the resulting thermal-wave signal have been found to be negligible due to the Lock-in-controlled repeated measurement and signal filtering of the thermal wave response at constant modulation frequencies, and thus the incoherent IR background fluctuations have been found to represent the main limitation for Modulated IR Radiometry [7]. In systematic studies on the detection limits of the described detection system thermal wave amplitudes above about 44 µK, 15 µK, and 9 µK have successfully been detected in a closed high-temperature cell at average sample background temperatures of 300 K, 400K, and 500 K, respectively.…”

“…The quantity γ D (T c ) is the detection efficiency for modulated IR radiation, depending on Planck's radiation law and the technical parameters of the IR focusing and detection system [7]. Using a two-phase Lock-in amplifier to amplify and filter the small periodical variations of the IR emission related to the thermal waves from the IR background radiation, the inphase and out-of-phase contributions [ …”

Thickness Control of Coatings by Means of Modulated IR Radiometry

“…The large errors observed for measurements on rather thin coatings can be due to the fact that at high modulation frequencies, corresponding to small thermal wave amplitudes, the detectable signals are affected by IR background fluctuations [8] and that only small deviations measured for the inverse calibrated phase lag signals F n ( f) can lead to larger shifts of the modulation frequency f min measured at the relative minimum F n min ( f ¼ f min ). This can be seen in Figure 4 for the data obtained in repeated measurements {TiCO(E1^), TiCO(E2^)} on the thinner coating, where only small deviations of the measured data (^,^) from the opaque two-layer model, observed above about ( f min /Hz) 1/2 > 150, already contribute to large errors (Table 2).…”

“…Apart from the limitations for thin coatings at high modulation frequencies, which are due to the IR background fluctuation limit of thermal waves, [8] there are also principal limitations for coatings of low thermal effusivity. With decrease in effusivity ratios coating-to-substrate, larger negative values are measured for the relative minima e.g., for the set of coatings with (e c /e b ) % 0.42 (Table 1) relative minima of about F n min % À15.48 are measured in Figure 3, whereas for the coatings with (e c /e b ) % 0.32 (Table 2) relative minima of about F n min % À19.68 are measured in Figure 4.…”