Measuring Thermal Emission Near Room Temperature Using Fourier-Transform Infrared Spectroscopy

Accurate characterization of thermal emitters can be challenging due to the presence of background thermal emission from components of the experimental setup and the surrounding environment. This is especially true for an emitter operating close to room temperature. Here, we explore the characterization of near-room-temperature thermal emitters using Fourier-transform infrared (FTIR) spectroscopy. We find that the thermal background arising from optical components placed between the beam splitter and the detector in an FTIR spectrometer appears as a “negative” contribution to the Fourier-transformed signal, leading to errors in thermal-emission measurements near room temperature. Awareness of this contribution will help properly calibrate low-temperature thermal-emission measurements.

Figure 1

(a) The optical path of our FTIR spectrometer. Light emitted from the sample (red arrows) is collected by a parabolic mirror, traverses the interferometer, passes through several optical components (mirrors and apertures), and is then sent to a liquid-nitrogen-cooled MCT detector. Most of the instrument components are at approximately room temperature, which results in several sources of background emission in both the forward (green and blue arrows) and backward (yellow arrows) directions. (b) Measured uncalibrated spectral intensity from the carbon nanotube (CNT) laboratory blackbody from 10 ^oC to 30 °C, which is minimized near 19 °C and increases when the sample is either heated or cooled. Inset: the integrated (but still uncalibrated) total spectral power over all wavelengths. The temperature of the room housing the experiment, $T_{\mathrm{room}}$, is 23 °C. (c) Selected interferogram traces corresponding to the spectra in (b). The amplitude of the interferogram trace is minimized when the sample temperature is near 19 °C, and there is a $\pi$ phase shift between the traces when the sample temperature is above (red lines) and below (blue) 19 °C. (d) Extracted contributions from the sample (solid lines) and the combined background (black dotted).

Figure 2

Interaction of two thermal emitters (“Sample” and “Backward background”) with an interferometer. The sample emission is modulated via transmission through the interferometer, whereas the backward background is modulated via reflection. The signal received by the detector is the sum of the two contributions.

Figure 3

(a) Measured polarization-averaged emissivity of a thick fused-silica sample at an angle of 10°, extracted from the magnitude of the measured spectrum (i.e., the magnitude of the Fourier transform of the interferogram), from different temperature ranges. The corresponding calculation based on optical constants from spectroscopic ellipsometry is shown in green. (b) Averaged relative error [Eq. (5)] in the extracted emissivity as a function of the central temperature at which the emission data was taken, using only the magnitude of the spectrum (blue) or the full complex value (red).