Fig. 1. Diagrammatic sketch of an ideal spectrum

Fig. 2. Raw spectrum of a low-temperature blackbody observed by FTS

Fig. 3. Measured spectra of the external blackbody at different temperatures observed by the FTS

Fig. 4. Response relationship between uncalibrated spectra and blackbody radiances observed by FTS. (a) Lower range; (b) higher range

Fig. 5. Relative coefficients calculation corresponding to measured spectra of the external blackbody observed by FTS at different temperatures. (a) 7 ℃; (b) 17 ℃; (c) 22 ℃; (d) 27 ℃; (e) 32 ℃; (f) 37 ℃; (g) 42℃; (h) 52 ℃

Fig. 6. Response relationship between uncalibrated spectra and blackbody radiances. (a) Before nonlinearity correction; (b) after nonlinearity correction

Fig. 7. Comparison of original and corrected spectra

Fig. 8. Radiance bias of the 47 ℃ blackbody radiance based on linear radiometric calibration after nonlinearity correction. (a) Bias; (b) relative bias

Fig. 9. Radiance bias of the 27 ℃ blackbody based on linear radiometric calibration after nonlinearity correction. (a) Bias; (b) relative bias

Fig. 10. Radiance bias of the -3 ℃ blackbody based on linear radiometric calibration. (a) Bias; (b) relative bias