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This method assumes that the operating parameters are not
changed between the measurement of the calibration spectra and that
of the object view spectra. The scanning speed and the optical con-
figuration must remain the same. Since the experiment requires the
acquisition of data in both directions of the interferometer stroke, two
sets of M Stray (σ) and K(σ) must be provided (one for each direction)
for the calibration.
The radiometrically corrected measurements are also complex
spectra. However, all spectral information should be contained in
the real part of the complex spectra. The imaginary part should only
contain noise. The imaginary part can be used to validate the spectra
and to estimate the noise levels. After verification, the imaginary part
can be dropped to free up storage space.

10.9.2 Multiple Point Calibrations
The calibration procedure can be extended to use multiple reference
points instead of two points. Multiple point calibrations are used to
improve the quality of the obtained calibrated spectra. Two types of
errors can be addressed with multiple point calibrations: random
errors due to noise, and systematic errors due to the nonlinear response
of the spectroradiometer.

10.9.3 Linear Multiple Point Calibrations
Linear multiple point calibrations are used to reduce the impact of ran-
dom uncertainty on the calibration curve. When a series of measure-
ments are taken for blackbodies at different temperatures, there is always
a fluctuation around the ideal calibration line, as shown in Fig. 10.14.
The best-calibrated spectra will be obtained by using the multiple
point calibration, which is a linear regression that uses all the availa-
ble points in order to more accurately characterize the system. If we

Scene Spectral Radiance 1 3 2 5 4

Spectral Power at Detector
2 point calibration using 1 and 4
2 point calibration using 1 and 5
Multiple point calibration
FIGURE 10.14 The multiple point linear relationship between the object view
radiance and the detector response.