Multiple reflections between the sample and the window can be corrected with a calibration routine.
We developed a simple model of multiple reflections between the sample and the sapphire window that we use to correct the measured spectra. In this model, we assume that when light encounters the sapphire window, a constant fraction of light, T, is transmitted in the forward direction while the complementary fraction of light, 1 – T, is reflected in the backward direction. We apply this assumption for all multiple reflections between sample and window, which gives the following relationship between the sample actual reflectance, R, and the apparent reflectance, M (measured with the window), at each wavelength:
Application of this simple model to the data gives excellent results in correcting both the photometric bias and spectral effects of the sapphire window. Fig. 2 provides an example of the comparison between measurements made on the same sample without the window and with the window (raw and corrected). For this particular example, we choose an organic material that presents extremely contrasted values of reflectance in the visible and near-infrared spectral ranges to insure that the correction method is efficient from the lowest to the highest reflectance value.
Figure: Comparison between reflectance spectra of an organic material (‘‘tholin”) measured in the simulation chamber successively without (red curve) and with (blue curve) the sapphire window. Reflectance spectrum corrected from photometric and spectral effects of the sapphire window using Eq. (1) is also plotted in black.