Complex sentence: To achieve the results below, since the "n" and "k" spectra of aluminum is known, it is fixed in the calculation, and the thickness and "n" and "k" spectra of the TiN is allowed to vary.
Split into simpler sentences: To achieve the results below, since the "n" and "k" spectra of aluminum is known a priori and is fixed in the calculation. The thickness and "n" and "k" spectra of the TiN is allowed to vary.