Index Variations (My TiO2)
Need better understanding of film index variations? FilmStar offers a simple way to calculate spectra as 'My TiO2' varies between 'Good TiO2' and 'Bad TiO2'. (Of course, the discussion is not limited to TiO2.) Consider two index files TiO2-Good.itw and TiO2-Bad.itw. We define User Index Function MyTiO2 as follows:
Note that coefficient A is expressed in percent for convenience. While component files require overlapping wavelength ranges, they do not need to have the same spacing. The Film Indices editor below illustrates a case of 'My TiO2' consisting of 50% 'Good' and 50% 'Bad'.
The FSPlot graph below illustrates how index variations affect spectra. Users can verify results by specifying each material file separately in the Film Indices dialog.
In the Interactor, click Setup...Adjust Indices to quickly visualize index variations. The Adjust Film Indices dialog can be left open while returning to the main Interactor screen for other adjustments. Use Compare or Difference Mode to plot both measured and calculated spectra. (Design Mode shown below.)
FilmStar INDEX Functions...Fit Index provides further means to understand and verify results.
As can be seen, extreme TiO2 files were utilized for purposes of illustration. The strength of this method is that only one coefficient is required, but obviously more complex models are possible. That raises the question whether it is reasonable to assume that 'My TiO2' is a linear combination of 'Good' and 'Bad'.
As with many practical issues in optical films, we cannot recall any papers on this subject. (Please let us know otherwise!) We suggest this represents an excellent subject for PhD student research in conjunction with an industrial optical coating facility. Furthermore, this provides another reason why coating measurements need to go beyond pass-fail.
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