The ability to deduce dispersive indices is crucial for the  successful optical coating
enterprise. Program INDEX  is included as part of the FilmStar design package.

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Main Screen

Index Values

Edit, plot and store tables of n,k vs. wavelength for subsequent use in DESIGN. Built-in index functions (Lorentz, Sellmeier, etc.) can be plotted and fitted to these n,k tables.

The extensive Sopra (537K) database of ellipsometrically determined index tables is included. Click here for information about importing n,k data and dispersion functions from other sources.

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Formula Editor

User-Defined Functions

Indices (n,k) can be represented by built-in and user-defined functions. Built-in functions include Buchdahl, Cauchy, Lorentz, Sellmeier, Forouhi-Bloomer, Tauc-Lorentz, GaAlAs (Afromowitz), etc. Users may add new functions in a built-in equation editor, in the FilmStar Workbook, in MS Excel, etc. These functions ultimately appear as if built into INDEX and DESIGN.

Mixed material models include Maxwell-Garnett, Bruggeman, and user-defined (shown here). A very important and practical example of user-defined functions is provided in Index Variations (My Tio2).

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Fitting Index Functions


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Using Functions in DESIGN

Index-Fitting Algorithms

  • n,k from %R and %T data for films of known thickness
  • n,k from %T data for k<<n films (envelope method)
  • n,k from %R data for k<<n films on reflective substrates such as Si (envelope method)

Spectral data can be obtained with FilmStar MEASURE or with any program that can output ASCII files. Index functions are further refined in DESIGN via inverse synthesis. The goal is to achieve the excellent agreement illustrated below:


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Filter Glass Table

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Filter Glass n,%T Plot

Filter Glass

FilmStar provides convenient handling of absorbing glass substrates. Transmittance (internal t or measured %T) values are automatically converted to k values or t/%T for other thicknesses. Multiple filter glass substrates with varying thickness can be used in a thin film design. Schott filter glass tables 200-1100 nm are included. These files were generated from Schott filter glass software (2.4M).

Because of differences between melts, careful
measurements may be required when designing
devices containing colored glass filters.

Copyright 2018 FTG Software Associates
Last updated on January 01, 2018