Technical Issues - Determining k in Filter Glasses and other Substrates (k<<n)

Schott filter glass data (C:\Winfilm\Index\Schott Filter\*.itw) were derived from n and Internal Transmittance (t) data supplied by Schott. As different batches may have slightly different k values, users need to verify data with measured %T or optical density (INDEX 2.51.0800).

INDEX 2.51.0800 Setup Parameters adds Optical Density option

It is assumed that Schott n values do not vary in different batches (melts). The Optical Density and % Transmittance plots shown below simulate measurable quantities.

Catalog t (internal transmittance) and measured %T/OD wavelengths must match index 'n' wavelengths. Implement with Functions.. Convert Table (INDEX) or Spectra..Convert Table (DESIGN and MEASURE).

The FilmStar installer includes 1999 Schott filter data (above). We have recently downloaded 2023 data supplied by Schott. Click here to download 2023 data converted to CIE color range (380-780x1 nm). Contact FTG if you require a glass not in the list.

As there is a possibility of errors in our conversion (let us know), please compare with Schott 'Optical Filter Glass - Calculation Program and Tutorial'. A comparison between 1999 and 2023 Schott data is displayed below. One assumes that differences are due to melt-to-melt variation, but improved spectroscopy is also possible. Be sure to inspect and characterize purchased filter glass before coating.

It is instructive to verify INDEX %T spectra in DESIGN. Download (no unzip password required) sample ITW/FAW files to verify that DESIGN %T and OD plots agree with those displayed in INDEX for various substrate thicknesses. Hint 1: Copy %T data in INDEX and paste into DESIGN to compare in FSPlot. Hint 2: INDEX OD/%T spectra always include Side 2 compensation.

Related Topics:

https://www.ftgsoftware.com/indx.htm#Filter_Glass

 https://www.ftgsoftware.com/issues_diabatic.htm

Historical comment: Years ago we fabricated a filter for a color copier. This included a highpass and lowpass deposited on two sides of clear glass. Replacing the lowpass with yellow Schott glass was far more productive.