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Baader 4 mm thick UBVRI filters feature a proprietary design that strictly adheres to the classic Johnson/Bessel transmission specification. Unlike the classic construction that regularly has a thickness of 5mm or more, the Baader-filters are made of dielectric layers as well as absorption glasses. This allows for a smaller filter thickness of 4 mm. The original stack of pure colored glass had a thickness of 5 or 6 mm and would be too thick and too heavy for modern filter wheels.
In the old days UBVIR-filters were used for photodetectors and no one would care if the huge stack of cemented absorption glasses would cause havoc in terms of inherent internal reflections. Our method of combining dielectric coatings with a combination of short pass and long pass filters does provide for perfect off-band blocking into the far ends of the spectrum. This steep off-band-blocking would not be possible with pure dielectric layers over such a large transmission range.