Alley in Pisa, Italy

by Lars Kotthoff

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They probably don't correspond to Wratten numbers at all. The experimentally-derived filter factor for the red filter, for instance, is 2, whereas the filter factor for a #25 is 5, so there's an extra stop and a third of light coming from somewhere, and that somewhere is probably in the shorter wavelengths. The other filters are quite similar. That suggests ...

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