Quote Originally Posted by hrst View Post
Well yes, the filter you show is not "blue", but more likely "light cyan-blue". It is probably meant for coloring tungsten light sources to give an impression of "blue" compared to unfiltered light. The reason why it passes so much green and even red is to increase light output level for tungsten light sources that are low in blue wavelengths. In fact, that curve is quite close to 80A color temperature conversion filter! No, it's probably more close to 80B or even 80C. There's so much red passed.

There are also real blue filters available, and when I speak about blue filter, I really mean blue filter.
No, this filter is advertized as blue filter for b&w and not as correction filter. This is from Heliopan using Schott glass. Older B&W filter catalogs show similar curves for the blue filter.
I found much narrower response for wratten gel filters but even these went up to 500nm. IDK if these are available as glass filters.