Quote Originally Posted by ath View Post
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.
Well, you are right; this is very problematic terminologically. As described before about human vision, words of colors can mean many different things, and all that matters is the shape of the absorption spectrum; the curve of response to different wavelengths. In this case, "blue" can mean many things. More descriptive words are used such as light blue, deep blue, dark blue etc., but they are not much of help. You really have to know what you buy by checking the absorption spectrum plot.

I wouldn't call the plot you showed blue! It's so light. It could be light blue, but definitely not blue. But, OTOH, it is not called blue - it's called "blau 38". It is a product name that must be thought as such.