I remember learning about bandpass filters in college physics so I understand the underlying theory and can definitly say it is not a bunch of hocus pocus. I think this could prove as a good measure of whether your newly purchased 660 nm leds are to good spec! https://www.google.com/search?q=660...m=93&ie=UTF-8#q=660+band+pass+filter&tbm=shop A bandpass filter is exactly what the name says. A filter that only lets a given band of wave lengths pass through. You'll notice the first bandpass filter on the list claims it is 660nm +-2nm. If you look at an led through one of these bandpass filters and it does not appear very bright you immediately know that it does not emit any wave lengths you want it to emit. If it does appear bright then it is good to go! I may personally purchase one of these filters and let you all know how well it works. It could serve as a very good/quick means of checking the quality of a led (or an ordinary light bulb for that matter) before you integrate it into your build! They have all sorts of wave length band pass filters by the way. So whatever wavelength your interested in checking, theoretically you can just look it up wave length you want online and buy it.