For an astronomy related book I've just finished, I needed to find exactly where the blue depression occurred. Just an FYI, H-beta is at 486 nm and O-III (forbidden lines of oxygen) are at 495 and 500 nm. For photography H-alpha is at 656nm. If these lines are left nicely isolated with LED conversions, it's very good for astronomers. Knowing exactly where the Sc and Hg lines are located, I lit a standard MH lamp in the background to get the figures to + or -1 nm accuracy.