What's the badwidth you're interested in?  Shooting from the hip, I'd recommend a photodiode http://media.digikey.com/pdf/Data%20Sheets/Opto%20Diode/ODD-470W.pdf if you just need a coarse signal that detects the band centered on 470nm.  Also, you could use an analog to digital converter or even just a properly dialed-in comparator to cut out the heads and tails of the spectrum.