Duel arc bulbs combine the MH spectrum and HPS spectrum into 1 bulb, Have not tried one as of yet but seems that you are suppose to get the benefits of both light spectrums during the entire grow. I would not think they would be a great idea if you were trying to keep your "moms" vegging indefinatly but if doing a start to finish grow they should work very well. From what I have seen they cost about double or so compared to a reg HPS bulb and not allot of places carry them atm.
Have a nice day!! :jointsmile: