Double ended. The bulbs are fired from both ends instead of one. Creates a stronger more even light, higher par levels. Everyone I know is switching or has already switched to them. I upgraded all my rooms to them last year, well worth it. Their hard to get right now because all the warehouse guys are buying them up by the 100's. Im getting a 15-20% increase in yield compared to my old setups (regular 1000w hps). It was a battle trying to get all the setups I needed in, I was on the phone for 3 weeks. Finally the hydroponics store I use got me nanolux de ballast in. Not my first choice but it was impossible to find gavita remote ballast or phantom de ballast. They have been working great so far, no complaints with them.
Their a game changer, more light for same amount of power.