Projectors, have 2 conclave mirror type covers over the H3 and H1 bulbs. These have a greater (wider) light range.. i think, and dependign on the style projector, the glass can give off a blueish tint.
Halo's have no cover over a open C cup for H1's surrounded by a "halo ring" which is just like 14 LED's that light up and appear to make a small halo. the H3 is covered partially only to block blindign people, there is a bulb protector cover thing, its only the size of a quater, and increases the light range. Once properly adjusted, these are the best for the sentra (IMHO)