Yes, inquiring mines want to know. Me too. I sure like the looks of the home made reflector with two horizontal bulbs. Am going to make one. Before I saw his video, I bought a four bulb vanity bar from E-bay. So I can go either way.
Kenny

Sideways.

Not as much light comes out the end of the bulb as from the sides, so aim the side at the plants and use reflectors to redirect what misses.

And inquiring "minds" too!

Unless you are trying to grow only a couple of plants, I would go with horizontal. But if it is one or two plants in a small hydro unit or similar, I would go vertical and buy one of those cheap cylindrical reflectors.

Mike

You can also hang them around the plant(s) vertically so you're lighting them from the sides.