There are many options for fixtures that will accomplish this look. These happen to be copper and brass fixtures from Vista (www.vistapro.com), specifically 5 2116-CSN's and 3 2151-CSN's. The key is getting quality fixtures that will protect the lamp inside, whether it is halogen (like I used on this oak) or LED. The lamp is what determines the look, and I used 3 different halogen lamps on this tree. The lights aimed up are 20w 36° (BAB) and 35w 36° (FWM) lamps. The light shining down, creating the shadows on the path are 20w 60° (BAB-60) lamps with light blue filters, but now I'd use 4000°K LEDs to achieve the same look with far easier maintenance. Same for uplighting the Oak, LED is a better option, and I'd stay in the 3000°K range (as opposed to something warmer in color like 2700°K) to bring out the white and grey in the bark. This will depend on your species of tree though, so play around with it until you get what you like.

You obviously did a great job as it looks amazing! Certainly showcases the beauty of that tree!
Is it okay to place the light in the tree without hurting the tree? What's the best way to secure it in place? Thank you!!!

This is beautiful. I am bookmarking it right now. Thank you for the detailed information. Just happened upon this post.

The fixtures and wires in trees should be fastened with stainless steel screws and hardware so as to not poison the tree. As long as the fasteners are stainless, the tree will heal around them and be fine.

I use zip-ties with eyelets like these: http://www.tnb.com/ps/fulltilt/index.cgi?part=TY34MX to fasten the wire, with 2.5" stainless deck screws. Leave the screws out about 1"-1.5" to allow space for the tree to grow. As the tree grows, you can back the screws out. If you don't, the tree will swallow the wire and pull it apart, usually leading to a short (which could lead to fire).

To mount the fixture, use stainless hanger-bolts, 0.25" X 3" or so. Again, this allows for a stand-off gap between the fixture and the tree, so that the tree won't grow around the fixture. It too can be backed out as the tree closes the gap. Not as big an issue with oaks since they grow so slowly. That 1" gap will usually give me 5-6 years at least before I need to make any adjustments on most oaks. Live oaks tend to need attention sooner than the valley oaks (like the one in the pic) or blue oaks we have here.