What is the best lamp for outdoor lighting?
There is no such thing. Lamps have their plusses and minuses. A high pressure sodium lamp may have a very good efficacy and a long life but its colour rendering is poor compared with a metal halide lamp, which in turn is less efficient and does not last as long. It is a matter of horses for courses and a good lighting engineer will be able to give you the best advice for your scheme.
There are lots of lamps that are made and approved for outdoor lighting. Patio Living Concepts makes some great models, but there are several brands out there. Just make sure that you look for a lamp that is specifically rated for outdoor use and is made of a waterproof or moisture proof material.
