I know this is an old post, but wanted to be sure to post a warning here about 'shatter-proof' bulbs. They are coated in PTFE, which is Teflon, and very toxic to birds. Many manufacturers, but not all, now have warnings not to use for poultry. Some say 'plastic coated'. Just had someone in my 4H poultry club go through 2 sets of 7 poults before figuring out it was the new heat bulb.
Google it and you will see that word is getting out to poultry growers not to use these new shatter-proof bulbs.
Murray McMurray has a great article about this and they sell a 'splatter proof' bulb that is coated in pyrex glass so that it will not explode when a hot bulb contacts water. Totally different than the 'shatter proof' ones.