If your PT40 will get a good amount of use (considering you're using nimh), get a 4 cell Mag bulb instead of the 3. The 3 will be brighter, but it's running a 25% voltage overdrive for sustained burns. This can be fragile in nature. Depending on how much you'll depend on this light, take this into consideration. If you do go with the 3 cell bulb, definitely have a spare on the lanyard.