Uhm ..... first, a lot of those "guns" for dental resin polymerization, are halogen lamps with a filter (usually a 10W 12V lamp with a glass filter) ..... only the new models uses leds, like P4 emitters ..... second, the usually frequencies of polymerization of the most part of the dental resins are in the 440/480nm range (opposite to the ones used for hobby or industrial works, that can have a polymerization range that vary from 350 to 550 nm) .....
This, also if the polymerization is more quick at 350nm, is made primarily for health safety ..... from a 10W halogen, the filter can left pass 1 or 2 W in the range of 440/480nm, and this range is just "blue light", where instead a pair of watt of 350nm for repeated or prolonged expositions, for the delicate tissues inside the mouth (that don't have any protective pigmentation against UV) can be dangerous, or at least, it was considered a risk.