This question comes up regularly on Candlepower Forums. Betalights are illegal in the US but they do sometimes show up on the forsale board over there anyway. For a PSK you're better off with an underdriven keychain coin cell LED light (i.e. use a CR2032 battery with a white led, instead of the usual two CR2016's). It will still: put out much more light than the Betalight with a more neutral color cast which is better for reading maps; have enough runtime for pretty much any situation; cost a lot less; take less space; and not have any legality problems. The shelf life of the lithium coin cell is about the same as the half life of the tritium in the betalight.