I have one tought ...on backup ... DRIS uses reflector ( which made best performance over lens ) .... but it is also very sensitive on oxidation , fingerprints...( I don't like that if you open backup it decomposes instantly at many parts )
For insctance one interesting detail .... I have mentioned Ammonite led one ... when you open head apart .... head has 3O rings and these O ring but these are also in conical nozzle !
What gives superb tightening. And when fully open head catches first battery with inside O ring .
At the moment I still have SCHULTZ GS35 which is good light with streamdesign , but it nerves me because it has fine thread , one exposed Oring ( when is of is there a little gap between head and body )
and when we have on surface equipment check it always is closed but between dive and between drils it often activate itself ...)
If you look for instance on well known Hesser ( a little overpriced ) which is best over these DIR lights is a insert of ( LED itself + lens ) compact and watertight.
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and IMO best led for BACKUP at the moment ( performance , beam , reliability, consumption ) is Cree XP-G2 ( and not XML and specialy not U2or T6)