That graph looks to have errors on the x axis to my eye. It should continue to jump in 12.5 year increments, not double like that for that curve. It is clearly wrong at 49, 98 years and upwards. While some active tritium remains indefinitely, in practice after 2 half lives ie 25 years there isn’t sufficient activity left to excite the phosphor in any meaningful way so the lume can be considered dead.
Originally Posted by PickleB
Just in case there is anyone in doubt, 12½ years isn't a cut-off time. The lume should be present for many more years in proportion to activity of the Tritium as it decays:
However, the phosphor will also degrade with time and make the situation worse. There is more in a previous thread: How long can tritium lume last?
In particular, the OP may be interested in:
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