Well, it means that the amount of radioactive tritium will be half of what it is today. I'm still not certain the it means the lume brightness will be half. And this might be more of an academic/theoretical concern... but...That means 12.3 years from now, the tritium lume brightness will be half what it is today.
I think I asked this in another thread, but does anyone know if it takes ALL of the initial radioactivity to reach some maximum brightness? Or is there "excess" radiation initially in the tubes? Do radioluminescent materials "max out" in brightness, or get continually brighter with more radiation?