What you observed is true - on the NF series, or any watch using this type of luminous material, the luminosity is very strong for the first 2 hours or so, drops off considerably over the next 3 hours, and then even more so after that.
What I think is important to note is, that even after 7 or 8 hours, when the luminosity is low, it's about as bright as Tritium.
Also, while bright luminosity is eye catching, there are very few situations where there is not enough ambient light to read a watch, even one which has no luminous material.
For years I wore vintage watches with no luminous markers or hands whatsoever and don't think I ever found myself unable to tell the time - but then again I don't rely on my watch to tell the time while I'm sleeping.
Best regards,
Steven
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