Well it looks like someone cracked it already.
http://www.reddit.com/r/ffxiv/comments/2gm1ru/nexus_light_information/
Feeble: 1 (Confirmed)
Gentle: 2 (Confirmed)
Bright: 4 (???)
Brilliant: 6 (???)
Blinding: 8
Newborn Star: 10 (???)
Tiers:
'No Activity': 0 - 199
'Indisctinct Activity': 200-399
'Faint Activity': 400-599
'Slight Activity': 600-799
'Modest Activity': 800-999
'Distinct Activity': 1000-1199
'Robust Activity': 1200-1399
'Vigorous Activity': 1400-1599
'Intense Activity': 1600-1799
'Extreme Activity': 1800-1999
'Bursting Activity': 2000+ (Complete)
There was another list I saw on what gave what type of light..
Figure we could discuss and add to it here..
2000 to complete.