I am under the impression that Light is just your ability to contain/obtain Motes of Light.
And as you fill up your Light bar, you get Motes of light, that are used for leveling.
I don't think Motes of Light are used for level past 20 are they?
I think they are the currency used by the Speaker NPC,
This what the wiki tells me:
A Mote of Light is an item Guardians collect by collecting experience after they've reached level 20. Motes of Light can be traded in to the Speaker for Legendary-level armor.
So if you are level 20, every time you ding to the next level, it will be a Mote of Light
Some of the armor requires 20+ Motes if I remember correctly, meaning you filled the XP bar 20 times after hitting level 20.
I think that Light stat that your armor gives, gives you your new level
So reaching 118 Light correlates to +9 levels from 20, making that Titan 29
If you divide 118/9 = it translates to every 13 light added to your stat you gain a level
(Maybe a piece has an extra+1 on it)
Most likely capping the +Light on any Guardian can have to 130 (maybe a little more), making you be a level 30 Guardian and set up for the level 31 Raid for Vault of Glass (Hard Edition)
Cause if they let you level to 31 and be equal to Raid level, the difficulty would change (hard would be normal, just as Devils' when you do it at 7 there's still hard, but be 8, it becomes normal), so I'm guessing Light will be hovering around 130-135, not much more than that
So you'll have to add the right amount to reach the max, in case those Exotic piece become a necessary item to hit the best level possible