It’s not a buff - it’s a passive damage proc that has its own discrete line item in scrolling combat text and logs.
If you have access to Combat Metrics, look at the log after a fight and see what damage Burning Light did. (Alternatively, you can turn on outgoing damage combat text in the native game settings.) The fight should be more than a single trash mob or pack if you are able to erase those in a couple of seconds, as Burning Light relies on reaching 4 stacks with a 0.5 second cooldown between stacks and a 3 second timer on maintaining them.
It also doesn’t hit multiple targets simultaneously. It’s single target direct damage.