Some meditation teachers take it to mean mental visualization of bright light while others take it to mean sitting with open eyes or looking at actual light, sun, lamp, overhead light etc. until drowsiness fades and then resume sitting. I find the second interpretation works well for me.
To study is to know the texts,
To practice is to know your defilements,
To attain the goal is to know and let go.
- Ajahn Lee Dhammadharo
With mindfulness immersed in the body
well established, restrained
with regard to the six media of contact,
always centered, the monk
can know Unbinding for himself.
- Ud 3.5