He would strip naked, grease himself up (both for warmth and lubrication), put a candle and lifeboat matches “in the one pocket of which man is still possessed in his natural state”, and squirm into spring outlets. More often than not, his illumination was a candle, or perhaps a lantern in a bigger cave.
He comes to mind as I recall him describing seeing cave paintings by candlelight, and the flickering light providing animation and life to them.
I also seem to recall Herzog talking about the same in his “Cave of forgotten dreams” documentary, along with the topology of the underlying rock sculpting light and dark to provide another layer of context and meaning to the paintings.
Anyway. Neat paper, and good to see this getting serious consideration.