Some must've cast more light than others, since objects, or even people, were known to spontaneously catch fire in the presence of certain individuals, while staying stone cold in the presence of others.
A very specific, exacting gaze could indeed ignite that which it fell upon within seconds, and had the capacity of doing major damage not only to the inflamed, but to all surrounding areas by association.
Surprisingly, the medievals did not live in constant fear of catching fire. They were enraptured with light and with incandescence, and they were given to the notion that we are each aflame and shining towards some great mystery that burns within.
Catching on fire was merely a good omen.
(image by Andrei Porfireanu via Unsplash)