In him was life, and the life was the light of men.(Jn 1:4)I am the light of the world: he that followeth me, walketh not in darkness, but shall have the light of life. (Jn 8:12) let your light shine before men, that they may see your good works, and glorify your Father who is in heaven (Mt 5:16)
you were heretofore darkness, but now light in the Lord. Walk then as children of the light (Eph 5:8)
The nimbus of a saint is a visual representation of the light that is in a holy man. It is never shown, in traditional iconography anyway, as a reflected light, but always as something that comes out of the saint himself. The face of the saint is painted in a similar way. The shadows around the nose and the cheeks are combined with bright strokes so that the overall impression is of the source of light being inside the person, rather than outside of him.
Observe that the eyes do not have glare that would reflect an outside source; they are in themselves luminous.