Why do people say the faces of saints glow?

Many people who meet saints, meet holy people, say that their faces glow, the light comes out from the center of their face. What is this? This is literally the indwelling of God's glory. This is the Old Testament Shekinah glory that God bestowed upon Moses when he was on Mount Sinai and came down, and it says his face shone like the sun. We read about this in the life of St Seraphim of Sarov and numerous other saints, whose faces literally shone and glowed with the glory of God. But this glory was not their own. This glory was given to them by God.