Jamieson, Fausset and Brown on Luke 9:29

29. as he prayed, the fashion, &c.--Before He cried He was answered, and while He was yet speaking He was heard. Blessed interruption to prayer this! Thanks to God, transfiguring manifestations are not quite strangers here. Ofttimes in the deepest depths, out of groanings which cannot be uttered, God's dear children are suddenly transported to a kind of heaven upon earth, and their soul is made as the chariots of Amminadab. Their prayers fetch down such light, strength, holy gladness, as make their face to shine, putting a kind of celestial radiance upon it (2Co 3:18, with Ex 34:29-35).

raiment white, &c.--Matthew says, "His face did shine as the sun" (Mt 17:2), and Mark says (Mr 9:3), "His raiment became shining, exceeding white as snow, so as no fuller on earth can white them" (Mr 9:3). The light, then, it would seem, shone not upon Him from without, but out of Him from within; He was all irradiated, was in one blaze of celestial glory. What a contrast to that "visage more marred than men, and His form than the sons of men!" (Is 52:14).