[In the year of the death of king Uzziah -- I see the Lord, sitting on a throne, high and lifted up, and His train is filling the temple/QUOTE]
from awpink
3. His Effulgency.
"Who being the brightness of (His) glory" (verse 3). In this verse the Holy Spirit continues to set forth the excellencies of Christ, and in the same order as in the preceding one. First, the Divine dignity of His person, His relation to the Father—He is the Brightness of His glory. The Greek verb from which "brightness" is derived, signifies "to send forth brightness or light," and the noun here used, such brightness as cometh from light, as the sunbeams issuing from the sun. The term is thus used metaphorically. So ably has this been developed by Dr. Gouge we transcribe from his excellent commentary of 1650: "No resemblance taken from any other creature can more fully set out the mutual relation between the Father and the Son: "1. The brightness issuing from the sun is the same nature that the sun is—cf. John 10:30. 2. It is of as long continuance as the sun: never was the sun without the brightness of it—cf. John 1:1. 3. The brightness cannot be separated from the sun: the sun may as well be made no sun, as have the brightness thereof severed from it—cf. Proverbs 8:30. 4. This brightness though from the sun is not the sun itself—cf. John 8:42. 5. The sun and the brightness are distinct from each other: the one is not the other—cf. John 5:17. 6. All the glory of the sun is this brightness—cf. John 17:5; 2 Corinthians 4:6. 7. The light which the sun giveth the world is by this brightness—cf. John 14:9 . . . Thus the Son is no whit inferior to the Father, but every way His equal. He was brightness, the brightness of His Father, yea, also the brightness of His Father’s glory. Whatever excellency soever was in the Father, the same likewise was in the Son, and that in the most transplendent manner. Glory sets out excellency; brightness of glory, the excellency of excellency."
That which is in view in this third item of our passage so far transcends the grasp of the finite mind that it is impossible to give it adequate expression in words. Christ is the irradiation of God’s glory. The Mediator’s relation to the Godhead is like that of the rays to the sun itself. We may conceive of the sun in the firmament, yet shining not: were there no rays, we should not see the sun. So, apart from Christ, the brightness of God’s "glory" could not be perceived by us. Without Christ, man is in the dark, utterly in the dark concerning God. It is in Christ that God is revealed.