Looking directly at the sun (darkened or not) is dangerous. That seems an apt picture of the danger sinful people face in the presence of holy God. When the Lord appeared to Israel on Mt. Sinai, His presence was so terrifying that they did not want to hear His voice, did not want to have Him come any closer. They begged Moses to represent them while they stayed safely at the foot of the mountain (see Exodus 19:18-19). Here’s one description of God’s presence: the appearance of the glory of the Lord was like a devouring fire on the top of the mountain (Exodus 24:17).
Moses, on the other hand, wanted to be closer to God. Having experienced His presence, Moses asked “Please show me your glory" (Exodus 33:18). But God was merciful. He said, “I will make all my goodness pass before you, and will proclaim before you my name … (v. 19). But “you cannot see my face, for man shall not see me and live” (v. 20).
Yes, God revealed Himself, but He shielded Moses from the full radiance of His glory. The prophets’ visions of God, His appearances at Sinai and in the Tabernacle - these were filtered images of His glory.
All of that is necessary background to our understanding of another of Christ’s titles from Hebrews 1. In v. 3 Jesus Christ is called THE RADIANCE OF GOD’S GLORY.
Radiance means that Jesus always, constantly, radiates God’s glory. Just as sunlight is distinct from the sun itself and yet is still the sun, so Christ radiates God’s glory, is distinct from it, and yet it is also His glory. Here's how John put it: And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth (John 1:14). Later he added, No one has ever seen God; the only God, who is at the Father’s side, he has made him known (v. 18).
The only radiance of God’s glory that can reach us and not destroy us comes through Jesus Christ. The perfect humanity of Jesus is the filter that allows God’s glory to come through in all its beauty without killing us. Colossians 2:9 says in him the whole fullness of deity dwells bodily, and that’s the glory we see. All the fullness is there, but it dwells in His humanity.
Christ's radiance is not just His stellar personality or superior intellect or moral purity. Yes, He is all those things in His humanity. He is the perfect man. But also, shining through the perfection of His humanity, is the glory of the eternal God.