Albert Barnes on Luke 2:9

The glory of the Lord. This is the same as a great glory--that is, a splendid appearance or light. The word glory is often the same as light, 1Cor 15:41; Lk 9:31; Acts 22:11. The words Lord and God are often used to denote greatness or intensity. Thus, trees of God mean great trees; hills of God, high or lofty hills, &c. So the glory of the Lord here means an exceedingly great or bright luminous appearance--perhaps not unlike what Paul saw on the way to Damascus.