The meaning of απαυγασμα in Heb 1:3 is disputed. Actively, the word can denote radiance or effulgence (Phil, Spec. Leg. iv.123), or passively, reflection or the light that is reflected (Wis 7:26; Philo Op. 146; Plant. 50). The sentence structure in Heb 1:3 favors understanding απαυγασμα and → χαραχτηρ as synonyms and, therefore, interpreting απαυγασμα as pass.: Christ “reflects the glory of God and bears the very stamp of his nature.” Both predicates characterize the Son as the perfect image of God and thus correspond to the expression → εικων του θεου (Col 1:15; 2 Cor 4:4). (Exegetical Dictionary of the New Testament, volume 1, eds Horst Balz and Gerhard Schneider [Edinburgh: T&T Clark, 1990], 118)
For more on why this is significant, see the discussion of Heb 1:3 (quoting D. Charles Pyle) at: