Ἀπαύγασμα, apaugasma, Goodrick-Kohlenberger [GK] G575 or Strong [S] G541, according to the New International Dictionary of New Testament Theology and Exegesis, second edition, revision editor Moisés Silva, volume one, pages 440—441 (slightly expanded on first edition by R.P. Martin, vol. 2, p. 290):
Special interest attaches to ἀπαύγασμα, apaugasma, GK G575 or S G541, a term applied to the Son of God in Hebrews 1:3, ὃς ὢν ἀπαύγασμα τῆς δόξης καὶ χαρακτὴρ τῆς ὑποστάσεως αὐτοῦ, hos ōn apaugasma tēs doxēs kai charaktēr tēs hypostaseōs autou, literally, “who being the radiance/reflection of [God’s] glory and the impress of his being.” This statement stands in a doxological, possibly hymnic, tribute to the cosmic Christ, who is described either (a) actively, as the radiance of the divine glory, comparable to the way sunlight conveys the brightness and intensity of the sun, or (b) passively, as the reflection of that glory, comparable with the thought of Christ as the “mirror” or “image” of God (Wisdom 7:26 in the Common English Bible [2011]: She’s the brightness that shines forth from eternal light. She’s a mirror that flawlessly reflects God’s activity. She’s the perfect image of God’s goodness.). Since χαρακτήρ, charaktēr, GK G5917 or S G5481 clearly has a passive sense, many scholars understand ἀπαύγασμα, apaugasma, GK G575 or S G541 here to mean “reflection,” but a firm decision is difficult. Indeed, the distinction between these two aspects should not be pressed; in either case, the verse may be viewed as a piece of “wisdom Christology” that interprets Christ as the supreme and final revelation of the Father. (The term is used with increased frequency by early writers such as Origen [almost 30 times] and Gregory of Nyssa [about 80 times], but especially Athanasius [over 150 times]. For a valuable summary of the patristic interpretation of this statement, see Philip Edgeumbe Hughes, Commentary on the Epistle to the Hebrews [1977], pages 41—45.)