In all other cases of the use of the word in the Bible its sense is metaphorical. The psalmist says that God lightens his darkness (18:28 KJV “enlighten”), and that God’s commandments are pure, enlightening the eyes (19:8 RSV). Pondering the omniscience of God, the prophet Isaiah asks, “Whom did he consult for his enlightenment?” (Isa 40:14).
It is evident from these passages that enlightenment is the intellectual and moral effect produced upon a person by the reception of the Christian revelation. It is not a mere intellectual illumination or understanding of divine truth, for this spiritual insight manifests itself in ethical action. Christians are “sons of light,” as Paul puts it (1 Thess 5:5).
International Standard Bible Encyclopedia (1915)
(1) ’or, "illumination" in every sense, used in the ordinary sense of giving natural light (Ps 97:4 the King James Version; see also Ezr 9:8) or as a sign of health and vigor (1Sa 14:27,29). "His eyes were enlightened," literally, "became bright." He had become weary and faint with the day’s exertions and anxieties, and now recovers (see Job 33:30 and compare Ps 13:3). Thus in sickness and grief, the eyes are dull and heavy; dying eyes are glazed; but health and joy render them bright and sparkling, as with a light from within.
(2) In Ps 18:28 the King James Version, The word naghah, figuratively describes the believer’s deliverance from the gloom of adversity and the restoration of joy in the knowledge of God.
(3) Most frequently the terms so translated mean the giving of spiritual light to the soul (Ps 19:8; Eph 1:18, photizo; Heb 6:4; 10:32). This spiritual enlightening the Spirit of God brings about through the Divine word (Ps 119:130; 2Ti 3:15; 2Pe 1:19). Sin mars the intellectual discernment; "but he that is spiritual discerneth all things" (1Co 2:15 King James Version, margin).