λαμπρός“shining”The word λαμπρός (lampros), the adjective meaning "gleaming" or "splendid," is by far the best-known form of this small family and the only one the New Testament uses more than once. It describes anything that catches the eye by giving off or throwing back light: the bright morning star, a river sparkling like crystal, the dazzling white linen of heaven, the shining clothes of an angel, and, at the other end of its range, the expensive clothes of a rich visitor and the luxury goods a doomed city loses. Its matching abstract noun λαμπρότης (lamprotēs) names that gleam on its own, and its matching adverb λαμπρῶς (lamprōs) says a life was lived in that showy style. All three are one word in different forms, so they are studied together under the adjective.
λαμπρότης“brightness”The word λαμπρότης (lamprotēs) is the abstract noun of this family, naming the gleam itself: brightness, radiance. It appears once, when Paul tells King Agrippa that the light he saw on the Damascus road was brighter than the sun. It carries exactly the idea of the adjective λαμπρός (lampros), "gleaming, splendid," only named as a quality in its own right rather than attached to a thing, which is why the λαμπρός entry explains it.