As I understand it (not being a native speaker), a beholder has a more active relation to the scene or object he is beholding. It is "in the eye of the beholder", but not in the eye of the spectator who is passively spectating, not relating him- or herself to the object in a way that necessarily implies a subjective evaluation. This personal distance is shared by the observer who, in contrast to the spectator, has an abstract or technical interest in the object of observance. A viewer seems to me the least specific superordinate concept for all three of them, of course also encompassing a non-personal, technical meaning.
Is this understanding correct and are there any other semantic differences I may have missed so far? Does the term "beholder" give an archaic impression?
Thank you for any contribution!