principal visual ray
principal visual ray[′prin·sə·pəl ′vish·ə·wəl ′rā]
A perpendicular extending from a station point to a perspective plane and theoretically passing exactly along the visual axis of a viewing eye. Also known as principal ray.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.