US7679802B2 - Optical Scanner Configured so a Principal Ray of an Incident Beam from a Light Source to a Deflector and a Principal Ray of a Scanning Beam from the Deflector to a Scanned Surface Travel on Opposite Sides of an Optical Axis of Optical Element(s) Having Transmissive Surfaces Both Concave Toward the Deflector in Sub-Scanning Section, and Image Forming Apparatus Using the Same | Patent Bear