An image reading apparatus and an image reading method are provided by which a top face image and a back face image of a document can both have an improved reading accuracy while simplifying a circuit configuration and providing a higher processing speed. The generator includes a plurality of birefringent stages, each stage including a differential group delay element and a phase-shifting element. In multi-bank dynamic memory systems, concurrent refresh systems allow memory refresh circuitry to refresh memory banks that are not currently involved in memory access operations. The vertical propulsion system is capable of providing a force to such wheels normal to the surface so that the vehicle separates from the surface. Where the secondary air concentration value exceeds a second predetermined threshold, an alarm signal may be activated or fluid delivery may be stopped.