An optical test apparatus comprises: a) a motor rotating a spindle which in turn rotates a workpiece such as a magnetic disk substrate; b) an upper test head comprising a laser for providing an upper laser beam to the upper surface of the workpiece, an upper lens for receiving light reflected from the upper surface, and an upper detector for generating a signal indicative of the received light; and c) a lower test head comprising a lower laser providing a laser beam to the lower surface of the workpiece, a lower lens for receiving light reflected from the lower surface and a lower detector for generating a signal indicative of the received light. The path of the lower laser beam is displaced from a radial direction of the workpiece so that the lower lens can be placed closer to the laser spot than if it was not displaced.