A lithographic apparatus in which beams of radiation are projected onto an array of individually controllable elements, such that the beams interfere. Also, a method of determining ink drop velocity including providing such a card, and printing a line including a plurality of dots thereon. A flat portion is formed on a rearward-facing surface of the protruding portion, and an inclined surface is formed radially outward of the flat portion. When the error is eliminated, that effect is notified to the actuating system to enable re-initiation of the usual command input from the remote operating system.