A proposed electromechanical display system would be capable of presenting as many as four different messages. Conventional electromechanical display systems are limited to three messages.
The three-message limit arises as follows: A typical electromechanical display system contains display elements with multiple flat faces that are rotated into view to present a message. Each display element could show, for example, a letter or number or part of an image. If the display elements have flat faces, then the number of messages is limited to three because three is the maximum number of sides of a polygon that can be placed contiguously with other, identical polygons along a common base line and that can be rotated without interfering with an adjacent polygon.
In the proposed system, each display element would have four concave cylindrical faces shaped so that the elements could be positioned contiguously on a display plane, yet could be rotated without mechanical interference with adjacent elements (see figure). Because of the curvature of the faces, messages could be partially visible at greater (in comparison with flat faces) angles off the perpendicular to the display plane.
There is one "catch": To prevent mechanical interference while a display element is being rotated, an adjacent display element must either remain stationary or else be rotated in the opposite direction with a 45° lag. This "catch" can be turned to an advantage in that one can select from a variety of allowable sequences to draw attention to the display. For example, display elements could be rotated one at a time in any sequence, or the odd-numbered elements could be rotated while the even-numbered ones were held stationary, and vice versa.
This work was done by Richard T. Howard of Marshall Space Flight Center.