In the early development of data display systems, it was customary to employ a cathode ray tube wherein a layer of phosphor material was made to luminesce by means of an electron beam that scanned across the layer of phosphor material. Although good quality images can be created in this manner, the physical size of the images that can be created by a cathode ray tube is severly limited by various factors, such as, for example, power required and distortion of the electron beam path. In order to provide images of greater size, numerous schemes have been proposed for optically enlarging an image created by cathode ray tube. Although an image of greater proportions can be obtained in this manner, the amount of light available from the phosphors present on the face of the tube is very limited. As a result, the quality of the enlarged image and particularly the brightness thereof has heretofore been very poor.
Cathode ray tubes have also been utilized in systems which employ a projection system having a light source which is independent of the cathode ray tube light emission. For example, in U.S. Pat. Nos. 3,667,830; 3,701,586; 3,609,222; and 3,746,785, large scale displays are provided by utilizing a display structure having a deformable, light reflective metalic film supported by a support grid which is situated within a cathode ray tube. When an electron beam scans across the display structure, charge accumulates on areas of the display structure in accordance with the information content of the electron beam. This charge accumulation causes small deformations or dimples to form in the metal film at the areas of the charge accumulation. When light from a projection system is directed upon the metal film, only light which strikes the deformed areas reaches a display screen. Thus, a light image is formed on the display screen corresponding to the dimpled image formed in the metal film by the electron beam. A flood gun must be provided within the cathode ray tube to neutralize the charge areas to thereby allow the deformations in the metal film to relax to the non-deformed or normal position.
The size limitations of display systems using cathode ray tubes has led to the use of matrix addressed displays when large displays are required. In a matrix display, pairs of conductors in a two-dimensional array of such conductors are utilized to address each elemental area of the display to thereby initiate the emission of light at a selected elemental area when the pair of conductors associated with that selected elemental area are properly biased. in such a display system, as described in U.S. Pat. No. 3,091,876, when a selected elemental area is properly biased, a pressure valve is opened which forces a portion of a flexible membrane out past the end of a tubular support member. In this outer position, the reflective surface of the membrane reflects incident light to provide a visible "bright spot" in the surface of the display to thereby provide a visible display. U.S. Pat. No. 3,091,876 also teaches using an electroluminescent panel in conjunction with the flexible membrane whereby when an elemental portion of the membrane is addressed, the membrane is forced by a pressure system into contact with an electrode of the electroluminescent panel whereby a voltage is applied across an elemental area of the electroluminescent panel to thereby cause it to emit visible light.
As noted, the display systems described which utilize cathode ray tubes suffer from size limitations and the expensive cathode ray tube component. Also, these systems do not use ambient light as the projection light source. The matrix display systems that utilize a pressure source also suffer from the requirement of expensive mechanical components and also, especially when using an electroluminescent panel, from the lack of a threshold behavior since the elemental areas adjacent a selected elemental area receive half the voltage applied across the selected elemental area and that voltage may be sufficient to initiate undesirable glow discharges and hence undesirable light output at areas adjacent the selected elemental area. Also, many of the display systems described do not have machine readable capabilities.