There are many applications where it is desired to provide feedback information for information displayed on a CRT screen. For example, it has become common practice with the use of computers to display on the screen a choice for the user to select from. The user is typically instructed to operate specific keys, on a keyboard or similar device, to select from among a menu of possible choices. In response to the user operating the selected key the menu is changed and the user is given a new choice, again making the choice by operating a particular key. Such an arrangement is tedious since a user must first lock at the screen and then go to a separate keyboard to find the proper key. This is time consuming and requires costly separate equipment.
One possible solution to the problem has been to arrange the menu of choices along a side of the viewing screen and to arrange next to the screen a series of buttons. As the labels on the screen change the buttons become dynamically relabeled. While this solves some of the problems it does not allow for complete flexibility of the visual display and still requires an artificial arrangement of the display.
Several attempts have been made to solve the problem, one such being the use of a light pen which is held over the point on the CRT screen corresponding to the desired response. Light from the CRT raster then enters the pen and the position of the raster is determined by coordinating the signal output from the pen with the position on the raster at the time of the signal. This arrangement, while performing properly, has the disadvantage of requiring the user to hold a pen and to properly direct the pen to the proper place on the screen.
Other touch sensitive screens used crossed wires, crossed beams of infra red light, reflection of acoustic surface waves, current division in resistive sheets, force balancing, or mechanical buttons on which a display image was superimposed by a half silvered mirror. When used with a CRT display, the foregoing methods require careful calibration to establish correspondence between points on the touch screen and poins on the display. The need for special transducers or many electrical connections increase complexity and cost. Also, most of the methods only allow activation of one point at a time.
Thus, it is desired to solve these problems in a manner which allows the visual display to be touched directly at any location on a dynamically changing basis with the position of the touch being easily determinable. These problems must also be solved in a manner which allows for fingers of varying degrees of dampness and for the use of screens in ambient light conditions which vary considerably from place to place.