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Browse Prior Art Database

Touch-Input Overlay for CRT or Other Scanning Displays

IP.com Disclosure Number: IPCOM000044628D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Garwin, RL: AUTHOR [+2]

Abstract

A touch-sensitive input device for use with a scanning display, such as a cathode ray tube (CRT), can be assembled using a transparent overlay or plate over the face of the CRT, which overlay has a light collector with frequency shifting capability spaced away and surrounding the periphery with an interruption for photodetection. The front view of the structure is shown in Fig. 1, with the side view shown in Fig. 2. The invention employs a principle that a transparent overlay as a light guide of uniform thickness with smooth faces will guide essentially no light to its edge from a display screen which is spaced so as not to be in optical contact with the light guide.

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Touch-Input Overlay for CRT or Other Scanning Displays

A touch-sensitive input device for use with a scanning display, such as a cathode ray tube (CRT), can be assembled using a transparent overlay or plate over the face of the CRT, which overlay has a light collector with frequency shifting capability spaced away and surrounding the periphery with an interruption for photodetection. The front view of the structure is shown in Fig. 1, with the side view shown in Fig. 2. The invention employs a principle that a transparent overlay as a light guide of uniform thickness with smooth faces will guide essentially no light to its edge from a display screen which is spaced so as not to be in optical contact with the light guide. When an object such as a finger or stylus makes optical contact with the light guide, light passing through the overlay from the display can scatter from the touching object and approximately 1/4 of the light scattered is in the angular range of total internal reflection and will be guided without loss to the edge where it can be detected by photodetectors. In operation, imagine first a uniformly illuminated scanning display, such as a CRT, showing a blank or white screen. Ideally, a finger in the middle of the screen is only very weakly illuminated until the scanning electron beam approaches the region of the finger. Thus, when the output of the photodetector is plotted on a monitor CRT slaved to the display CRT, the monitor screen will be almost dark except for a small region somewhat larger than a finger in the center of the screen. If the CRT has a long-persistence phosphor, such as 500 microseconds or about 7 scan lines, the bright region on the monitor screen will start at approximately the position of the finger, but it will extend all the way to the right margin and then provide a band of somewhat decreasing intensity all across the screen for the height of the finger and 7 or 10 lines beyond. Even for a short- persistence phosphor, every speck of dust or grease will provide a signal as well, although it will not be as strong or as bright on the monitor as the finger touch. Rough-surface anti-glare treatment which is inexpensive and effective on other displays cannot be used. Roughening the surface of the light guide would scatter display light into the guide everywhere, but more importantly would scatter guided light out everywhere. Multilayer non-reflective coatings could be used, however. The effects of scratches and dirt on the overlay can be minimized by digitizing the photosignal and storing in memory, thus maintaining a record of imperfections so that the signal from the object that touches can be compared with the stored data. The photodetector signal is stored vs. time with a time resolution comparable with the size of the finger. Arranging for the display to be unchangi...