Arun Narayan Netravali

A new lightpen for raster displays

Abstract We describe a lightpen with a position accuracy that is better than the size of a picture element. This is achieved by computing, at field rate, the centroid of the spatial pattern of light received by the photosensor of the lightpen. A necessary condition for the implementation as described is that the phosphor of the CRT display be shorter in persistence than the horizontal blanking interval. We show that the rightward bias due to persistence depends only on the phosphor time constant and can be allowed for. We also show that random errors as well as bias errors can in total be made less than a quarter of the width of a scanning line.