Storage oscilloscopes are largely used as output instruments of analog and digital computer. The large diffusion of this type of display is due to its high resolution versus its limited cost. For comparison, a standard CRT display used for graphical application requires either a large special purpose memory or continuous image refreshing by computer; other applications of storage oscilloscopes are the analysis of non periodic events and the monitoring of slow signals. The main disadvantage of such oscilloscopes has been up to now the difficulty of interacting with the image, while for standard CRT display several light pen systems are available for image interaction. The light pen operates by correlating the time phase of the light, as detected by a photodetector, with the period of the image refreshing, as performed by a flying spot of light on the CRT screen. This time phase is strictly related with the position of a pen holding the photodetector optically oriented toward the screen.
"A Simple Light Pen For Storage Oscilloscope", Proc. SPIE 0038, Electro-Optics Principles and Applications, (1 June 1973); doi: 10.1117/12.953758; https://doi.org/10.1117/12.953758