Explain the working of light pens

Light pens still exist, but due to several disadvantages they are not as popular as other input devices that have been developed. For example, when a light pen is pointed at the screen, part of the screen image is obscured by the hand and pen. And prolonged use of the light pen can cause arm fatigue. Also, light pens require special implementations for some applications because they cannot detect positions within back areas. To be able to select positions in any screen area with a light pen, we must have some non-zero intensity assigned to each screen pixel. In addition, light pens sometimes give false readings due to background lighting in a room.