The light pen consists of a photocell that senses the light radiated from the phosphor on a cathode ray tube (CRT) screen. The light pen reacts every time a pixel on the screen is lit up by the electron ray within the tube. The signal passes from the light pen to the computer, which at the same time receives information on where and when the spot passes different places on the screen. In this way the computer can identify where the pen is on the screen. The light pen can be used for pointing to parts of the screen one wishes to know more about. It can also be used to activate different functions. If, for example, one pointed to a valve and, at the same time, pressed a button on the side of the pen, this may cause the valve to close. The light pen is suitable for moving cursors on a screen. However, it is difficult to see any operational advantages of light pens over other controls.
If a light pen is used over a long period, it is necessary to have a specially designed armrest to prevent discomfort. The light pen has to come close up against the screen, which means that it is impossible to have any form of reflection shield or filter on the screen, and this can give rise to visual problems. Positioning of the light pen must be exact, which makes considerable demands on vision and also contributes to bad working posture in many instances.