Light pen
A light pen, also called a light pen or light pen, is a computer pointing device to work directly on a CRT screen. It was developed in 1949 at Lincoln Laboratory ( Massachusetts Institute of Technology). ( Some sources speak of 1955, the first CAD application Sketchpad with light pen was introduced in 1963. )
If you hold the narrow, provided with a photo-transistor end of the rod-shaped device on the screen, then the produced on the phosphor layer upon impact of the electron beam " flash of light " ( perceiving the human eye as a continuous light ) is converted into an electrical signal. Applies the electron beam on the other hand at a position other than that of the light pen on the phototransistor remain inactive. Based on the timing of the signal triggered by the phototransistor, the start time and the velocity of the electron beam across the screen performed finally the position of the light pen can be calculated by the computer; the position determination is therefore indirectly. In black or very dark parts of the light pen can not work due to lack of light pulse.
Professional use experienced light pen, among others in control stations as of air traffic control and data centers, which is still occasionally seen today in movies and documentaries.
A certain degree of popularity gained the light pen in the early 1980s ( in particular by using a universal pointer device at Thomson -home computers such as the TO7 -70 and as a kit for the C -64), but was of a few years later by the much more ergonomic and production displaces more favorable computer mouse.
On modern screens (LCD, plasma, 100 - Hz - tube ), the light pen is unusable due to lack of time or not assignable light flash.
The light gun used for some computer games ( light gun ) based on the same principle, but also has a focusing optics in order to detect its generated by the electron beam " flash of light " at a greater distance to the screen can. Today's models work partly different, which is also due to the increasing popularity of LCD televisions.