3.7. Light Pens.   

Input device shaped like pen so can fit in users hand. Used to point at graphics and to locate positions within display area. Most light pens track by means of moving a cross hair cursor on screen. (In fact, cross hair follows light pen.) Uses light detector in end of pen. 


     
A light pen

    
                     Fig. 3.10 : A light pen



 When beam of electrons strikes phosphor, pulse of light is emitted

which the photodiode detects. Since display processor knows what

region it just illuminated (because of hardware timers) it can determine

x, y coords of light pen hit.

Cross hair is moved to that position.  This is done repetitively.

The smaller the opening of the light pen, the smaller the field of view, therefore higher resolution detectable.

Switch can be used to enable the cross hair or to accept/select current position.

Light pen can be used as a PICK device on a vector display since it "knows" which line segment just drawn caused a light pen hit. If it knows which line segment, it knows which object was just being drawn and hence can tell host computer what the objects name or number is: i.e. a little man or a duck or a transistor caused the light pen hit. The object has been PICKed.