R Fiber Optics Amplifier FIGURE 89.3(a)Light pen.( Courtesy of FTG Data Systems. )(b) Light pen schematic( Source: After S. Sherr, Electronic Displays, New York: John Wiley Sons, 1979, P. 388. With permission. somewhat difficult to use with raster systems. This device goes by a misleading name, as it does not emit light and is not a pen other than being somewhat similar to one in its physical appearance, as shown in Fig. 89.3(a) However, when we consider its functional characteristics, the validity of the term becomes apparent, as it is used to cause the electron beam to "write "patterns on the cathode ray tube( Crt) that are defined by the motion of the light pen on the CRT faceplate The light pen operates by sensing the existence or nonexistence of a pulse of light at the point on the screen f the CRT or surface of any other light-emitting device where the point of the pen is placed. This is accomplished by means of the circuit shown in Fig. 89.3(b), where the light pulse is collected and transmitted through the fiber optics to a light-sensitive device that converts the light pulse into an electrical pulse which is shaped by some form of electronics(of which a Schmitt trigger is one example). We need not concern ourselves with the exact form of the electronics except to note that this pulse is then sent to the computer, as shown in Fig. 89.4 and provides a complete, closed-loop system. As the electronic pulse occurs at the time when the light pulse passes under the light pen, the computer is informed of the location at which the designated operation is to be performed and may proceed accordingly. Thus, the light pen is a pointing device that designates a point on the display screen and can be used as an input device. Various light pen programs have been written to expand the capabilities of the original one, and it should be noted that the light pen is coming back into favor improvements in accuracy, ease of operation, and reliability occur There are two characteristics of light pen operation that affect the capabilities of this input device. The first is the sensitivity, given by e 2000 by CRC Press LLC© 2000 by CRC Press LLC somewhat difficult to use with raster systems. This device goes by a misleading name, as it does not emit light and is not a pen other than being somewhat similar to one in its physical appearance, as shown in Fig. 89.3(a). However, when we consider its functional characteristics, the validity of the term becomes apparent, as it is used to cause the electron beam to “write” patterns on the cathode ray tube (CRT) that are defined by the motion of the light pen on the CRT faceplate. The light pen operates by sensing the existence or nonexistence of a pulse of light at the point on the screen of the CRT or surface of any other light-emitting device where the point of the pen is placed. This is accomplished by means of the circuit shown in Fig. 89.3(b), where the light pulse is collected and transmitted through the fiber optics to a light-sensitive device that converts the light pulse into an electrical pulse which is shaped by some form of electronics (of which a Schmitt trigger is one example). We need not concern ourselves with the exact form of the electronics except to note that this pulse is then sent to the computer, as shown in Fig. 89.4, and provides a complete, closed-loop system. As the electronic pulse occurs at the time when the light pulse passes under the light pen, the computer is informed of the location at which the designated operation is to be performed and may proceed accordingly. Thus, the light pen is a pointing device that designates a point on the display screen and can be used as an input device. Various light pen programs have been written to expand the capabilities of the original one, and it should be noted that the light pen is coming back into favor as improvements in accuracy, ease of operation, and reliability occur. There are two characteristics of light pen operation that affect the capabilities of this input device. The first is the sensitivity, given by FIGURE 89.3 (a) Light pen. (Courtesy of FTG Data Systems.) (b) Light pen schematic. (Source: After S. Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p. 388. With permission.)