Intensity decode (op=需_]一“mh FIGURE 89.4 Block diagram of light pen computer system. Source: S Sherr, Electronic Displays, New York: John wiley Sons, 1979, P. 389. with permission pAAm具 where E,= illuminance at photodetector, H,= photodetector sensitivity, A, preamplifier gain, A= main amplifier gain, u,- Schmitt trigger sensitivity, u,=flip-flop sensitivity, and ',=optical loss Equation(89.1)may be used to calculate the light output required from the display surface, which may be a CRt or other light-emitting device, but with the limitation that most of the flat panel units are matrix driven and must track the drive sequence in order to know the location of the light pen from the drive pulse timing When phosphors are involved as for the CRT, vacuum fluorescent displays(VFDs), thin-film electroluminescent (TFEL)units, and color liquid crystal displays(LCDs), the phosphor delays must be entered into the timing, and the total delay is given by E。=E( t/t) (892) where E.= voltage at triggering element, E: voltage equivalent of phosphor light output, t= time, and t of all delay These delays set limits to the positional accuracy, as the computer tracking the signal will be in error by this amount. TABLE 89.3 Light Pen Data Other inaccuracies are due to the dimensions of the optical Field of View Response Time sensitivity pickup surface, all of which somewhat negate the simplicity of operation. The result is the parameter values shown in Table 89.3 Data Tablet(Graphics, Digitizer) A very convenient means for data entry, retaining some of the ease of operation of the light pen but with much better accuracy, are the various forms of data tablets available. These tablets differ from the light pen in another ignificant way in that they do not require a moving spot of light to detect the location of the beam or direct it to a new location. This need for a moving light spot made the light pen difficult to use with the data tablets itially designed to overcome this limitation while still using a device with a pen-like input. The first successful example was the Rand tablet, a digital device that used an X-Y assembly from which a wand placed above some point on the X-y wire matrix could pick up pulse generator output that fed X and Y electrical pulses into the matrix By determining the number of pulses in a time period, the location of the wand is established. Another similar device used magnetostrictive rather than electrical signals to accomplish the same result, and this location is converted into display coordinates used to position a cursor on the CRT screen. The cursor may then be e 2000 by CRC Press LLC© 2000 by CRC Press LLC S = ELmpApAmmsmf tL (89.1) where EL = illuminance at photodetector, mp = photodetector sensitivity, Ap = preamplifier gain, Am = main amplifier gain, ms = Schmitt trigger sensitivity, mf = flip-flop sensitivity, and tL = optical loss. Equation (89.1) may be used to calculate the light output required from the display surface, which may be a CRT or other light-emitting device, but with the limitation that most of the flat panel units are matrix driven and must track the drive sequence in order to know the location of the light pen from the drive pulse timing. When phosphors are involved as for the CRT, vacuum fluorescent displays (VFDs), thin-film electroluminescent (TFEL) units, and color liquid crystal displays (LCDs), the phosphor delays must be entered into the timing, and the total delay is given by Eo = Ei (1 – e – t/t) (89.2) where Eo = voltage at triggering element, Ei = voltage equivalent of phosphor light output, t = time, and t = sum of all delays. These delays set limits to the positional accuracy, as the computer tracking the signal will be in error by this amount. Other inaccuracies are due to the dimensions of the optical pickup surface, all of which somewhat negate the simplicity of operation. The result is the parameter values shown in Table 89.3. Data Tablet (Graphics, Digitizer) A very convenient means for data entry, retaining some of the ease of operation of the light pen but with much better accuracy, are the various forms of data tablets available. These tablets differ from the light pen in another significant way in that they do not require a moving spot of light to detect the location of the beam or direct it to a new location. This need for a moving light spot made the light pen difficult to use with the data tablets initially designed to overcome this limitation while still using a device with a pen-like input. The first successful example was the Rand tablet, a digital device that used an X–Y assembly from which a wand placed above some point on the X–Y wire matrix could pick up pulse generator output that fed X and Y electrical pulses into the matrix. By determining the number of pulses in a time period, the location of the wand is established. Another similar device used magnetostrictive rather than electrical signals to accomplish the same result, and this location is converted into display coordinates used to position a cursor on the CRT screen. The cursor may then be FIGURE 89.4 Block diagram of light pen computer system. (Source: S. Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p. 389. With permission.) TABLE 89.3 Light Pen Data Field of View Response Time Sensitivity 0.02–0.08 in. 120–150 ns 0.02–0.04 ft.L