The image dissector provides an output dependent on the distribution of light 

 on the photocathode. For flexibility in design the initial light flux is pro- 

 jected onto a ground glass screen. This screen is the back face of the dis- 

 sector housing window. The resulting light distribution is then imaged onto 

 the dissector photocathode through an interference filter. The dissector is 

 chosen as the scanning instrument principally because of its wide dynamic 

 range. The device uses a photomultiplier amplifier and high gain with low 

 noise is an important characteristic. The output current from the dissector is 



M 

 Id(^) =i%skDpGDAp (\)^ (-1^)^ P(r). 



The photomultipliers collect scattered radiation, integrated over a small solid 

 angle of view and over the beam volume. 



P = 



"^ JonJ% <ri9) dpdn 



and a sufficiently accurage approximation gives 



The output current of the photomultiplier is then 



% = ^n'^^N^N '^(^n) Po • 



The basic instrimient electronics are shown in block form in Fig. 4. Power comes 

 from a battery pack located in the shipboard power supply. An inverter and 

 regulator produce an output of 115V, 400 Hz. The output voltage is regulated to 

 .017o. This power is transmitted to the instrument through the power cable and 

 is used as input to the laser and high voltage power supplies. The high voltage 

 for the photomultiplier and dissector bias is generated in a separate supply. 

 Each component has a stacked zener diode supply for individual regulation. With 

 the regulated input power the final high voltage is sufficient constant that 

 variation in the gains are negligible. 



The signal current from the photomultiplier is a pulse instantaneously propor- 

 tional to the input light pulse. The signal processing circuit integrates this 

 current during the dissector sweep. The capacitance of the signal cable from 

 the instrument and an added variable input capacitor collect the current pulse. 

 An FET source follower and second transistor emitter follower maintain signal 

 voltage while reducing output impedance. At the end of the dissector sweep the 

 capacitor is discharged to zero. Hence, the gradient of the output voltage is 

 proportional to the integrated or average laser power. The dissector output is 

 treated differently. The output pulse is fed into a small capacitor and a 

 source follower used for impedance charge. However, the circuit acts as a box- 

 car maintaining a voltage proportional to the integrated pulse current. 



The dissector output, as mentioned earlier, has a wide dynamic range. Hence, 

 after the final emitter follower output a log amplifier is used to reduce the 

 dynamic range of the signal. All signals were recorded continuously on a 14 

 track Sangamo recorder. Frequency modulation was used to eliminate drop out. 

 The tape was supplied with power directly from the battery pack. 



VII-36 



