CHOPPER MOTOR 



PHOTOTRANSISTOR 

 REFERENCE (PRP) 



REFLECTIVE OPTICAL 

 CHOPPER BLADE- 



□ 



LIGHT 



SOURCE 



CHOPPER 

 BLADE 



PREAMPLIFIER 



CHOPPER BLADE SHAPE 



Fig. k. Optical system layout. 



reflecting in the infrared. When the cavity is 

 closed to incoming radiation the detector 

 receives the cavity black body radiation as 

 reflected by the gold plated surfaces. Thus, 

 as the blade rotates, the detector alternately 

 receives the target and the black body radiation. 



thermometers . The control circuit for the cavity 

 is designed to maintain its temperature at 50°C 

 +0.02°C. 



Considerable care has been taken with radi- 

 ometer design to provide a thermally-stabilized 

 optical structure. This has been accomplished by 

 placing the detector and signal chopping elements 

 within an enclosure consisting of the cavity and 

 germanium lenses in a compact in-line thermal 

 structure, thereby achieving, as far as possible, 

 an isothermal entity. This is done to minimize 

 the effects on the system of thermal variations 

 due to changes in lens emission and chopper mask 

 emission as well as changing lens transparency. 

 Changes in detector responsivity also occur with 

 varying detector temperature. 



A point of particular interest is that in 

 order to achieve the in-line structure the chopper 

 blade drive shaft actually passes through the 

 germanium lenses through an on-axis hole ground 

 in each lens . 



Electronic Processing System 



Details of the electronic processing system 

 are shown on the right hand half of Fig. 5- The 

 output of the preamplifier is a 20 cps signal 

 composed of the thermistor generated signal and 

 an out-of -phase offsetting signal. This com- 

 posite signal is fed to a synchronous rectifier 

 where it is converted to a DC signal. Synchron- 

 ous rectification, as used here, provides the 

 equivalent of a very narrow band-pass circuit; 

 hence, it affords a great reduction in noise 

 signal while operating at low signal levels. 



The detector consists of a thermistor bridge 

 network which produces an output signal propor- 

 tional to the difference between the incoming 

 radiation and reference black body radiation. 

 The radiation is chopped at a rate producing a 

 20 cps signal which enters a high gain, highly 

 stable hybrid preamplifier which amplifies the 

 signal approximately 2,000 times. 



A second chopper blade, external to the opti- 

 cal system, is driven by the chopper blade motor 

 through a timing belt to provide a square wave 

 signal in phase with the detector signal. The 

 signal is generated as the second chopper blade 

 interrupts a light beam directed on a photo- 

 transistor. The reference generator is called 

 a "phototransistor reference pickup" (PRP) . 

 The PRP signal is used later on in the electronic 

 circuits for synchronous rectification of the 

 signal generated by the thermistor detector. An 

 electronic temperature offsetting signal (E.T.O.) 

 for zero suppression in range selection is also 

 derived from the reference signal. 



The black body reference cavity is of the 

 conical type and its temperature is precisely 

 controlled and monitored with thermistor beads 

 embedded in the cavity and used as resistance 



As mentioned earlier, the signal used for both 

 offsetting and as a reference for the synchronous 

 rectifier is obtained from a second chopper in 

 the radiometer structure. Linearizing and range 

 determination occur automatically during range 

 selection by simultaneously switching in the 

 proper amount of E.T.O. and appropriately 

 changing the system gain. The resultant output 

 signal is fed to the panel meter through a meter 

 amplifier and to the strip chart recorder. 



Since black body radiation normally varies as 

 the fourth power of its temperature, linearizing 

 is necessary if the total temperature range is to 

 be presented on a linear scale. The linearizing, 

 and range determination and selection, are accom- 

 plished in the following way. Referring to 

 Fig. 6 the output of the synchronous rectifier is 

 fed to an adjustable attenuator controlled by the 

 range selector switch. A second deck on the 

 range switch receives the E.T.O. signal and feeds 

 a selected portion of it to the preamplifier. 

 The E.T.O. signal reduces the output signal to 

 zero at the beginning of each range (-2, +7, +16, 

 +25) and each section of the gain attenuator 

 reduces the slope of the output function from 

 that of the fourth power curve to that of the 

 linearized curve, providing a signal to the 



6^ 



