Energy Considerations 



The least amount of radiation available for 

 detection is from the ocean at a water tempera- 

 ture of -2°C (271°K) . It is further required to 

 detect a 0.2 C change at this temperature. The 

 change in radiation emission for a small change 

 in temperature can be found from the differen- 

 tiated form of Stefan-Boltzmann Law 



AW = liaeTS^T 



(1) 



Thus at T = 271°K, ^T = 0.2°C or 0.2°K, 

 Aw = 8-5 x 10"5 w/cm^ (total into a hemisphere). 



Experience with detectors in previous appli- 

 cations indicates that it is quite practical to 

 detect radiation differentials of this order. 

 In fact, in the spectral region of interest 

 (7-5 to 12.5 microns) a thermistor bolometer has 

 been found to be quite capable of detecting con- 

 siderably smaller radiation differentials than 

 that calculated . 



DESCRIPTION OF AIRBORNE RADIATION THERMOMETER 



General 



The airborne radiation thermometer equipment 

 consists of 3 basic parts: a radiometer optical 

 head, an electronic processing system and an 

 indicating-recording system. The radiometer 

 optical head collects radiation from the sea sur- 

 face and generates an electrical signal propor- 

 tional to the difference between this radiation 

 and the radiation from a precisely controlled 

 internal black body reference source. This sig- 

 nal is processed by the electronic circuits to 

 produce a precise DC signal. This output is 

 linearized and broken up into k overlapping 

 temperature ranges, each spanning a 10 C interval 

 with 1°C overlap. Finally the measured signal 

 output is monitored on a panel meter while a 

 continuously operating strip chart recorder simul- 

 taneously produces an accurate and permanent 

 record of the sea surface temperature data. 



As shown in Fig. 3 the airborne radiation 

 thermometer is integrated into a single cabinet. 

 Controls, panel meters and the recorder are 

 arranged on the sloping control panel of the 

 cabinet for maximum operating convenience. The 

 radiation collecting and detecting unit is 

 located at the base of the cabinet behind the 

 forward-bottom access plate and faces out through 

 an opening in the bottom of the cabinet . Its 

 line of sight is directed vertically downward 

 and a shutter mechanism in the base of the cabi- 

 net covers the entrance aperture when the radi- 

 ometer is not in use. 



Printed circuit and component boards, 

 mounted primarily at the rear of the cabinet, 

 contain the electronic circuitry. Access plates 

 are provided and some panels are hinged so they 



Fig. 3- Model ll+- 320 airborne radiation 

 thermometer . 



may be serviced on either side without removing 

 them from the cabinet. When the unit is in 

 operation these panels are secured into position 

 by spring lock fasteners. 



Special provisions have been made to keep a 

 record of the selected temperature range while 

 the instrument is in use and a marking system 

 is incorporated into the recorder to mark the 

 range setting of the instrument in code along 

 the edge of the strip chart. 



Radiometer Optical Head 



A layout of the optical system is shown in 

 Fig. k. Radiation enters the system through a 

 germanium doublet which focuses the energy onto 

 a thermistor detector. The detector is mounted 

 at the apex of a temperature controlled black 

 body cavity operating at 50 C and assumes the 

 temperature of the black body. A chopper blade 

 and mask, each consisting of two opposed 90° 

 sectors, are placed at the front end of the 

 cavity. The chopper blade rotates, and as it 

 rotates it either completely blocks the incoming 

 radiation by closing the 90° sector openings in 

 the mask or allows entering radiation to fall on 

 the detector when the chopper blade sectors are 

 aligned with the mask sectors. The inner surface 

 of the chopper blade is gold plated and highly 



63 



