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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />,. <br /> <br />- 27 - <br /> <br />I <br /> <br />crossed polaroids set for maximum extinction through which a narrow beam of <br /> <br />light was passed. If an ice crystal passed between the polaroids, it caused <br /> <br />a slight rotation of the plane of polarization of the light beam emerging <br /> <br />from the first polaroid so that there was a momentary increase in the amount <br /> <br />of light passing through the second polaroid. The increases in light <br />intensity were detected by a photomultiplier tube. Water droplets are not <br />detected because they do not rotate the plane of polarization. However, <br />airborne testing of this device showed that the filament light source which <br /> <br />was used was of insufficient intensity compared to the high ambient lights. <br /> <br />Moreover, due to vibrations, the intensity of the light source was not steady. <br /> <br />In the fall of 1970 further developmental work on this instrument was <br /> <br />started. by Mr. F. Turner of our research group. The light source was changed <br /> <br />to a mercury high pressure arc lamp which was physically very rigid. This <br /> <br />eliminated vibrationally induced noise but inherent arc noise was still <br /> <br />considered excessive. To eliminate this problem the mode of operation was <br /> <br />changed from one of light transmission to forward scattering (Fig. 1.9). <br /> <br />Tests in a cold room showed that this instrument was capable of detecting <br /> <br />stellar ice crystals and hexagonal ice plates greater than 100 ~m in diameter <br /> <br />but that it was completely insensitive to even large quantities of liquid <br /> <br />water. Wind tunnel tests showed that the velocity of air through the <br /> <br />instrument was about 16% less than that in the face airstream. To compensate <br /> <br />for this a venturi was added which raised the airflow through the instrument <br /> <br />to that in the free airstream. At an aircraft speed of 70 m sec-l the <br /> <br />instrument views about 3 liter sec-l of air. <br /> <br />A system of electronics provides control functions and two separate <br /> <br />