The development of a thermal imaging device which operates on the principle of differential evaporation (or condensation) of oil on a thin membrane is described. Section I,A summarizes the requirements of any thermal imaging method and develops the theory as applied to the particular case of the Evaporograph, emphasizing the consideration of scene temperatures near 20°C. It is shown that the greatest component of irradiance at the membrane is utilized in the evaporation of the oil layer. A presentation system which forms a visible image based on the phenomenon of light interference to detect differences in the thickness of the oil film is described. Theoretical calculations based on this system, assuming a reasonable minimum detectable thickness difference, indicate that a temperature difference of 1°C in the scene can be detected with an f = 2 optical system. Experimental results confirm this. The application of the Evaporograph to quantitative measurements is indicated. Section I,B describes experimental work on the three components of the Evaporograph, the infrared optical system, the transducer or cell, and the visual optical system. The two commercial models which have evolved from this work are described. These Evaporographs can detect a temperature difference of 1°C from a 20°C background and have a resolution of 10 lines per mm. Various applications are pictured and described.

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