Image Converter for Thermal Radiation*

The development of the phothermionic image converter is described, a device which reproduces scenes by virtue of differences in their thermal radiation. The method uses the principle of projecting a temperature image on a free multilayer film about 0.05 μ thick which contains a photosurface with thermally sensitive yield. Image conversion is achieved by probing the temperature distribution with a flying light spot and displaying synchronously the thermally modulated photocurrent on a monitor. Section 1 discusses the limits of temperature imaging. Under idealized shielding conditions, the relative temperature distributions in the image assume a very simple and general relationship to that of the object, e.g., are equal in radiation equilibrium. Practical heat exchange processes will present other signal limitations which, in combination with the random noise inherent in any conversion process, determine the system performance. Section 2 deals with the experimental work of searching for, and improving on, the phenomena on which the operation of the image converter is based. This includes theory and experiments on temperature sensitive photoelectric yields and on the electrical and optical properties of thin films. Certain components such as sealed crystal windows, cooled radiation shields, and free thin films had to be developed for the image tube. Several models were built and accommodated by suitable optical and electronic systems. When completed, the device had television bandwidth and motion portrayal capabilities and the ability to detect, or image, temperature differences of 10° to 50°C.