It is shown that the temperature-dependence of the luminescent efficiency of numerous phosphors may be used for the observation, measurement, and recording of temperatures and temperature distributions. Remarkably high temperature-sensitivities can be obtained by exciting the phosphors under such conditions that their efficiency is far below the optimal efficiency obtainable with the same phosphors. Thus, they are used under conditions which are most unfavorable for any other practical application and for most investigations. In particular, the strong temperature-dependence of superlinear phosphors, and the variation of this dependence with the exciting intensity, have been used for the development of thermometric and thermographic methods.
Two methods are described: In the first, the surface to be observed is coated with a luminescent material and evenly illuminated with ultraviolet radiation. The temperature distribution on the surface of the object under test becomes visible as a pattern of brightness or color of the luminescent coating. This pattern can be observed visually, measured by visual or photoelectric photometry, or recorded photographically. The result may be displayed in various ways. Isotherm charts can be obtained easily by purely photographic means. In the second method, a temperature-sensitive phosphor screen is placed in the focus of a suitable optical system. An image of the distribution of the thermal radiation from the object becomes visible on the screen as a brightness pattern which is easily recorded photographically and on which measurements can be carried out. While the second method does not require any coating of the object, the first is by far the more sensitive. A number of pictures illustrating the two methods and a discussion of their performance are given.
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