5.1 The significant features are typified by a discussion of the limitations of the technique. With the description and arrangement given in the following portions of this test method, the instrument will record directly the normal spectral emittance of a specimen. However, the following conditions must be met within acceptable tolerance:
5.1.1 The effective temperatures of the specimen and blackbody must be within 1 K of each other. Practical limitations arise, however, because the temperature uniformities are often not better than a few degrees Kelvin.
5.1.2 The optical path length in the two beams must be equal, or the instrument should operate in a nonabsorbing atmosphere or a vacuum, in order to eliminate the effects of differential atmospheric absorption in the two beams. Measurements in air are in many cases important, and will not necessarily give the same results as in a vacuum, thus the equality of the optical paths for dual beam instruments becomes very critical.
5.1.3 Front-surface mirror optics must be used throughout, except for the prism in prism monochromators and the grating in grating monochromators, and it should be emphasized that equivalent optical elements must be used in the two beams in order to reduce and balance attenuation of the beams by absorption in the optical elements. It is recommended that optical surfaces be free of SiO2 and SiO coatings; MgF2 may be used to stabilize mirror surfaces for extended periods of time. The optical characteristics of these coatings are critical, but can be relaxed if all optical paths are fixed during measurements or the incident angles are not changed between modes of operation (during “08201;% line,” “1008201;% line,” and sample measurements). It is recommended that all optical elements be adequately filled with energy.
5.1.4 The source and field apertures of the two beams must be equal in order to ensure that radiant flux in the two beams compared by the apparatus will pertain to equal areas of the sources and equal solid angles of emission. In some cases it may be desirable to define the solid angle of the source and sample when comparing alternative measurement techniques.
5.1.5 The response of the detector-amplifier system must vary linearly with the incident radiant flux.
1.1 This test method describes a highly accurate technique for measuring the normal spectral emittance of electrically conducting materials or materials with electrically conducting substrates, in the temperature range from 600 to 1400 K, and at wavelengths from 1 to 35 μm.
1.2 The test method requires expensive equipment and rather elaborate precautions, but pr......
Copyright ©2024 All Rights Reserved