The precision, accuracy and wide range of usefulness of the disappearing-filament optical pyrometer depend on perfect disappearance of the filament with high resolving power and magnification by the eyepiece. Older forms have not accomplished this. Nondisappearance of the filament is caused by diffraction, and if the filament is a wire, by reflection at its edges. Perfect disappearance of a flat filament with sharp edges is obtained by using an entrance aperture sufficiently large compared to the exit aperture. A round filament is made to disappear by decreasing the entrance aperture until diffraction and reflection nearly balance, in their effects at the filament.
An extensive study of diffraction revealed that diffraction by an obstacle in an image plane is a special case which has not been previously recognized as such. It is shown that a new law of diffraction is consistent with theory and experiment, viz., Of light incident on an obstacle in an image plane, only that component is diffracted which represents the diffracted rays radiating from the edge of the aperture. Thus with the flat filament there is no visible effect of diffraction if the aperture of the observing eyepiece (exit aperture) is far outside the shadow of the entrance aperture.
Precise measurement shows that when the filament is made properly to disappear, the ratio of brightness of filament and image when an apparent match is obtained, is unity.
A laboratory form of the pyrometer is described, which has been used with success as a tele-pyrometer and as a micropyrometer. Two special improved forms of micropyrometer, or microphotometer are described in principle.
© 1923 Optical Society of AmericaFull Article | PDF Article
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