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This paper treats in detail the problem of transmitting light from a diffuse source, such as a flame, to a place some distance away for the purpose of making spectroscopic measurements. The particular cases which have been analyzed are: the simple uniform tube, the simple tapered tube, the uniform tube conttaining one lens placed at its focal distance from the ligh-source, the uniform tube with one lens of focal length equal to one-fourth the length of the tube placed at the midpoint of the tube, the uniform tube with a series of lenses spaced at intervals of twice their focal length, and uniform simple tubes with reflecting walls. In each case expressions for the total amount of light transmitted have been derived and the relative strength of the contribution from different zones of the source have been investigated. The greater part of the work is theoretical but most of the conclusions have been tested experimentally.
It is found that a tube with reflecting walls and no lenses gives the greatest amount of transmitted light. This light emerges from the output end of the tube nearly parallel to the axis but is disordered enough that the formation of sharp images is impossible. However, this is usually satisfactory in transmitting light to instruments such as interferometers and spectrographs when sharp images are not necessary.
A tube fitted with lenses spaced two focal lengths apart gives efficient transmission of light as well as formation of sharp images of the source at intervals of four focal lengths along the tube. Thus if it is desired to use light from specific parts of the diffuse source the “2F” lens system should be used even though it transmits less light than the open tubes with reflecting walls.
© 1949 Optical Society of America
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