Resolving Power in the Near Infrared of the Fabry–Perot Interferometer with Gold and with Silver Coatings

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Abstract
In order to determine the resolving power and the luminosity in the near infrared of the Fabry-Perot interferometer with plates coated with evaporated layers of gold and of silver, the reflectance, transmittance, and absorptance of thick layers of these metals, produced by evaporation onto quartz plates, have been measured over the range 0.6 to 2.4 µ. The absorptance of gold layers decreases a little with increasing wavelength, from a value of 0.053 at 1.0 µ, to 0.043 at 2 µ; the absorptance of silver layers has a value of approximately 0.02 over the whole range 1.0 µ to 2.1 µ. Consequently, for a luminosity of 25%, the finesse of the fringes produced by the interferometer with gold coatings is 30 at a wavelength of 1 µ increasing to 35 at 2 µ, while with silver coatings it is approximately 80, over the range 1.0 to 2.0 µ. The Fabry-Perot interferometer with reflecting surfaces either of gold or of silver (preferably the latter) can be used to increase the resolving power of a spectrometer, in this region, without loss of luminosity; or alternately to increase the luminosity without decreasing the resolving power. A description is given of suitable optical systems for these applications of the Fabry-Perot interferometer, and estimates are given of the amount by which the resolving power or the luminosity of the spectrometer can be increased.

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