In Fabry-Perot interferometry a large fraction of light can be reflected by the etalon; thus, it is lost for spectroscopic purposes. The present article shows the possibility of recovering this light, collecting it by means of reflectors and sending it back to the etalon under a different angle. Some devices are examined and some experimental proofs of the device are reported. The gain achieved is of the order of five.
U. Ascoli-Bartoli, G. Benedetti-Michelangeli, and F. DeMarco, "An Improvement in Fabry-Perot Spectrometry," Appl. Opt. 6, 467-470 (1967)