This paper discusses a novel method for including a 100 Å bandpass filter in an f/l optical system. The system is a gimbaled optical system comprising a video tracker and a laser spot tracker operating at f/l. Conventional spectral beam splitting of these two trackers would be done with a dichroic mirror, resulting in the laser narrow bandpass filter being placed in the f/l converging beam. Because of angle shift characteristics of inter-ference filters, this would require a minimum half power bandwidth of no less than 350 Å. The author shows that a novel filter which is both a bandpass at the laser wavelength and a broad band transmitter at the vidicon response wavelengths can be placed in the entrance pupil of the system. This reduces the incidence angles to a minimum and allows half power bandwidths of less than 100 Å.
"How To Realize A 100 Å Bandpass Filter In An F/1 Optical System", Proc. SPIE 0050, Optical Coatings: Applications and Utilization I, (1 March 1974); doi: 10.1117/12.954114; https://doi.org/10.1117/12.954114