High altitude chemical vapor trails may not be photographed by day because of the intense background light of the sky. A comparison of several possible emission line detectors is given. An instrument incorporating an ultra-narrow interference filter in a differential radiometer is described and the results of a successful field test are given.
G. T. Best, "Optical Instrumentation for Tracking High Altitude Vapor Releases by Day," Appl. Opt. 9, 2666-2672 (1970)