Photometer for detection of sodium day airglow.

Daytime, ground based measurements of sodium airglow emission are particularly difficult to perform. The ratio of airglow signal to Rayleigh and Mie scattering by the atmosphere as well as the very pronounced Fraunhofer structure lead to extraneous signals that are difficult to evaluate and require specialized techniques to eliminate. The photometer described can be characterized by the following principal features: (1) a narrow (4.5-A) interference filter for initial discrimination; (2) cooled photomultiplier detector to reduce noise from dark current fluctuations and chopping to eliminate the average dark current; (3) a sodium vapor resonance cell to provide an effective bandpass comparable to the Doppler line width; (4) separate detection of all light transmitted by the interference filter to evaluate the Rayleigh and Mie components within the Doppler width of the resonance cell; (5) temperature quenching of the resonance cell to evaluate and account for instrumental imperfections.