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Emission line galaxies are an invaluable tool for our understanding of theevolution of galaxies in the Universe. Imaging of deep fields with narrow-bandfilters allows not only the selection of these objects, but also to infer theline flux and the equivalent width of the emission line with some assumptions.The narrow-band filter technique provides homogeneous samples of galaxies insmall comoving volumes in the sky. We present an analysis of the selection ofemission-line galaxies using narrow-band filters. Different methods ofobservation are considered: broad-band -- narrow-band filters and twobroad-band and one narrow-band filters.  We study also the effect of several lines entering simultaneously inside thefilters (this is the case of Halpha). In each case the equations to obtain theequivalent width and line flux from the photometry are obtained. Candidates toemission-line objects are selected by their color excess in a magnitude-colordiagram. For different narrow-band filters, we compute the mean colors of starsand galaxies, showing that, apart from galaxies, some types of stars could beselected with certain filter sets. We show how to compute the standarddeviation of the colors of the objects even in the usual case when there arenot enough objects to determine the standard deviation from the data. Wepresent also helpful equations to compute the narrow-band and the broad-bandexposure times in order to obtain minimum dispersion in the ratio of fluxes ofboth bands with minimum total exposure time.Comment: Accepted for publication in PASP 48 pages, 10 figures Corrected  typos, fixed references. Updated reference to T

A Contribution to the Selection of Emission‐Line Galaxies Using Narrowband Filters in the Optical Airglow Windows