In four-color, intermediate-band photometry of late-type stars, difficulty has been experienced in reproducing the Str5mgren system as defined by standard stars published by Crawford and Barnes. The difficulty is believed to be connected with the fact that an international, bulk order for filters has produced a considerably narrower V filter than that used in the original observations of the standard stars. Because of this difficulty a new system has been defined which is characterized by (b - y) = (b - y) Str6mgren, M1 = m1 (Str5mgren) for (b-y) < and C1 (Str6mgren) for (b-y) < 020. The use of M1 as a metal-abundance parameter is demonstrated with Hyades-cluster and group stars and members of the Arcturus group, as well as with some very metal-weak subdwarfs.
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O. J. Eggen (1976) studied this question.