A fifth filter (fwhm = 90 A) centered on Ca II H and K has been developed for use with the standard uvby system. The filter, called Ca, is designed primarily for applications to metal-poor dwarfs and red giants, regions where the uvby metallicity index, m(l), loses some sensitivity. An index, hk, is defined by replacing v in m(l) by Ca. The effects of interstellar extinction on the index are modeled and demonstrated to be modest and relatively insensitive to spectral type. Observations of V, (b-y), and hk for 163 primary standards are detailed and transformed to the standard V and (b-y) system. A qualitative analysis using only the primary standards indicates that hk is more sensitive than m(l) over the regions of interest by about a factor of 3.
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Anthony-Twarog et al. (1991) studied this question.