Strömgren photometry is widely used in many fields of research: analysis of open clusters and associations, binary systems, the structure and kinematics of the Galaxy, test of stellar evolutionary models, among others. Although it was primarily designed to study early main sequence stars, it has also been used to investigate late type, metal deficient, supergiant stars, etc. This system has become a key in determining physical parameters such as absolute magnitudes and effective temperatures and enables the age of stars to be determined.
Standard relations among intrinsic colour indices have been established using unreddened and non-evolved stars and stars from open clusters, with reference to a given metallicity. The lower envelope of non-evolved stars in a suitable colour-colour diagram defines an observational ZAMS. Furthermore, the calibrations take in account the evolutionary status and the specific metallicity, and hence reddening and absolute magnitudes can be derived from observed colours. Effective temperatures and surface gravities can be obtained from the intrinsic colours through models of stellar atmosphere and subsequently, the stellar evolutionary models allow the age and mass of the stars to be computed.
However, since different colour indices are related with effective temperature and luminosity depending on the region of the HR diagram, different calibrations have been built for each of these regions.
The aim of the present paper is to analyze the agreement among these
calibrations for main sequence stars by comparing the
mean values of reddening and the distance modulus of open cluster members.
We built a sample containing 24 open clusters with 
photometry
available, covering a wide range of ages, and
the selection of cluster members was made by following photometric criteria.
The calibrations analyzed are those used most extensively at present.
Unfortunately, the lack
of independent photometric data for supergiant stars belonging to open clusters
meant it was impossible to extend
the analysis to the calibrations of supergiants.
Empirical or observational ZAMS are compared with theoretical ZAMS from
stellar evolutionary models.
In addition, this paper deals with the algorithm of star classification into
"photometric'' regions of the HR diagram and the description of the
standard relations used to compute reddenings and absolute magnitudes
in each of the regions.
In a previous paper, Figueras et al. (1991) discussed the
determination of physical parameters for the main sequence A-type stars
while Jordi et al. (1992) and Comerón et al. (1993) used calibrations
to treat main sequence B- and F-type stars and B supergiants, respectively.
Thus, the present paper seeks to extend our previous studies by building
a tool which can determine the stars' physical parameters from
photometry covering as much of the HR diagram as possible.