We have described an algorithm to classify stars into photometric regions based on Strömgren indices. The algorithm enlarges that described in Figueras et al. (1991) and covers as much of the HR diagram as possible, including the supergiant stars. The application of the algorithm to a large sample of stars shows its validity: about 90% of "normal'' single stars are photometrically classified in agreement with their spectral type and luminosity class; for peculiar, emission line or binary stars this percentage is slightly lower.
The calibrations used to calculate intrinsic colours and absolute magnitudes
in each photometric region have been presented and discussed. To study the
coherence of the calibrations we applied them to stars
from open clusters. Some differences were found between
the mean values for each photometric region. In the case of the absolute
magnitudes there was a discrepancy between the results obtained
for the early and intermediate regions
and those for the late region.
We can conclude that there is a good agreement among currently
available calibrations for
E(b-y), but not for those of 
. This would suggest the
need for a correction in the
calibration for early and intermediate regions.
The agreement between observational and theoretical ZAMS has also been studied.
It was found to be satisfactory within the uncertainty of the determinations in
and log g, for B and A type stars.
However, for F-type stars there is an evident discrepancy between
observational and theoretical ZAMS.
The availability of new data, especially accurate trigonometric parallaxes from Hipparcos satellite, and the revision of stellar atmosphere models, should help to improve the calibrations and thus avoid existing discrepancies.
We would like to thank J.C. Mermilliod for kindly suppling us with his Open Cluster Data Base. This work was supported by the CICYT under contract ESP95-0180. E.M. acknowledges financial support from the Ministerio de Educación y Ciencia.