Strömgren 
photometry, coupled with Kurucz (1979, 1991) model
atmospheres, provides a powerful technique for the determination of
in B, A and F stars. Napiwotzki et al. (1993) have provided a
discussion of the various calibrations developed over the past decade, with
the reddening free Strömgren colour-indices
[
] and [u-b] widely employed. Fig. 1 demonstrates
tghe temperature sensitivity of the Balmer jump for B stars based on
Kurucz model atmospheres, and includes the Strömgren
filter bandpasses. Temperatures of hot stars are
poorly constrained using the Johnson reddening free index
(e.g. Moehler et al. 1990).
Photometric calibrations applied to supergiants and
stars showing non-solar He-contents can lead to significant
discrepancies since their colours typically lie either at the edge of, or
beyond current calibrations and the strength of their
Balmer jumps are affected
by composition effects (Kudritzki et al. 1989). For example,
Venn (1995) obtained 
=9.7kK
from a detailed spectral analysis of HD46300 (A0Ib) using MgI-II
lines, which compares with 8.4kK from
the 
calibration of Napiwotzki et al. (1993), and 10.0kK from
the 
versus [
] relation of Lester et al. (1986).
Saffer et al. (1994) have discussed the systematic errors obtained using
standard photometric calibrations (e.g. Lester et al. 1986) for
temperatures of (He-poor) sdB stars.
Figure 2: The position of hot stars in effective temperature and gravity
discussed in this review. The Eddington limit
for radiative stability is
indicated (thin solid), as are the LTE and non-LTE domains (thick solid)
and OB dwarf/supergiant (dashed/dotted) scales