The soft X ray telescope (SXT) on Yohkoh produces calibrated images which a
re used to derive X ray irradiance and coronal temperature. In this paper w
e compute the mean temperature and spectral irradiance for 25 coronal diffe
rential emission measure (DEM) models ranging from very quiet solar conditi
ons to the main phase of large flares. These results are compared to what w
ould have been derived from an SXT observation of the solar conditions repr
esented by the DEM models. After applying an empirical correction algorithm
, derived as part of this work, 24 of 25 SXT filter-ratio temperatures fall
within 0.1 in log10(T) of the mean DEM temperatures. The irradiance error
depends both on the chosen spectral band and the spectrum, here represented
by a single plasma temperature. As expected, the irradiance is more accura
te and less sensitive to spectrum for bands that match the band pass of the
sensor. Even a crude color temperature approximation to the coronal X ray
spectrum greatly improves the accuracy of derived radiance over values deri
ved for an assumed temperature of 3 x 10(6) K: the mean temperature of all
DEM. An RMS improvement of a factor of 12-17 was found for the cases studie
d. For two bands, which match the sensor band pass, the RMS accuracy is bet
ter than 10%. Yohkoh X ray irradiance data, converted with SXT color temper
atures, will be placed in the National Geophysical Data Center for use by t
he solar and atmospheric physics communities.