  The measurement of precise absolute fluxes for stellar sources has been
pursued with increased vigor since the discovery of the dark energy and the
realization that its detailed understanding requires accurate spectral energy
distributions (SEDs) of redshifted Ia supernovae in the rest frame. The flux
distributions of spectrophotometric standard stars were initially derived from
the comparison of stars to laboratory sources of known flux but are now mostly
based on calculated model atmospheres. For example, pure hydrogen white dwarf
(WD) models provide the basis for the HST CALSPEC archive of flux standards.
The basic equations for quantitative spectrophotometry and photometry are
explained in detail. Several historical lab based flux calibrations are
reviewed; and the SEDs of stars in the major on-line astronomical databases are
compared to the CALSPEC reference standard spectrophotometry. There is good
evidence that relative fluxes from the visible to the near-IR wavelength of
~2.5 micron are currently accurate to 1% for the primary reference standards;
and new comparisons with lab flux standards show promise for improving that
precision.
