  In $\delta$ Scuti star models, the photometric amplitudes and phases exhibit
a strong dependence on convection, which enters through the complex parameter,
$f$, which describes the bolometric flux variation. We present a new method of
extracting simultaneously $\ell$ and $f$ from multi-colour data and apply it to
several $\delta$ Scuti stars. The inferred values of $f$ are sufficiently
accurate to yield an useful constraint on models of stellar convection.
  In addition, if the identified mode is radial, the multi-passband data may be
used to refine global stellar parameters. Finally, the usage of radial velocity
measurements to our method improves significantly determination of $\ell$ and
$f$.
