  We have measured the penetration depth $\lambda(T)$ on $\rm
YBa_{2}Cu_{3}O_{7}$ thin films from transmission at 120, 330 and 510~GHz,
between 5 and 50~K. Our data yield simultaneously the absolute value and the
temperature dependence of $\lambda(T)$. In high quality films $\lambda(T)$
exhibits the same linear temperature dependence as single crystals, showing its
intrinsic nature, and $\lambda(0)=1750\,{\rm \AA}$. In a lower quality one, the
more usual $T^2$ dependence is found, and $\lambda(0)=3600\,{\rm \AA}$. This
suggests that the $T^2$ variation is of extrinsic origin. Our results put the
$d$-wave like interpretation in a much better position.
