We have measured the penetration depth λ(T) on YBa2Cu3O7 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 λ(T). In high quality films λ(T)
exhibits the same linear temperature dependence as single crystals, showing its
intrinsic nature, and λ(0)=1750A˚. In a lower quality one, the
more usual T2 dependence is found, and λ(0)=3600A˚. This
suggests that the T2 variation is of extrinsic origin. Our results put the
d-wave like interpretation in a much better position.Comment: 12 pages, revtex, 4 uuencoded figure