Measurement of the penetration depth λ(T) as a function of temperature using millimeter wave transmission in the range 130-500GHz are reported for three YBa<Subscript>2</Subscript> (YBCO) laser ablated thin films. Two films, deposited on a LaAlO<Subscript>3</Subscript> substrate (T <Subscript> c </Subscript>=90.2 K), exhibit a narrow resistive transition (0.3 K). One has been subsequently irradiated with He<Superscript>+</Superscript> ions in order to increase the scattering rate of the quasi-particles (T <Subscript> c </Subscript>=87.8 K). The third film, grown on MgO (T <Subscript> c </Subscript>=88.5 K), exhibits also a fairly narrow transition (0.8 K) and a high crystalline quality. The experiment provides the absolute value λ(T≤ 30K) for the penetration depth at low temperature: the derivation from the transmission data and the experimental uncertainty are discussed. We find a zero temperature penetration depth λ<Subscript>0</Subscript>=1990 ± 200 Å, 2180 ± 200 Å and 2180 ± 200Å, for YBCO-500 Å/LaAlO<Subscript>3</Subscript> (pristine), YBCO-1300 Å/MgO and YBCO-500 Å/LaAlO<Subscript>3</Subscript> (irradiated) respectively. λ(T≤ 30 K) exhibits a different behavior for the three films. In the pristine sample, λ(T ≤ 30 K) shows a clear temperature and frequency dependence, namely the temperature dependence is consistent with a linear variation, whose slope decreases with frequency: this is considered as an evidence for the scattering rate being of the order of the measuring frequency. A two fluids analysis yields 1/τ (T ≤ 30 K) ~ 1.7 × 10<Superscript>12</Superscript> s<Superscript>-1</Superscript>. In the two other samples, λ (T ≤ 30 K) does not display any frequency dependence, suggesting a significantly larger scattering rate. The temperature dependence is different in these latter samples. It is consistent with a linear variation for the YBCO/MgO sample, not for the YBCO/LaAlO<Subscript>3</Subscript> irradiated one, which exhibits a T<Superscript>2</Superscript> dependence up to 40 K. We have compared our data to the predictions of the <Emphasis Type="Bold">d-wave model incorporating resonant scattering and we do not find a satisfactory agreement. However, the large value of λ<Subscript>0</Subscript> in the pristine sample is a puzzle and sheds some doubt on a straightforward comparison with the theory of data from thin films, if considered as dirty <Emphasis Type="Bold">d-wave superconductors. Copyright EDP Sciences, Springer-Verlag 1998
