Infrared reflectivity of an oxygen reduced YBa2Cu3O6.7 thin film Was measur
ed in the frequency range 30 cm(-1) < v < 7000 cm(-1) and for temperatures
5 K < T < 300 K. At lower frequencies, 3 cm(-1) < v < 40 cm(-1), the comple
x conductivity sigma*=sigma (1)+i sigma (2) was measured directly using a t
ransmission technique. Both data sets were combined to obtain the reflectiv
ity in a broad frequency range and thus to improve substantially the qualit
y of the Kramers-Kronig analysis at low frequencies. The analysis of the co
nductivity spectra reveals that the low-frequency spectral weight strongly
depends on temperature directly above TC but saturates for T greater than o
r similar to 150 K.