  The real part of the optical in-plane conductivity of p-- and n--type
cuprates thin films at various doping levels was deduced from highly accurate
reflectivity measurements. We present here a comprehensive set of optical
spectral weight data as a function of the temperature $T (> T_c$), for
underdoped and overdoped samples. The temperature dependence of the spectral
weight is not universal. Using various cut-off frequencies for the spectral
weight, we show that n--type Pr$_{2-x}$Ce$_x$CuO$_4$ and p--type
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ exhibit both similarities and striking
differences.
  The Fermi surface is closed in overdoped metallic samples. In underdoped
Pr$_{2-x}$Ce$_x$CuO$_4$ samples, it clearly breaks into arcs, giving rise to a
"pseudogap" signature. It is argued that such a signature is subtle in
underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$.
