The far-infrared electronic c-axis conductivity, σ 1 c el, of underdoped to overdoped Y 1− x Ca x Ba 2Cu 3O 7− δ crystals has been studied by ellipsometry. We find that the peculiar temperature and frequency dependence of σ 1 c el, in particular, the appearance of the normal state gap, are determined by the CuO 2 planes and their hole doping state. In addition, we find that the spectral gap in the c-axis conductivity exhibits a very similar temperature and doping dependence like the gap that has been observed by angle-resolved photo-emission experiments around the X-point of the 2D Brillouin zone (the so called `hot spots'). This finding implies that the c-axis transport may be determined mainly by the carriers around the `hot spots'.