The enhancement of the photothermal conversion efficiency of solar energy in solar collectors can be achieved using spectrally selective absorbing surfaces limiting the infrared radiative losses. The interest and the limits of the spectral selectivity, in terms of con-centration ratio and temperature, are discussed and the global quantities a (solar absorptance) and ε (infrared emittance) characteristic of the selectivity are pointed out, considering the energy balance. The useful parameters λ (wavelength) and θ , cp (incidence) related to real problems and real surfaces are then considered. The connections between the measured characteristic quantities a' (λ,θ,Φ, c'(λ,θ,Φ and the optical properties of so-lids are established for the case of homogeneous, inhomogeneous and rough films. Finally the measurement techniques of these quantities are presented. An original system, including integrating spheres, developed at Laboratoire d'Optique des Solides, is described in detail.