  The Extragalactic Background Light (EBL) is the integrated light from all the
stars that have ever formed, and spans the IR-UV range. The interaction of
very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at
cosmological distances, with the intervening EBL results in electron-positron
pair production that leads to energy-dependent attenuation of the observed VHE
flux. This introduces a fundamental ambiguity in the interpretation of the
measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are
separately known - only their combination is. In this paper we propose a method
to measure the EBL photon number density. It relies on using simultaneous
observations of blazars in the optical, X-ray, high-energy (HE: E>100 MeV)
gamma-ray (from the Fermi telescope), and VHE gamma-ray (from Cherenkov
telescopes) bands. For each source, the method involves best-fitting the
spectral energy distribution (SED) from optical through HE gamma-rays (the
latter being largely unaffected by EBL attenuation as long as z<1) with a
Synchrotron Self-Compton (SSC) model. We extrapolate such best-fitting models
into the VHE regime, and assume they represent the blazars' intrinsic emission.
Contrasting measured versus intrinsic emission leads to a determination of the
gamma-gamma opacity to VHE photons - hence, upon assuming a specific cosmology,
we derive the EBL photon number density. Using, for each given source,
different states of emission will only improve the accuracy of the proposed
method. We demonstrate this method using recent simultaneous multi-frequency
observations of the blazar PKS2155-304 and discuss how similar observations can
more accurately probe the EBL.
