  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 measured
VHE gamma-ray spectra: neither the intrinsic spectrum, 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 BL Lac objects 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 BL Lacs' intrinsic emission.
Contrasting measured versus intrinsic emission leads to a determination of the
photon-photon opacity to VHE photons. 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 high-frequency-peaked BL Lac object PKS 2155-304 and discuss how similar
observations can more accurately probe the EBL.
