The observed high energy gamma ray spectrum of PKS 2155-304 modified by the optical depth at three different heights, assuming for illustration that the observed spectrum is emitted at a height of 300   r_g above the accretion disk. The filled circles represent the observed spectrum. The various symbols represent the absorbed spectrum at the following heights: open circles z = 50   r_g; Diamonds: z = 100   r_g; Crosses: z = 150   r_g.