The TeV energy spectrum of the Blazar type galaxy Mrk-501 extends beyond 16 TeV and constitutes the most energetic photons observed from an extragalactic object. A fraction of the emitted spectrum is possibly absorbed in interactions with low energy photons of the diffuse extragalactic infrared radiation, which in turn offers the unique possibility to measure the diffuse infrared radiation density by TeV spectroscopy. One of the recently published ideas to reduce the absorption of TeV photons is based upon the assumption that sources like Mrk-501 could produce Bose-Einstein condensates (BEC) of coherent photons. The condensates would have a higher survival probability during the transport in the diffuse radiation field and could mimic TeV air shower events. The powerful stereoscopic technique of the HEGRA air Cherenkov telescopes allows to test this hypothesis by reconstructing the penetration depths of TeV air shower events: Air showers initiated by BECs are expected to reach the maximum of the shower development in the atmosphere earlier than single photon events. By comparing the energy dependent penetration depths of TeV photons from Mrk-501 with those from the TeV standard candle Crab Nebula and simulated events, we can reject the hypothesis that TeV photons from Mrk-501 are pure BECs. ©2001 American Institute of Physics.