  Determining redshifts for BL Lacertae (BL Lac) objects using the traditional
spectroscopic method is challenging due to the absence of strong emission lines
in their optical spectra. We employ the photometric dropout technique to
determine redshifts for this class of blazars using the combined 13 broad-band
filters from Swift-UVOT and the multi-channel imager GROND at the MPG 2.2 m
telescope at ESO's La Silla Observatory. The wavelength range covered by these
13 filters extends from far ultraviolet to the near-Infrared. We report results
on 40 new Fermi detected BL Lacs with the photometric redshifts determinations
for 5 sources, with 3FGL J1918.2-4110 being the most distant in our sample at
z=2.16. Reliable upper limits are provided for 20 sources in this sample. Using
the highest energy photons for these Fermi-LAT sources, we evaluate the
consistency with the Gamma-ray horizon due to the extragalactic background
light.
