The Rapid Drop is because of the Battery Chemistry, if you are using Alkaline or Zinc-Carbon batteries.
I doubt if that drop will be there if we use Ni-MH or Ni-Cd D-Size Batteries.
Also, with the Energizer Lithium D batteries (which are not out yet) i guess, the output will stay there for much much longer duration.
Do we have Comparitive Runtime Graphs with Ni-MH, Ni-CD, Alkaline, Zinc-Carbon batteries for the said 3D Light to verify the said things.