The Effect of Brick Walls on the Black Hole Radiation
  In order to understand the physical effect of the brick wall boundary condition, we compute the distribution of the zero-point energy of the massless scalar fields minimally coupled to the Schwarzschild and Reissner-Nordstr\"{o}m black hole backgrounds. We find that the black hole radiation spectrum depends on the positions of the brick wall and the observer, and reveals the interference effect due to the reflected field by the brick wall. 
Sang Pyo Kim, Sung Ku Kim, Kwang-Sup Soh, Jae Hyung Yee