  We have measured the reflectance of rhenium in the visible region to
pressures up to 100 GPa in a diamond anvil cell (DAC). By photographing the
reflecting surface, we visually show that there are challenges to obtaining
accurate values in a DAC for several reasons and propose some useful
procedures. We also show that knowledge of the reflectance of rhenium can
overcome the problem of absorption of light by diamonds when studying the
reflectance of materials at high pressure in a DAC.
