  Polytetrafluoroethylene (PTFE) is an excellent diffuse reflector widely used
in light collection systems for particle physics experiments. However, the
reflectance of PTFE is a function of its thickness. In this work, we
investigate this dependence in air for light of wavelengths 260 nm and 450 nm
using two complementary methods. We find that PTFE reflectance for thicknesses
from 5 mm to 10 mm ranges from 92.5% to 94.5% at 450 nm, and from 90.0% to
92.0% at 260 nm. We also see that the reflectance of PTFE of a given thickness
can vary by as much as 2.7% within the same piece of material. Finally, we show
that placing a specular reflector behind the PTFE can recover the loss of
reflectance in the visible without introducing a specular component in the
reflectance.
