Dependence of polytetrafluoroethylene reflectance on thickness at visible and ultraviolet wavelengths in air

TL;DR

This study examines how the thickness of PTFE affects its reflectance at visible and ultraviolet wavelengths, providing detailed measurements and methods to optimize light collection in particle physics detectors.

Contribution

It offers the first detailed analysis of PTFE reflectance dependence on thickness at specific wavelengths, including methods to recover reflectance loss with a specular reflector.

Findings

PTFE reflectance varies with thickness from 92.5% to 94.5% at 450 nm

Reflectance variation within the same piece can reach 2.7%

Using a specular reflector behind PTFE can recover reflectance loss

Abstract

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.