  The next generation of experiments for the measurement of the Cosmic
Microwave Background (CMB) requires more and more the use of advanced
materials, with specific physical and structural properties. An example is the
material used for receiver's cryostat windows and internal lenses. The large
throughput of current CMB experiments requires a large diameter (of the order
of 0.5m) of these parts, resulting in heavy structural and optical requirements
on the material to be used. Ultra High Molecular Weight (UHMW) polyethylene
(PE) features high resistance to traction and good transmissivity in the
frequency range of interest. In this paper, we discuss the possibility of using
UHMW PE for windows and lenses in experiments working at millimeter
wavelengths, by measuring its optical properties: emissivity, transmission and
refraction index. Our measurements show that the material is well suited to
this purpose.
