The transmission of sub-millimetre radiation by copper light pipes

It is important in many experiments where electromagnetic radiation is guided by means of mirrors, light pipes or waveguides, to know the reflective properties of the surfaces involved. The authors extend the data available for the reflectivity of copper surfaces for sub-millimetre radiation in the wavelength range 0.2-1 mm (50-10 cm-1). The experimental data are compared with theoretical values based on expressions for the reflectivity of a metal surface derived from electromagnetic theory. It is found that although the general trend of both theory and experiment is the same-a decrease of reflectivity with increasing wave number-the decrease of the experimental values is the more rapid. A reflectivity of 98% at a wave number of 20 cm-1 is obtained.