At long wavelengths, diffraction effects cause condensing lightpipes to have a significant response in directions within the geometric optics shadow zone. Here, using an analogy with the corrugated "scalar" horns often used in antenna engineering, we point out that a lightpipe with anisotropic surface impedance on its interior surfaces can have substantially reduced sidelobe levels. This may be important for measurements requiring high dynamic range, particularly in the far-infrared.