This paper derives expressions for the transmission of mirror light pipes (MLP) with cross sections exhibiting congruent symmetry. These include rectangular, triangular, rhombic and hexagonal cross sections. For the case of rectangular MLP the transmissions calculated by this method are the same as the transmissions calculated by integrated ray trace methods. Measurements of the transmission of rectangular and triangular MLP were made and compared with calculated transmissions. It is shown that calculated transmissions are consistent with measured transmissions provided a reflectance that varies with incidence angle on the aperture of the MLP is used. A simple expression for this variation was found from the experimental measurements. (C) 2010 Elsevier Ltd. All rights reserved.