Using an integrated-ray approach an expression for the transmission of rectangular section mirror light pipe (MLP) has been derived for the case of collimated light input. The transmittance and the irradiance distribution at the exit aperture of rectangular-section MLPs have been measured experimentally and calculated theoretically for the case of collimated light input. The results presented extend the description of MLPs from the cylindrical case. Measured and calculated transmittances and irradiance distributions are in good agreement. (c) 2008 Elsevier B.V. All rights reserved