Modem building simulation tools depend more and more on the availability of accurate data describing the solar energy and daylight performance of glazing under realistic conditions. As a result, the need for reliable measurement techniques for directional optical properties is growing. Most existing spectrophotometer accessories for measuring directional optical properties of coated glass samples proved to have serious shortcomings. After investigating all problems involved with this type of measurement, a new spectrophotometer accessory has been developed by the author, taking into account all major measurement problems. The accessory is capable of performing measurements of directional reflectance as well as transmittance. Measurement procedures have been developed for maximum accuracy and efficiency. Results of an inter-laboratory comparison have validated the performance of the accessory, which has recently been turned into a commercially available product for the glazing industry. (C) 2002 Elsevier Science Ltd. All rights reserved.