To correctly measure the solar transmittance of a material that scatters light, a detector with uniform directional response must be used, and the integrating sphere is often used in combination with an optical detector for this purpose. However, no real optical detector system has a completely uniform directional response, thus a directional response error will always exist. This note analyzes the directional response error in directional-hemispherical transmittance measurements at solar wavelengths and determines this error for transmittance measurements on materials that scatter light in different ways, made with a state-of-the-art integrating-sphere-spectrophotometer system. It is shown that for certain types of materials, the directional response error is the dominant error in the measurement of directional-hemispherical transmittance.