Fluorescent materials become increasingly important for signalling and warning purposes. Measurement facilities, however, are still very poor. This article reviews the radiometric aspects of fluorescent materials and the measurement of their radiometric and colorimetric characteristics. The precision of colorimeters and one-monochromator instruments can be improved by applying luminescent reflectance standards, calibrated with a two monochromator, system. Problems of radiation feedback with sphere geometry and a way to solve this problem with a two-monochromator system are discussed.