The question is understood to refer to means of measuring the intensity of fluorescence. This is a somewhat difficult problem as it involves measuring the shade of an area and the intensity of radiation in the shade. Diffuse reflectance values are measured and related to the shade which is characterized in x, y, z parameters using colorimeters. Several factors render precise determination difficult. Because of these difficulties, manufacturers of fluorescent paints do not publish intensity values in their data sheets.
The emission peak and UV excitation are given in listings of current types. In the manufacture of bumilux luminescent pigments, for example, the luminance of fluorescing pigments is measured with a luminance meter. LD values can be read directly in cd/m’. Emission spectra are determined as x, y colour coordinates with a spectrophotometer. With phosphorescent pigments, luminance is measured to DIN 67510 with specially designed equipment.
Standard samples with known absolute values are measured at the same time and values calculated directly. When measuring coloured non-luminescent pigments, values for xy and y are determined to DIN 5033 (luminosity). Standard illuminant's are used for excitation. The "Chroma meter" is an example of apparatus suitable for direct measurement of xy and y coordinates.