An optical fluorescent composite which controls fluorescence intensity comprises a solid layer containing fluorescent material (such as a phosphor) formed on an optically transparent base or substrate and optically coupled to a spectrally selective filter, also formed on the base or substrate, that modifies the intensity of either or both the excitation or emission spectra of the phosphor. Devices constructed in this manner serve as stable references for fluorescent measurements. Sets of such device is made with filters having different transmittance amplitudes, can characterize fluorescent measurement response over several orders of magnitude. In one of numerous alternative structure, the fluorescent layer can be formed on one substrate, the filter formed on a second substrate and the two substrates joined together.