In this paper we report the use of Fraunhofer Line Radiometry and the Fraunhofer Line Depth (FLD) method to measure the passive fluorescence of select, high quantum yield (> 0.7) fluorophores. The measurements we describe involved direct solar illumination of both a non-fluorescent standard material (NIST, Spectralon), and the fluorescent target materials (in solution) within several relevant Fraunhofer lines: 486 nm (H-beta), 518 nm (Mg), 589 nm (Na), and 656 nm (H-alpha). Of the selected labels, Rhodamine B demonstrated the highest fluorescence coefficient of 2.2, at 589 nm, and possessed a fluorescence intensity of 1.1 × 106 cps at an optical density of 1.0. These results show fluorescent labels can be selectively measured and observed passively in relevant Fraunhofer lines. The implications for such findings indicate the potential daytime measurement of environmental labels and probes as a promising sensing technology.