Measurement of goniofluorescence in photoluminescent materials

In recent years, there has been an increasing use of fluorescent materials, which has increased the demand for reference standards and procedures for accurate characterisation of colour appearance under specific illumination conditions. Fluorescence emission is a common phenomenon occurring in many objects. These materials absorb light at a certain wavelength interval and then reemit it at longer wavelengths, after about 10-8 seconds. For a fluorescent material, the colour appearance depends upon the combined effect of the fluoresced and reflected radiation. This work is focused in the measurement of the fluorescence of fluorescent standard materials as a function of the irradiation and detection directions, which may be called as "goniofluorescence". IO-CSIC has measured and provided uncertainty budgets for spectral bidirectional scattering distribution function (BSDF) of six fluorescent samples at different combinations of irradiation and detection directions. The measurements were carried out using monochromatic irradiation and a spectrorradiometer as detector, which allows the spectral BSDF to be evaluated as a function of excitation wavelengths. The set of all spectral BSDF at different excitation wavelengths includes not only the spectral BRDF, but also a fluorescence distribution which could be called bidirectional luminescence distribution function (BLDF). The measurements were performed with the goniospectrophotometer GEFE, a device designed and developed at IO-CSIC to measure spectral BRDF at any geometry. It was upgraded by including a monochromator in front of the Xenon lamp. The spectral FWHM was estimated in 7 nm for irradiation and in 3nm for detection (between 380 nm and 780 nm). The measurement area is a circle with a 3 mm diameter, corresponding to a field of view of 1o. The solid angles of irradiation and detection in the measurement setup are formed by cones of 0.5 deg and 2.5 deg respectively. The spectral BRDFs of six fluorescent samples were measured at the geometries resultant from combining the irradiation and detection directions given by the following spherical coordinates (subscripts: i for irradiation and d for detection): Polar angle ?i = 0o, 8o, 15o, 30o, 45o, 60o; Polar angle ?d from 0o to 80o (in steps of 5o); azimuth angle ?i = 0o; azimuth angle ?s = 0o and 180o. A 0o/45o diffuse reflectance standard certificated by NPL (National Physical Laboratory) was used to determine the calibration factor of our system, and all spectral BSDF data were calibrated using that reference. The estimated total relative standard uncertainty was around 0.5 % The dependence of the fluorescence emission on the measurement geometries will be shown in this contribution for every sample. We found a general behavior when representing the fluorescence peak of the data as a function of the geometry. At the three excitation wavelengths, the relative dependence of the fluorescence on the detection direction seems independent from the irradiation direction. The dependence on ?d is symmetrical respect to ?d = 0 o, although an increase of the fluorescence around retro-reflectance geometries were observed in some cases. A very simple physical model was fitted to the experimental data, assuming that fluorescence depends on the distance the fluorescent light has to travel within the material before returning to the air. The dependence of the measured fluorescence on the detection direction can be explained by this simple model. However, its complete dependence on the irradiation direction is needed to be integrated in the model yet and it is part of the ongoing work. Authors are grateful to EMRP for funding the project "Multidimensional reflectometry for industry". The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Authors are also grateful to Comunidad de Madrid for funding the project SINFOTON-CM: S2013/MIT-2790.