Poly(ethylene-terephthalate) (PET) film is widely used in photovoltaic module backsheets for its dielectric break- down strength, and in applications requiring high optical clarity for its high transmission in the visible region. However, PET degrades and loses optical clarity under exposure to ultraviolet (UV) irradiance, heat, and moisture. Stabilizers are often included in PET formulation to increase its longevity; however, even these are subject to degradation and further reduce optical clarity. To study the weathering induced changes in the optical properties in PET films, samples of a UV-stabilized grade of PET were exposed to heat, moisture, and UV irradiance as prescribed by ASTM-G154 Cycle 4 for 168 hour time intervals. UV-Vis reflection and transmission spectra were collected via Multi-Angle, Polarization-Dependent, Reflection, Transmission, and Scattering (MaPd:RTS) spectroscopy after each exposure interval. The resulting spectra were used to calculate the complex index of refraction throughout the UV-Vis spectral region via an iterative optimization process based upon the Fresnel equations. The index of refraction and extinction coefficient were found to vary throughout the UV-Vis region with time under exposure. The spectra were also used to investigate changes in light scattering behavior with increasing exposure time. The intensity of scattered light was found to increase at higher angles with time under exposure.
The use of fluorescent-retroreflective materials in transportation safety applications (traffic signing and personal safety garments) has increased significantly over the last decade. The improved photostability of modern fluorescent colorant systems finally allows transportation agencies to make full use of the safety benefits of high visibility fluorescent signing and garments. As the use of fluorescent colored materials has grown, so has the need for accurate colorimetry to reliably and reproducibly describe the properties of these materials. Specifications for materials used in visual signaling are written in terms of absolute chromaticity and luminance factor limits. Therefore, accurate color measurement based on technically sound procedures is necessary to describe these materials. The bispectral or two-monochromator method is the referee procedure for determining the general (illuminant-independent) colorimetric properties of fluorescent materials. Until recently the ability to do bispectral fluorescent colorimetry was limited to a few high level photometric laboratories, almost solely National Standards Laboratories. Commercial bispectral fluorescent colorimeters are now available. The purpose of this paper is to examine the state-of-the-art in the colorimetry of fluorescent-retroreflective high visibility materials. Inter-comparisons were made between bispectral instruments and commercials 1-monochromator instruments. Measurements made on a series of fluorescent-retroreflective and non-retroreflective (diffusely reflecting) fluorescent materials were made on both types of instrument. Ordinary colored retroreflective and diffuse reflective materials were also measured. Representative results of the measurement inter-comparisons are presented and the effects of instrument geometry and illumination are discussed. The results are presented in terms of uncertainty in the determination of luminance factors and chromaticity coordinates. Accuracy is assessed relative to colored fluorescent reference materials calibrated by National Standards Laboratories.
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