Factors affecting the whiteness of optically brightened material

The whiteness of fluorescent white materials is in part due to the absorption of ultraviolet (UV) light and subsequent emission of visible blue light. The UV content of light sources in viewing booths and in spectrophotometers can thus significantly affect the perceived whiteness (PW) and measured sum of reflected and emitted light of fluorescent materials. The effect of UV content on the spectral radiance factor of fluorescent white materials containing different amounts of a fluorescent brightening agent and the subsequent assessment of their PW were evaluated. The UV content of sources in two calibrated viewing booths that simulated D65 and D75 illuminants, separately, was changed by selectively blocking UV emission of the source by approximately 0%, 25%, 50%, 75%, and 100%. The radiance spectra of a series of white fabrics were also obtained using a reflectance spectrophotometer at 0%, 25%, 50%, 75%, and 100% UV transmittance. The CIE and Uchida whiteness indices (WIs) were calculated for white samples and compared to perceptual results under varying illumination and UV conditions. Results indicate relatively modest agreement between perceptual assessments of fluorescent samples and whiteness metrics examined. Results also show that when the UV content of sources used in the viewing booths is adjusted to be similar to that used in measurements, improved correlations between perceptual and calculated results are obtained. The CIE WI was found to outperform the Uchida index under both sources.