NOVOA, S.; WERNAND, M. R.; VAN DER WOERD, H. J.. The Forel-Ule scale revisited spectrally: preparation protocol, transmission measurements and chromaticity. Journal of the European Optical Society - Rapid publications, Europe, v. 8, aug. 2013. ISSN 1990-2573. Available at: <http://www.jeos.org/index.php/jeos_rp/article/view/13057>. Date accessed: 07 Jun. 2017. doi:434.