The modern Forel-Ule scale: a ‘do-it-yourself’ colour comparator for water monitoringS. Novoa, M. R. Wernand and H. J. van der WoerdJ. Eur. Opt. Soc.-Rapid Publ., 9 (2014) 14025DOI: https://doi.org/10.2971/jeos.2014.14025