The Forel-Ule scale revisited spectrally: preparation protocol, transmission measurements and chromaticity

Abstract

The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers and limnologists since the 19th century,providing one of the oldest oceanographic data sets. Present and future Forel-Ule classifications of global oceanic, coastal and continentalwaters can facilitate the interpretation of these long-term ocean colour data series and provide a connection between the present and thepast that will be valuable for climate-related studies. Within the EC-funded project CITLOPS (Citizens’ Observatory for Coast and Ocean Optical Monitoring), with its main goal to empower endusers,willing to employ community-based environmental monitoring, our aim is to digitalize the colours of the Forel-Ule scale to establishthe colour of natural waters through smartphone imaging. The objective of this study was to reproduce the Forel-Ule scale following theoriginal recipes, measure the transmission of the solutions and calculate the chromaticity coordinates of the scale as Wernand and Vander Woerd did in 2010, for the future development of a smartphone application. Some difficulties were encountered when producing thescale, so a protocol for its consistent reproduction was developed and is described in this study. Recalculated chromaticity coordinates arepresented and compared to measurements conducted by former scientists. An error analysis of the spectral and colourimetric informationshows negligible experimental errors.